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Dedolomitization: Review and case study of uncommon mesogenetic formation conditions
... Les coquilles aragonitiques sont remplacées par de la calcite selon deux mécanismes (Martin et al., 1986 ;Maliva, 1998) : (1) la dissolution partielle ou complète de la coquille permettant la création de pores moldiques et pouvant ensuite être remplis par des ciments de calcite ou (2) la transformation aragonitecalcite par remplacement moléculaire avec préservation de la microstructure originelle de la coquille (Folk, 1965 ;Kendall 1975 ;Sandberg 1975 ;Sandberg et Hudson 1983). Le processus de dédolomitisation peut résulter soit du remplacement moléculaire de la dolomite par des cristaux de calcites (sans création de pores), soit d'un processus de dissolution-précipitation avec création de pores moldiques (Kenny, 1992 ;James et al., 1993), dépendant principalement de la texture des dolomites ( Fig. 1.11) (Fu et al., 2008 ;Nader et al., 2008 ;Schoenherr et al., 2018). De plus, de nombreuses études ont montré que les processus de dédolomitisation sont étroitement liés aux processus de karstification (Nader et al., 2008 ;Huaguo et al., 2016). ...
... mainly depend on texture of the original dolomite rock (Al-Hashimi and Hemingway, 1973; Ayora et al., 1998;Fu et al., 2008;Nader et al., 2008;Schoenherr et al., 2018). Moreover, many studies showed that dedolomitization process are indicators of surface-related diagenetic transformations and demonstrated the close relationship with karstification processes (Kenny, 1992;Bischoff et al., 1994;Canaveras et al., 1996;Guo et al., 1996;Raines and Dewers, 1997;Sanz-Rubio et al., 2001;Nader et al., 2008;Huaguo et al., 2016). ...
... At that time the chemical conditions change once again, downward meteoric fluids dissolved the Jurassic limestones and carbonate cements (non-luminescent prismatic calcite cements and dolomite crystals) which may provide oversaturation with respect to calcite resulting to the subsequent infilling of mouldic pores and mm-scale vug spaces (samples KCVE5 and SMA6) (Fig. 7.14B). Therefore, dedolomitization process is mainly localized at the vicinity of karstic network during the telogenesis, as previously demonstrated by Nader et al. (2008), Fu et al. (2008), and Schoenherr et al. (2018). The telogenetic dedolomites precipitated during the exhumation of the platform in the Quercy area, 250 meters below the topographical surface (Fig. 7.14B). ...
Dans un contexte de changement climatique et de pression croissante sur la ressource en eau des aquifères carbonatés, une meilleure compréhension du fonctionnement et de la recharge des réservoirs est nécessaire afin d'optimiser leur exploitation. Une caractérisation sédimentaire et diagénétique des calcaires s'avère indispensable. Ce travail pluridisciplinaire se concentre sur les roches carbonatées du Jurassique moyen et de l'Oxfordien de la bordure nord-est du Bassin Aquitain. Le premier objectif est de quantifier l'influence couplée des fluctuations climatiques à long terme et de la création d'accommodation sur les producteurs carbonatés et l'accumulation de carbonates. Le second objectif est de reconstruire l'histoire diagénétique et paléohydrologique du bassin et de proposer une méthode pour dater les phases de dissolution et les périodes de création de porosité dans les calcaires. Les études menées dans le Bassin Aquitain et l'ouest de la France montrent que l'augmentation du taux de création d'accommodation provoque (1) l'augmentation du taux d'accumulation de carbonates et (2) des changements de producteurs carbonatés, quel que soit le climat. En période de climat aride, la création d'espace disponible est comblée par une production micritique-microbienne. En climat humide, les taux d'accumulation de carbonate diminuent drastiquement et provoque le déclin des producteurs oolithiques, photozoan et micrite-microbien. La combinaison d'observation pétrographique et de la méthode géochronologique U-Pb permet de dater les phases de dissolution et de création de pores, mais également d'établir une paragenèse à grande échelle. Les processus de néomorphisme et dolomitisation sont datés au cours de la période Jurassique supérieur à Crétacé inférieur. Les processus de dissolution postérieure affectent préférentiellement les corps dolomitiques bajociens. Les âges U-Pb mettent en évidences une succession de phases de dissolution-recristallisation au Paléocène-Éocène. Les phases de cimentation calcitique sont synchrones des processus de dissolution lors de périodes de karstification et peuvent se produire en profondeur formant et comblant des pores vacuolaires de taille millimétriques à métriques.
... On the other hand, stratabound geobodies extend laterally away from faults, forming elongate lobes that may extend for long distances away from the faults that fed the dolomitisation fluids (e.g. Hirani et al., 2018;Gasparrini et al., 2017;Gomez-Rivas et al., 2014Lapponi et al., 2011;Martín-Martín et al., 2013, 2018Nader & Swennen, 2004;Sharp et al., 2010). In some cases, both geometries appear connected forming a continuum (e.g. ...
... During recent years, numerous examples of fault-related dolostones from outcrops have been used as reservoir analogues to evaluate the spatial controls on the formation and distribution of reservoir and non-reservoir rock textures in three dimensions (e.g. Beckert et al., 2015;Hirani et al., 2018;Hollis et al., 2017;Koehrer et al., 2010;Lapponi et al., 2011;López-Horgue et al., 2010;Martín-Martín et al., 2013, 2018Rustichelli et al., 2017;Stacey et al., 2020;Sharp et al., 2010;Slater & Smith, 2012;Vandeginste et al., 2015;Yao et al., 2020). These studies typically highlight the depositional controls on dolostone distribution, the role of faults as conduits and/or barriers to fluid flow, and the relationship between fracture networks and dolostone geometry. ...
... thicknesses) that extend laterally for several kilometres away from these structures. Work by Gomez-Rivas et al. (2014, Martín-Martín et al. (2013, 2018 and Yao et al. (2020) established spatial variations in dolostone geometry associated with proximity to feeding faults, overlying sealing units, host rock facies, as well as sub-seismic structural features such as stylolites and fractures. ...
Fault-related dolomitisation is responsible for the development of numerous hydrocarbon reservoirs hosted in diagenetically-altered carbonates and is therefore critical to hydrocarbon exploration, subsurface storage (i.e., CO 2), the formation of associated mineralisation (i.e., MVT-deposits) and for understanding the key controls on subsurface fluid flow. Multiple dolomitised outcrop analogues have been characterised in recent years, but uncertainty still remains as to the controls on dolomitisation in terms of dolostone geobody size and geometries, their distribution and how they impact on reservoir quality. Late Tithonian shallow-marine carbonates at Serra Esparreguera in the Maestrat Basin (E. Spain) were partially dolomitised on the seismic scale, resulting in a spectrum of geobodies with varying degrees of spatial connectivity. Dolostone predominantly replaces Polpís Fm wackestones and packstones, and bioclastic grainstones of the Bovalar Fm. Dolostone geobodies transition through vertical stratigraphy from being massive and spatially extensive to localised stratabound bodies as textural heterogeneity increases. Irregular dolostone geometries occur in the Polpís Fm, which is texturally homogenous relative to the overlying Bovalar Fm, cross-cutting bedding in areas with high abundance of faults. Faults occur adjacent to dolostone and constrain its lateral extent across the outcrop. Dolomitisation fronts are typically sharp with morphologies affected by small-scale faults and bedding-parallel stylolites. Dolomitisation occurred under burial conditions and dolostones were later overprinted by phases of calcite and saddle dolomite cementation. The spatial distribution of dolostone is strongly influenced by the depositional heterogeneity and faults, whilst smaller structures (i.e., metre-scale fractures and stylolites) and bedding surfaces controlled the dolomitisation front geometry. Dolostone geobodies at Serra Esparreguera provide new insights into the structural, depositional and diagenetic controls on dolomitisation at a seismic scale, which can be used as a predictive guide to improve the understanding of carbonate reservoirs with complex paragenetic histories.
... Dolomitization is relatively well documented, whereas little attention has been paid to dedolomitization. Dedolomitization occurs under a wide range of diagenetic conditions, such as during burial-diagenesis (Stoessell et al. 1987;Schoenherr et al. 2018), related to contact metamorphism (Faust 1949;Wood and Armstrong 1975), and associated with hydrothermal activity (Ronchi et al. 2004;Huang et al. 2013;Lyu et al. 2013). However, most of the reported dedolomite occurrences are indicative of near-surface conditions, and associated with the effects of meteoric water along 1 3 78 ...
... Most convincing and commonly known of dedolomitization is certainly a dolomite rhombohedron, being partially replaced by equicrystalline anhedral calcite. Schoenherr et al. (2018) summarizes the most common dedolomite microtextures: (1) the dolomite is totally replaced by anhedral calcite in mosaic texture; (2) the dolomite is selectively replaced by calcite at rims or cores of the rhombohedron; (3) the dolomite is partly replaced by calcite with dolomite poikilotopic inclusions in dedolomite rhombohedra. The above petrographic features are quite different from those of this study. ...
... Dedolomitization occurs in a wide range of diagenetic environments, including the surface environment (Chilingar 1956), early burial environment (De Groot 1967), and deep burial environment (Stoessell et al. 1987). Most case studies conclude that an overwhelming majority of dedolomites form from meteoric waters in the near-surface environment (Ronchi et al. 2004;Nader et al. 2008;Reuning et al. 2009;Schoenherr et al. 2018). In the present study, the dedolomites are only observed in areas where the evaporite deposits are removed or residue-anhydritic breccias are present, suggesting that dedolomitization of the Majiagou Formation is probably related to the dissolution of anhydrite by fresh water, which is supported by the petrographic features and geochemical data of the dedolomite. ...
Dedolomites, the replacement of dolomite by calcite, are widely distributed in the Ordovician Majiagou Formation of the southeastern Ordos Basin, China, which critically affects reservoir quality throughout the region. Two types of dedolomites were recognized in the upper 100 m of the Majiagou Formation, namely brecciated dedolomite and crystalline dedolomite. The petrographic and geochemical data reveal that the investigated dedolomites are often associated with clay minerals and dissolution-collapse breccia, indicating their close relationship with erosional unconformities. The relative negative δ ¹⁸ O and δ ¹³ C values and relative high Fe, Mn contents of dedolomite with respect to corresponding dolomite are interpreted as being result from meteoric phreatic water involvement. The dedolomitization process related to subaerial exposure is conventionally interpreted to be formed by dolomite dissolution and calcite precipitation. The rhombic shape crystals in the brecciated dedolomite was formed by an Mg ²⁺ -loss process under evaporite solution conditions, which retain the rhombic shape of the dolomite. The crystalline dedolomites, however, were formed by dissolution/precipitation and recrystallization process to form medium to coarse calcite mosaic with very low intercrystalline porosity. The resulting fabrics are mainly controlled by the paleo-topography. The precursor dolomite of the crystalline dedolomite is primarily recrystallized mudstone, which is more soluble and favorable for calcite precipitation. The crystalline dedolomite mostly occurs in the low paleotopographic locations, where the karst-saturated water with respect to CaCO 3 is more concentrated, occluding the remaining porosity when excess calcium is supplied. It’s, therefore, recommended to implement drilling in tectonic highland and avoid low paleotopographic locations.
... In addition, dolomites have commonly been dedolomitized, i.e. calcitized and converted to a secondary limestone. This is considered to have taken place at two discrete intervals (discussed in more detail later): (1) during shallow-to-moderate to deeper burial, possibly through interaction with Ca-rich fluids derived from gypsum dehydration to anhydrite (as is the case in the Zechstein of Germany, Schoenherr et al. 2018); and (2) during uplift related to dissolution of gypsum-anhydrite, contact with meteoric water and formation of collapse breccias (Smith 1972). During burial, anhydrite was commonly precipitated in cavities and between grains as a cement, occluding porosity. ...
... The isotopic data support formation of these secondary limestones from shallow to moderate burial depths through neomorphism of an original mixedmineralogy sediment to calcite. Dedolomite that formed during moderate burial, i.e. before uplift and evaporite dissolution, is well documented from the subsurface in Germany (Schoenherr et al. 2018) and occurs in the Roker slope facies at Marsden. The δ 13 C values for this dedolomite range from +6 to +5‰, and the δ 18 O values from 0 to −4.0‰ (Fig. 18). ...
... It is thus envisaged that limestone recrystallization, dolomite recrystallization, dolomite calcitization (dedolomitization) and the neomorphism of carbonate to form concretionary limestone all took place during burial at shallow to moderate to deeper depths at increasing temperatures. A possible driving mechanism for these processes is the dehydration and dewatering of gypsum on its conversion to anhydrite, which can begin at a few hundred metres and continue to around 1000 m (Jowett et al. 1993;Schoenherr et al. 2018). ...
Hydrocarbon reservoirs hosted in Permian strata were some of the first to have been discovered in Europe. With discoveries in the Zechstein carbonates of Norway in recent years, and with exploration of Zechstein prospects both onshore and offshore UK, as well as in Dutch, Danish and Norwegian offshore sectors, understanding the architecture of the Zechstein carbonates remains very relevant. Here we study outcrops of Roker Formation carbonates (Z2, Ca2) in NE England to better understand geological processes associated with deformation following evaporite dissolution, with implications for exploration and production.
Collapse of Z2 Roker Formation strata in NE England, following the dissolution of c. 100 m or more of the Z1 Hartlepool Anhydrite, resulted in fundamental changes to the architecture of the succession. Complete dissolution of the anhydrite removed an effective regional seal and dramatically enhanced matrix and fracture permeability of the overlying Roker Formation. The collapsed Roker Formation can be vertically divided into three zones, based upon the degree of deformation. The lower zone and vertical collapse-breccia pipes that can extend across all zones have the highest permeabilities. The process of collapse was gradual, with local variations in the degree of brecciation. We derive a schematic sequence of collapse, recognizing the impact of mechanical barriers within the succession in retarding deformation up-section and it is this that ultimately leads to the vertical zonation.
Timing of evaporite dissolution is poorly constrained: it could have occurred soon after deposition, at the end of the Permian or during Tertiary uplift. It is known that evaporite dissolution has occurred offshore, with the oil fields Auk and Argyll (UK Central North Sea) given as examples of dissolution collapse-brecciated reservoirs. Reservoir quality is typically improved, with both matrix and fracture porosity and permeability enhanced. Complete evaporite dissolution could in some cases lead to the potential breach of the seal.
... Hallenberger et al. (2018) recently focused on unravelling the origin of these calcitising fluids and estimated mass balance constraints, determining that calcitising fluid generated during gypsum -anhydrite dehydration would only account for <1% of slope calcitisation in the Ca2. Schoenherr et al. (2018) indicates that pressure solution of the surrounding anhydrite beds is likely to be the dominant source of calcitising fluids. ...
... Dolomitic matrix preserved between stylolite teeth was not observed in this study. However, patchy remnants of dolomitic matrix have been observed between stylolites elsewhere in the Ca2 by Schoenherr et al., (2018). Stylolites could alternatively have nucleated at layer interfaces (Sheppard, 2002) after calcitisation. ...
... Stylolites also preserve partially calcitised dolomite matrix, which suggests that further stylolitisation took place after calcitisation (Fig. 16b). This is further supported by observations from Schoenherr et al. (2018), where stylolites are found in calcitised dolomite and not exclusively dolomite-calcitised dolomite contacts, suggesting that stylolites also postdated calcitisation. Based on evidence from the present study, together with work by Schoenherr et al. (2018), stylolitisation was an ongoing process during calcitisation in the Ca2, facilitated by prolonged subsidence commencing in the Early Triassic. ...
Stylolites are rough dissolution surfaces that form due to intergranular pressure-solution resulting from burial compaction or tectonic stress. Despite being ubiquitous in most carbonate rocks, their potential impact on structural diagenesis and fluid flow remains unclear. The Zechstein 2 Carbonate (Ca2) is a diagenetically complex reservoir in the Southern Permian Basin and represents one of the most prolific gas reservoirs in NW Germany. This investigation focuses on evaluating the relationship between stylolites, fractures/veins and their subsequent influence on the spatial variations in reservoir quality. We utilise drill core samples to carry out a combined analysis of cross-cutting relationships between different structures and diagenetic products. We therefore use a combination of petrography and statistical analyses on stylolite networks, focusing on their occurrence, morphology and sealing capacity. In the study area, the Ca2 carbonate mudstone was deposited in a slope environment and dolomitised under shallow burial conditions, followed by bedding-parallel stylolitisation during burial. Results indicate that calcium-rich fluids percolated from neighbouring evaporite units causing widespread calcitisation within the more distal environments of deposition. Some stylolites locally acted as barriers to affect the migration of the calcitising fluids, resulting in a macroscopic diagenetic stratification of relatively porous dolomite and areas of calcitised dolomite with lower porosity. However, pressure-solution continued during burial and bedding-parallel stylolites also appear postdating calcitisation. During inversion, horizontal stylolites were reopened to act as conduits to enable fluid migration that precipitated metal sulphides. This indicates that stylolites acted as both barriers and conduits for fluid flow depending on variations of the overburden pressure and regional stress regime. Stylolites present a range of sealing capacities between 63 and 89%, depending on their morphology, and can result in partial leakage and subsequent invasive calcitisation in their vicinity. This study highlights the importance of understanding the impact of stylolites on structural diagenesis and spatial variations in petrophysical rock properties that determine reservoir quality.
... In addition, dolomites have commonly been dedolomitized, i.e. calcitized and converted to a secondary limestone. This is considered to have taken place at two discrete intervals (discussed in more detail later): (1) during shallow-to-moderate to deeper burial, possibly through interaction with Ca-rich fluids derived from gypsum dehydration to anhydrite (as is the case in the Zechstein of Germany, Schoenherr et al. 2018); and (2) during uplift related to dissolution of gypsum-anhydrite, contact with meteoric water and formation of collapse breccias (Smith 1972). During burial, anhydrite was commonly precipitated in cavities and between grains as a cement, occluding porosity. ...
... The isotopic data support formation of these secondary limestones from shallow to moderate burial depths through neomorphism of an original mixedmineralogy sediment to calcite. Dedolomite that formed during moderate burial, i.e. before uplift and evaporite dissolution, is well documented from the subsurface in Germany (Schoenherr et al. 2018) and occurs in the Roker slope facies at Marsden. The δ 13 C values for this dedolomite range from +6t o+5‰, and the δ 18 O values from 0 to −4.0‰ (Fig. 18). ...
... It is thus envisaged that limestone recrystallization, dolomite recrystallization, dolomite calcitization (dedolomitization) and the neomorphism of carbonate to form concretionary limestone all took place during burial at shallow to moderate to deeper depths at increasing temperatures. A possible driving mechanism for these processes is the dehydration and dewatering of gypsum on its conversion to anhydrite, which can begin at a few hundred metres and continue to around 1000 m (Jowett et al. 1993;Schoenherr et al. 2018). ...
In this workshop we take a virtual tour to key clastic, carbonate, basement and shale outcrops to look more closely at the main types of fractured reservoirs.
... In this period, the primary limestone was dolomitized by surrounding magnesium-rich hydrothermal fluid, forming the dolomitized crust in contemporaneous and quasi contemporaneous phases ( Fig. 8b; Guo et al. 2018). The magnesium content in the fluid decreases with increasing dolomitization, while calcium levels increase due to the fluid's infiltration and dissolution of carbonates (Schoenherr et al. 2018). The Ca/Mg ratio is a decisive factor in the dedolomitization (Khalaf and Abdal 1993;Zhang et al. 2012). ...
... Fluid-rock interaction could play a pivotal role in the alteration of rock(Piccoli et al. 2016;Deng et al. 2020a; Ahmat et al. 2022;Li et al. 2023; Qiu et al. 2023a, b). Dedolomitization occurs in both supergene and burial environments(Budai et al. 1984;Schoenherr et al. 2018) and can function in thermal settings up to200℃ (DeGroot 1967;Stoessell et al. 1987). Dedolomitization typically involves the release of magnesium ions from dolomite during dissolution and the incorporation of calcium ions from calcium-rich fluids into the dolomite. ...
The knife-like structure generally develops on the weathered surfaces of dolostone, characterized by irregular distribution of karst ditches, whereas the mechanism driving its development remains unclear. This study aims to elucidate these mechanisms by investigating the roles of dedolomitization and subsequent dissolution, which are hypothesized to be crucial in the formation of this distinctive structure. We employ petrographic and geochemical analyses of oolitic dolostone from the Cambrian Zhangxia Formation, North China Craton, to explore these processes. The oolitic dolostone contains primary limestone, dolomitized crust, calcium-rich metasomatic zones, and calcite veins. The calcium-rich metasomatic zones exhibit a high degree of dissolution-related reformation. Micro-XRF analysis demonstrates that the metasomatic zones of dolostone are more calcium-rich than the undedolomitized dolostone. The high Ca/Mg ratio (19.38) and strong dedolomitization within the calcium-rich metasomatic zones indicate a low degree of resistance to dissolution. The process of dedolomitization was induced by a hydrothermal fluid at a paleotemperature of ca. 65.55℃ as indicated by the carbon and oxygen (C-O) isotopes of the calcium-rich metasomatic zones. Our results demonstrate that dedolomitization, facilitated by the interaction of primary limestone with calcium-rich hydrothermal fluids, led to the formation of calcium-rich metasomatic zones within the fractures. The calcite veins are generated from calcium-saturated hydrothermal fluids. We propose that the knife-like structure results from two processes: dedolomitization, which decreases the resistance to dissolution, followed by an increased degree of dissolution in the calcium-rich metasomatic zones.
... The calcite cement is restricted to intercrystalline pores without any precursor dolomite fabrics (e.g., Fu et al., 2008). The lack of textural evidence, meteoric diagenetic assemblages at this diagenetic stage (e.g., Al-Hashimi and Hemingway, 1973;Ronchi et al., 2004;Nader et al., 2008), and absence of local anhydrite unit at the Sangerhausen site as a Ca 2+ fluid source (Schoenherr et al., 2018) imply dedolomitization is not the likely process for the formation of the vuggy pores in the Ca1. Dedolomitization is a slow process that can increase porosity (e.g., Lucia, 1995Lucia, , 2004 or occlude porosity, depending on the presence (e.g., anhydrite dissolution) or absence of Ca 2+ -rich fluid sources (Escorcia et al., 2013;Schoenherr et al., 2018). ...
... The lack of textural evidence, meteoric diagenetic assemblages at this diagenetic stage (e.g., Al-Hashimi and Hemingway, 1973;Ronchi et al., 2004;Nader et al., 2008), and absence of local anhydrite unit at the Sangerhausen site as a Ca 2+ fluid source (Schoenherr et al., 2018) imply dedolomitization is not the likely process for the formation of the vuggy pores in the Ca1. Dedolomitization is a slow process that can increase porosity (e.g., Lucia, 1995Lucia, , 2004 or occlude porosity, depending on the presence (e.g., anhydrite dissolution) or absence of Ca 2+ -rich fluid sources (Escorcia et al., 2013;Schoenherr et al., 2018). In a hand-specimen (cm) scale, these vuggy pores are aligned in specific directions in the lowermost samples of the Ca1 and disappear upward. ...
The Southern Permian basin in central Europe contains a number of important high-grade sediment-hosted Cu deposits. Laterally extensive stratabound Cu and Zn-Pb sulfide mineralized rocks are located at a major stratigraphic redox boundary, where coarse-grained continental sandstones of the uppermost Rotliegend Group are overlain by carbonaceous mudstones (T1) and limestones (Ca1) of the Zechstein Formation. This study investigates the diagenetic evolution and style of sulfide mineralization in three drill cores that intersect Cu and Zn-Pb sulfide mineralized rocks at three locations (Sangerhausen, Allstedt, and Wallendorf) in the Saale subbasin (Eastern Germany), which is located at the southern margin of the Southern Permian basin. We combine macro- to microscale petrographic data (binocular, transmitted and reflected light, and scanning electron microscopy) with quantitative X-ray diffractometry and bulk-rock geochemical analyses. Petrographic results show extensive, primary-porosity-occluding, early diagenetic calcite cementation that predates both the diagenetic alteration of detrital clasts and sulfide mineralization. The highest-grade Cu and Zn-Pb sulfides (bornite, sphalerite, and galena) replace the calcite cement, with subordinate replacement of dolomite and detrital clasts. Quantitative mineralogical and geochemical data demonstrate that the highest base metal (Cu, Zn, and Pb) concentrations are associated with carbonate-rich samples, mostly as disseminated mineralization in the middle T1. Bulk-rock geochemical results show enrichment and covariation of redox-sensitive trace elements (RSTEs, e.g., Mo) with total organic carbon content toward the lower T1, consistent with highly reducing depositional conditions. Overall, the distribution and dissolution of calcite cement across this stratigraphic redox boundary provided the main control on the lateral migration of base metal-bearing fluids and high-grade Cu and Zn-Pb sulfide mineralization in the Saale subbasin.
... Mapping lithology and mineralogy in outcrops is especially important when mineralogical associations are controlled by porosity-enhancing or porosity-occluding diagenetic events, such as dolomitization and dedolomitization processes (Swart et al., 2005;Lindsay et al., 2006;Al-Emadi et al., 2009;Morad et al., 2012;van Koppen et al., 2015;Schoenherr et al., 2018). In many outcrop studies, mapping and quantifying mineralogy is desirable for improved understanding of the diagenetic history and for differentiating facies that may exhibit variations in petrophysical properties, such as permeability or porosity. ...
... Dolomitization can produce locally enhanced porosity and permeability in carbonate rocks (Schoenherr et al., 2018). Subsequent percolation of mineralizing fluids can trigger calcite recrystallization of dolomites as well as silica and micron-scale crystal halite filling dolomite euhedral crystals through intracrystalline pores. ...
Geological models from outcrop analogues are often utilized as a guide, or soft constraint, for distributing reservoir properties in subsurface models. In carbonate outcrops, combined sequence stratigraphic, sedimentological, and petrographic
studies constrain the heterogeneity of geobodies and diagenetic processes, including dolomitization, at multiple scales. High-resolution digital outcrop modeling further aids geometric mapping, geobody definition, and statistical analysis, though its usefulness for detailed mineralogical and lithological mapping is limited. Hyperspectral imaging offers enhanced spectral resolution for mapping subtle mineralogical differences. In both outcrops and subsurface, differences in carbonate composition can provide key information for distributing porosity and permeability, yet this mapping is highly challenging in field studies due to access difficulties, visible material differences and sampling resolution. Spectral analysis of limestone– dolomite ratios conducted in laboratory studies indicates theoretical measures for quantitative identification and mapping of dolomite degrees within carbonate rocks. In this study, close-range hyperspectral imaging is applied to outcrops of the Alacón Member, Barranco del Mortero, northeastern Spain, to identify exposed limestone–dolomite geobodies
and to quantify the degree of dolomitization across outcrop faces. Hyperspectral imaging is supplemented with photogrammetric outcrop modeling, field spectroscopy, and laboratory sample analysis for empirical validation and uncertainty analysis. Hyperspectral mapping shows that earlier fieldwork utilizing visual inspection of difficult
to access outcrop surfaces had overestimated the amount of dolomite in the outcrop. Results indicate that hyperspectral imaging identified dolomite bodies more accurately and reliably than conventional field methods and facilitates the mapping of dolomite contribution in areas modified by dedolomitization, where dolomite content changes by
more than ~20%.
... A further rock type at Niue was observed to be derived from dolomite, but is a further stage in diagenesis. Some of the minerals in dolomite in one layer studied (Schofield and Nelson 1978) were observed being elementally replaced, and the authors called this "dedolomite" a common usage (Schoenherr et al. 2018). The supposition is that rain has reacted with the dolomite. ...
... All saddle dolomites from the lower part of the GL outcrop (lower than the layer 40) were dedolomitized (Figures 8e,f and 14). This phenomenon might be attributed to the intrusion of Ca-rich fluids into the system, resulting from the conversion of gypsumanhydrite (gypsum lost 49% of its volume in the form of intercrystalline water, which is saturated with respect to CaSO 4 [128]), as well as by pressure dissolution within anhydrite in the burial setting [129]. The reported occurrence of Early-Middle Jurassic gypsum and anhydrite [105] may lend support to this hypothesis. ...
The age and dolomitization processes in the Paleo-oil reservoir zone, which is composed of massive dolostones found in the Qiangtang Basin (SW China), are still debated. In this research, the Long’eni-Geluguanna Area was selected. Macroscopic information, thin sections, and geochemical methods were used to investigate the dolomitization characteristics and the processes that controlled dolomitization. Five types of replacive dolomites and two types of dolomite cement were observed. Some of the dolomites displayed ghosts of primary sedimentary structures. Saddle dolomites were prevalent, occurring in the interparticle and moldic pores of the limestone which should have been filled at an early diagenetic stage. Ten microfacies types were identified. The foraminifera assemblage provides evidence that the studied interval is of Early Jurassic age. The δ13C values are similar to the contemporaneous seawater signature. The REE+Y patterns of limestones and dolostones exhibit similarities to that of seawater. The mean Na and Sr values are comparable to those of other near-normal seawater dolomites. The δ18O values of all lithologies are markedly depleted. The dolomitization started penecontemporaneously, with deposition. A general sand shoal setting with patch reefs developed. The dolomitizing fluids, near-normal seawater, was probably formed by slight evaporation on top of the shoal. Saddle dolomites in the interparticle and moldic pores might indicate hydrothermal activity, which also caused the recrystallization of some pre-existing dolomites. The recrystallization might have slightly increased the crystal size, demolished the ghost structures, formed saddle dolomites, and altered the REE+Y patterns. The recrystallization extent diminished with increasing distance from the fluids-providing fracture. Furthermore, the existence of protected areas within the sand shoal settings could enhance the vertical and horizontal heterogeneity of dolostone reservoirs.
... Mg 2? versus SO 4 2-? HCO 3 - (Fig. 3g) shows that for spring S5, water undergoes a dedolomitization process that converts dolomite into calcite (Schoenherr et al. 2018): ...
The regular hydrochemical monitoring of groundwater in the Mila basin over an extended period has provided valuable insights into the origin of dissolved salts and the hydrogeochemical processes controlling water salinization. The data reveals that the shallow Karst aquifer shows an increase in TDS of 162 mg L−1 while the thermal carbonate aquifer that is also used for drinking water supply exhibits an increase of 178 mg L−1. Additionally, significant temperature variations are recorded at the surface in the shallow aquifers and the waters are carbogaseous. Analysis of dissolved major and minor elements has identified several processes influencing the chemical composition namely: dissolution of evaporitic minerals, reduction of sulphates, congruent and incongruent carbonates’ dissolution, dedolomitization and silicates’ weathering. The hydrogeochemical and geothermometric results show a mixing of saline thermal water with recharge water of meteoric origin. Two main geothermal fields have been identified, a partially evolved water reservoir and a water reservoir whose fluid interacts with sulphuric acid (H2S) of magmatic origin. These hot waters that are characterized by a strong hydrothermal alteration do ascend through faults and fractures and contribute to the contamination of shallower aquifers. Understanding the geothermometry and the hydrogeochemistry of waters is crucial for managing and protecting the quality of groundwater resources in the Mila basin, in order to ensure sustainable water supply for the region. A conceptual model for groundwater circulation and mineralization acquisition has been established to further enhance understanding in this regard.
... The floor and roof anhydrites originally were deposited as gypsum [17]. Up to 40 vol% of structural bound water is expelled during recrystallization from gypsum to anhydrite, which likely takes place in burial depth of less than 250 m [49,50]. The escaping water is oversaturated with respect to CaSO 4 [51] and might have led to a second pulse of anhydrite cementation and replacement during shallow burial. ...
... EDX analysis of dolomite rhombs reveal dedolomitization features, with the dissolution of disordered metastable dolomite zones and the subsequent replacement by calcite. This selective dissolution and re-precipitation is probably the result of the intrusion of Ca 2+ -rich meteoric fluids (Schoenherr et al., 2018). However, other dolomite cements are not affected by dissolution. ...
Sandstones with carbonate cements are found in a variety of depositional settings (e.g. fluvial, lacustrine, deltaic, shallow marine) with abundances ranging from just a few volume percent to pervasive pore-filling cements and are of major interest due to their impact on reservoir quality. Despite the widespread occurrence of such cements, predictive models for eogenetic carbonate cementation have yet to be developed. Hence, investigations on the controls and distribution of early carbonate precipitation are necessary to improve the accuracy of predictions of the nature and distribution of diagenetic alterations. We conducted a petrological and petrophysical analysis of early near-surface carbonate cementation in siliciclastic and mixed siliciclastic-carbonate sandstones from fluvial and shallow marine environments to reconstruct eogenetic processes and their associated controls. Furthermore, sediment burial scenarios up to a loading of 50 MPa were simulated with the reservoir quality prediction software Touchstone to evaluate the impact of carbonate cements on the compaction behaviour of the rocks. Results highlight the importance of climate, depositional environment and detrital composition and texture in regards to the precipitation processes of near-surfaces carbonate cements and burial development. Our data shows that the presence and distribution of early carbonate cements reflects a complex interaction of several parameters, such as the composition and texture of detrital material in the source area, the availability and infiltration of meteoric or marine waters and the presence of precursor carbonate during deposition. Burial simulations demonstrate the strengthening effect of early carbonate cementation and its influence on the development of intergranular volume and porosity during burial with up to five times less porosity loss due to compaction than without near-surface cementation.
... Es por tanto significativo que en el estudio de (Díaz-Puga et al., 2016) se señale que los acuíferos karsticos del noroeste de Sierra de Gádor estén desarrollando un proceso de dedolomitización. Esto quiere decir que en esa zona el sistema karstico formado por las rocas metamórficas del complejo Alpujárride está desarrollando un proceso mineralógico que modifica sus características petrofísicas, es decir, porosidad y permeabilidad.Por tanto, teniendo en cuenta la conclusión desarrollada por (Escorcia et al., 2013) en base a la correlación negativa o positiva de los factores de permeabilidad y porosidad con la dedolomitización(Schoenherr et al., 2018; Choi et al., 2012), la concentración anómala de altos valores de b en el régimen tectónico local del Campo de Dalías se debería a una reducción en la permeabilidad de los acuíferos naturales de la Sierra de Gádor. ...
Southeast Spain is a focus of interest for many interdisciplinary works applied to characterize their active seismic sources. From a tectonic point of view, Almería province is one of the areas with a higher number of hypotheses due to the already proven coexistence of extensional and compressional regimes. This work is aimed at characterizing the seismic stress of Campo de Dalías, a litoral plain on the southwest coast of Almería, by using the IGN Spanish seismic catalogue for the period 2002-2019. The catalogue completitude was found, according to the statistic method of maximum curvature, to settle on a value of MC 1.6. The variation of b-values was observed within the expected values for a local and regional system of faults. However, the concentration of b-value anomalies above 1.2 relates the NW Sierra de Gádor seismicity to dedolomitization processes in the Alpujárride nappe.
... Dissolution of CaSO 4 during the incursion of undersaturated groundwater releases Ca-ions, and the elevated Ca/Mg ratio results in dedolomitzation (e.g. Canarveras et al., 1996;Schoenherr et al., 2018 for an overview). The most likely source for Ca is CaSO 4 units within the Zechstein succession. ...
The preserved Zechstein succession on the Utsira High in the NE part of the Norwegian North Sea is 25-100 m thick and is dominated by shelf carbonates. Internal subdivision of
the succession is based on the recognition of key surfaces in petrophysical logs and cores, and suggests that the carbonates mainly consist of ZS2 and ZS3 deposits and that younger
ZS4 and ZS5 deposits are only locally preserved. The carbonates have undergone early, syn-depositional dolomitization followed by later dolomite recrystallization and calcitization. Calcitization, interpreted as dedolomitization, is restricted to the upper part of the ZS3 carbonate unit and based on U/Pb dating took place during the Triassic, with a later phase of recrystallization linked to mid-Jurassic uplift. Both dedolomitization and dolomite
recrystallization relate to fresh-water infiltration with the resetting of dO18 values prior to the Late Jurassic drowning of the Utsira High. The reservoir quality of the carbonates is directly linked to post-depositional meteoric diagenesis, and the best reservoir properties are recorded in intervals dominated by recrystallized dolomites in ZS2 and lower ZS3 carbonates. Dedolomitization significantly reduced porosity in the upper ZS3 carbonates.
... Dedolomitisation probably results from the migration of Ca-rich fluids during burial, and conceptual models for dedolomitization predict its impact to be greatest in slope settings where thick anhydrites are juxtaposed with dolomites, whereas it is rare in platform margin and interior settings. In the Hauptdolomit of NE Germany, extensive dedolomitization of the lower slope has been documented, locally extending to the upper slope and platform margin, and resulting in average 5-10% reduction in matrix porosity (Schoenherr et al., 2018). ...
The Mid North Sea High (MNSH) region represents one of the least explored areas for the Late Permian Zechstein Hauptdolomit play in the Southern Permian Basin although some of the first offshore wells drilled in the UK were located here. In other parts of the basin such as onshore Poland, the Hauptdolomit Formation (“Hauptdolomit”) is an active and attractive exploration target, with oil and gas production from commercial‐sized fields. In the UK, the play has been overshadowed by drilling campaigns in areas to the south of the MNSH which tested plays in the underlying Rotliegend and Carboniferous successions. However, with these areas now in decline, there is increased exploration interest in the Hauptdolomit in the MNSH region, particularly since 2019 when 3D seismic data were acquired and the first hydrocarbon discovery was made at Ossian (well 42/04‐01/1Z). Geochemical data from the latter discovery have pointed to the presence of a prolific petroleum system with the potential for Hauptdolomit reservoirs to be charged both by Zechstein‐generated oils and Carboniferous condensate/gas. With regard to hydrocarbon migration and preservation in the southern MNSH, a detailed evaluation of the effects of the Mid Miocene Unconformity has allowed for a greater understanding of the main factors controlling hydrocarbon preservation and remigration. Reservoir characterization of the Hauptdolomit play has been achieved by integrating petrographic microfacies analyses, core data and petrophysical interpretations. The most important factors controlling reservoir quality are the presence and extent of anhydrite cementation and the presence of high energy shoal facies. Thicker and coarser grained shoal facies are expected to occur along the yet‐to‐be explored Orchard platform margin where numerous prospects have been mapped and refined using recently acquired 3D seismic data.
... Dedolomitization, the process of converting dolomite to calcite, occurs in the epigenetic environment of gypsum-bearing carbonate formations, providing additional Ca 2+ by dissolving gypsum minerals from atmospheric freshwater to replace Mg 2+ in dolomite [53,54]. The concave-convex contact between calcite and dolomite in the matrix is a critical feature of dedolomitization (Figure 2b), and calcite cement is a key by-product [55]. The C and O isotopes of the calcite cements also indicated a significant presence of atmospheric freshwater [13]; in addition, there was a positive correlation between them (Figure 3), and the δ 18 O showed an obvious negative bias. ...
Karst reservoirs have always been a key field of oil and gas exploration. However, quantifying the process of meteoric transformation remains a persistent challenge that limits the accuracy of reservoir quality prediction. To explore the controlling factors of meteoric cementation on karst reservoirs, the Majiagou Formation of the Ordos Basin in China was selected as an example. The petrology; carbon, oxygen, and strontium isotopes; and in situ major, trace, and rare earth elements were used, types and origins of calcite cements were analyzed in detail. The results revealed five types of calcite cements (Cal-1~Cal-5), four types of cathodoluminescence (CL) intensities (dull, dull red, deep red, and bright red luminescence), and six types of rare earth element patterns (Pattern-1~Pattern-6). These five types of calcite cements developed in three periods. Cal-1 (transition CL) and Cal-2 (dull CL) were precipitated during the Early Pennsylvanian period, the meteoric freshwater was clean; Cal-3 (transition CL) and Cal-4 (bright red CL) were precipitated at the end of the Late Carboniferous period, the fluids had strong dissolution ability and were polluted by terrigenous debris; Cal-5 (transition CL) was deposited during the burial period, the fluid was pure pore water or groundwater. The control of the cement on the reservoir during the burial period was much weaker than that of meteoric cements. Therefore, explorations of karst reservoirs should be focused on weak cementation during the epigenetic period.
... Among fluid-assisted metasomatic reactions, the mechanism of dolomite calcitization (i.e., dedolomitization) is a very common process that can convert a dolostone into a limestone. Most dedolomites form at, or near to, the Earth's surface where meteoric water infiltrates and gets enriched in calcium by dissolving carbonates (see the complete review of Schoenherr et al., (2018)). In some cases, dedolomitization can happen in burial conditions (Budai et al., 1984). ...
... Esto quiere decir que en esa zona el sistema karstico formado por las rocas metamórficas del complejo Alpujárride esta desarrollando un proceso mineralógico que modifica sus características petrofísicas, es decir, porosidad y permeabilidad. Por tanto, teniendo en cuenta la conclusión desarrollada por (Escorcia et al., 2013) en base a la correlación negativa o positiva de los factores de permeabilidad y porosidad con la dedolomitización (Schoenherr et al., 2018;Choi et al., 2012), la concentración anómala de altos valores de b en el regimen tectónico local del Campo de Dalías se debería a una reducción en la permeabilidad de los acuíferos naturales de la Sierra de Gádor. ...
Southeast Spain is a focus of interest for many interdisciplinary works applied to characterize their active seismic sources. From a tectonic point of view, Almería province is one of the areas with a higher number of hypotheses due to the already proven coexistence of extensional and compressional regimes. This work is aimed at characterizing the seismic stress of Campo de Dalías, a litoral plain on the southwest coast of Almería, by using the IGN Spanish seismic catalogue for the period 2002-2019. The catalogue completitude was found, according to the statistic method of maximum curvature, to settle on a value of MC 1.6. The variation of b-values was observed within the expected values for a local and regional system of faults. However, the concentration of b-value anomalies above 1.2 relates the NW Sierra de Gádor seismicity to dedolomitization processes in the Alpujárride nappe.
... Calcium values differs from 297-0.8 mg/L. It is derived from evaporite and carbonate minerals; calcite, aragonite, dolomite, anhydrite, and gypsum (Schoenherr et al., 2018;Teboul et al., 2016). ...
Multiple interactions of geogenic and anthropogenic activities can trigger groundwater pollution in the tropical savanna watershed. These interactions and resultant contamination have been studied using applied geochemical modeling, conventional hydrochemical plots, and multivariate geochemometric methods, and the results are presented in this paper. The high alkalinity values recorded for the studied groundwater samples might emanate from the leaching of carbonate soil derived from limestone coupled with low rainfall and high temperature in the area. The principal component analysis (PCA) unveils three components with an eigenvalue > 1 and a total dataset variance of 67.37%; this implies that the temporary hardness of the groundwater and water–rock interaction with evaporite minerals (gypsum, halite, calcite, and trona) is the dominant factor affecting groundwater geochemistry. Likewise, the PCA revealed anthropogenic contamination by discharging SO42-,NO3-,Cl- and K+ from agricultural activities and probable sewage leakages. Hierarchical cluster analysis (HCA) also revealed three clusters; cluster I reflects the dissolution of gypsum and halite with a high elevated load of NO3- released by anthropogenic activities. However, cluster II exhibited high KHCO32- and KCl- loading in the groundwater from weathering of bicarbonate and sylvite minerals. Sulfate (SO42-) dominated cluster III mineralogy resulting from weathering of anhydrite. The three clusters in the Maiganga watershed indicated anhydrite, gypsum, and halite undersaturation. These results suggest that combined anthropogenic and natural processes in the study area are linked with saturation indexes that regulate the modification of groundwater quality.
... The dissolution of gypsum consequently increased the concentration of Ca 2+ by the same Ca 2+ /Mg 2+ ratio. This ratio once greater than 0.5 thermodynamically causes dedolomitization [43]. All samples from the study area show a Ca 2+ /Mg 2+ ratio greater than 0.5, thus explaining that the dedolomitization process seems to mark the water chemistry. ...
Despite efforts to protect the hydrosystems from increasing pollution, nitrate (NO3−) remains a major groundwater pollutant worldwide, and determining its origin is still crucial and challenging. To disentangle the origins and fate of high NO3− (>900 mg/L) in the Sidi Bouzid North basin (Tunisia), a numerical groundwater flow model (MODFLOW-2005) and an advective particle tracking (MODPATH) have been combined with geostatistical analyses on groundwater quality and hydrogeological characterization. Correlations between chemical elements and Principal Component Analysis (PCA) suggested that groundwater quality was primarily controlled by evaporite dissolution and subsequently driven by processes like dedolomitization and ion exchange. PCA indicated that NO3− origin is linked to anthropic (unconfined aquifer) and geogenic (semi-confined aquifer) sources. To suggest the geogenic origin of NO3− in the semi-confined aquifer, the multi-aquifer groundwater flow system and the forward and backward particle tracking was simulated. The observed and calculated hydraulic heads displayed a good correlation (R2 of 0.93). The residence time of groundwater with high NO3− concentrations was more significant than the timespan during which chemical fertilizers were used, and urban settlements expansion began. This confirmed the natural origin of NO3− associated with pre-Triassic embankment landscapes and located on domed geomorphic surfaces with a gypsum, phosphate, or clay cover.
... Since dolostones are often multistage dolomitization (Guo et al., 2016;Kırmacı et al., 2018;Li et al., 2020) or, sometimes, de-dolomitization (Schoenherr et al., 2018;Makhloufi and Samankassou, 2019;Hajri and Abdallah, 2020) products that underwent complex diagenesis/epigenesis in their geological history, the identification of dolomitizing fluid sources and their nature is considerably complicated (Gregg and Shelton, 1990;Al-Aasm and Packard, 2000;Jiang et al., 2019;Mueller et al., 2020). Geochemical approaches to obtaining reliable chemostratigraphic and isotopegeochronological characteristics of the initial sedimentary material have not been found yet for these rocks. ...
... Both the c and d show low concentration of the Mg in the dolomitized horizons.EDS, energy-dispersive X-ray spectrometer dolomitization in burrows can be explained by the reduction in grain size during burrowing offers the enhanced surface area for dolomitization by percolating fluids(Gingras, Pemberton, Muelenbachs, & Machel, 2004). The subsequent dedolomitization also indicates an influence of meteoric waters and a continuous supply of calcium associated with short-term events, like sequence boundaries and tectonic events(Schoenherr et al., 2018;Vandenginste & John, 2012). It either represents mole-to-mole replacement of the dolomite by calcite (as in peloids) or by dissolution of the host dolomitized grain and contemporaneous or subsequent precipitation of calcite, as in bioclasts(James, Bone, & Kyser, 1993;Kenny, 1992). ...
The physical and geochemical character of the post‐depositional changes present in the Early‐Middle Jurassic carbonate‐dominated units (Samana Suk, Shinawari, Chiltan and Loralai formations) of the Indus Basin, Pakistan have been addressed. The study is based on 11 sections along a N‐S transect on the western margin of the Indian Plate to provide an insight into the nature of cementation and dolomitization behavior by using an integrated petrographic and geochemical analysis. The carbonates located in the northern proximity are dominated by the eogenetic signatures of hardgrounds, biogenic micritization, and iron‐containing burrows. The pattern and yellowish‐brown coloration of the burrows is distinct, and shows a positive relationship with the Fe, Si, Al, K, Mn concentrations, and meteoric phase. The transition from the Early to Middle Jurassic is marked by a correspondent increase in the dolomitization and bioturbation but its intensity decreases from north to the south. The packing index and marine phreatic environments generally increase towards the south. The planar‐s to planar‐e protodolomite and dolomite proper have particularly targeted the micritic facies and allochems, courtesy of retaining high‐magnesian calcite waters, and the geochemical analysis also reveals some dedolomitization, associated with sequence boundaries. The pore occluding cements of the Samana Suk and Chiltan formations are Fe deficient and have low Sr and Mn levels, as compared to the Shinawari Formation. The latter has an elevated concentration of Fe, Mg, Si, Al, Sr, Mn, and the Fe + Mn versus Sr relationship and Mn* values suggest that cementation dominantly occurred in the oxic‐dysoxic zones of sub‐tropical climate.
... Red arrow denotes gas migration pathway from the Ca2 into the Bunter Fm. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) represents the ultimate top seal for gas stored in the Ca2 (Steinhoff and Strohmenger, 1996;Schoenherr et al., 2018). The Lower Triassic to middle Bunter formations dominantly consist of competent sandstones with a cumulative thickness of 420 m-520 m. ...
Inversion of the Lower Saxony Basin produced large-scale broad antiforms, hosting Germany's largest gas accumulations in stacked Permian and Triassic reservoir systems. Locally, however, several exploration and production wells failed in the upper Permian Zechstein-2-Carbonte (Ca2) due to missing reservoir (hiatus) or formation-water wet conditions despite of drilling the topmost position of the trap structure. Seismic interpretation indicates lateral shear of the structural topmost Ca2 reservoir by oblique inversion-related collision with the overlying rigid sandstone unit of the Triassic Bunter Formation (Fm.), triggering a halokinetic redistribution of Permian Zechstein (Z2-Z4) salt in between. Lateral shear was enabled along a thin Zechstein (Z1) salt decollement, underlying the Ca2 reservoir. This ultimately led to a full detachment of large intra-salt Ca2 allochthonous gas reservoirs (‘stringers’), leaving behind areas of Ca2 autochthonous absence (‘bald highs’). These kinematics triggered hydraulic connection and gas leakage from both, the ‘bald high’ margins of the autochthonous Ca2 and the Ca2 ‘stringers’ into the sandstone reservoir of the Bunter Fm. Methane-rich fluid inclusions in sub-horizontal tectonic veins and from within vertical stylolites sampled from cores of wells, which tested the Ca2 reservoir to be formation water-wet, proof gas leakage as a result of inversion. Identification criteria for gas leakage and reservoir presence have been established from this study to frame geological subsurface scenarios for pre-drill risk assessments of inverted settings. The generalized conclusion from this study is that the topmost position of structural highs in inverted salt basins may be actually exposed to the highest geological risk.
... Other examples of burial dedolomitization are thought to relate to the upward-directed influx of deep basinal calcium-rich fluids through fractures (Faust 1949;Wood and Armstrong 1975;Land and Prezbindowski 1981;Wierzbicki et al. 2006). The gypsum-to-anhydrite conversion in burial diagenetic system, introducing Ca 2+ into the dolostone, could also lead to dedolomitization (Schoenherr et al. 2018). ...
Calcitized Ordovician dolostones in the Yichuan-Huanglong area of the Ordos Basin (China) are discussed and evaluated by utilizing petrographic, mineralogical and geochemical data. Symbiotic dedolomites with red and white colors are separated and studied. The new data demonstrate that the red dedolomites (Dd1) occurred as the result of the migration of karst-related meteoric water that can be characterized by karst breccias, low Sr and high Mn contents, low δ13C values, high 87Sr/ 86Sr ratios and pure liquid–fluid inclusions. The occurrence of the white dedolomites (Dd2) around stylolites or pores, their low Mn contents, 110–120 °C homogenization temperature and hydrocarbon inclusions, associated with less depletion of δ13C than Dd1, suggest that Dd2 were formed under a burial hydrocarbon expulsion environment. Due to an insufficient amount of Ca ions from the dissolution of residual gypsum, the dedolomitization of Dd1 is a mole per mole replacement process along the rim interfaces, which retains the original pore system and provides pathways for hydrocarbon migration. The formation of Dd2 is a result of the dissolution/precipitation process of Dd1 and/or dolostones by the acidic fluids of mature hydrocarbons in the burial stage that completely convert the rock. The calcitized Ordovician dolostones in Ordos Basin is the result of continuous dedolomitization in an evolving environments from meteoric to burial.
... A close look at dolomite calcitization fronts at the microscale indicates that this process is progressive, as revealed by the strong variability in its intensity depending on the sample chosen or even within the same sample (Fig. 7). The presence of stylolites acting as barriers to dolomite calcitization fluids has also been reported in studies of mudstones of the Zechstein Ca2 Stassfurt carbonate of the Southern Permian Basin in NW Germany (Koehn et al., 2016;Schoenherr et al., 2018;Humphrey et al., 2019). Although the extent and distribution of dolomite recrystallization and calcitization products are difficult to quantify, the Benicàssim case study demonstrates that stylolites can act as barriers for these fluids. ...
There is ongoing debate on whether stylolites act as barriers, conduits, or play no role in fluid transport. This problem can be tackled by examining the spatial and temporal relationships between stylolites and other diagenetic products at multiple scales. Using the well-known Lower Cretaceous Benicàssim case study area (Maestrat Basin, E. Spain), we provide new field and petrographic observations of how bedding-parallel stylolites can influence different diagenetic processes during basin evolution. The results reveal that stylolites can serve as baffles or inhibitors for different carbonate diagenetic reactions, and act as fronts for dolomitization, dolomite recrystallization and dolomite calcitization processes. Anastomosing stylolites that pre-date burial dolomitization probably acted as a collective baffle for dolomitization fluids in the study area, resulting in stratabound replacement geometries at the metre-to-kilometre scale. The dolomitization front coincides with stylolites, and can be traced along consecutive anastomosing ones. Such anastomosing stylolites are typical of mud-dominated facies that characterize limestone-dolostone transition zones. Conversely, dolostone bodies tend to correspond to grain-dominated facies characterized by parallel (non-anastomosing) stylolites. The same stylolites subsequently acted as fluid flow conduits and barriers when the burial and stress conditions changed. Partly dissolved stylolites within dolostones are found close to faults and are filled with saddle dolomite riming the stylolite pore, and high-temperature blocky calcite cements filling the remaining porosity. The fluids responsible for these reactions were likely released from below at high pressure, causing hydraulic brecciation, and were channelised through stylolites, which acted as fluid conduits. Stylolites are also found acting as baffles for subsequent dolomite calcitization reactions during meteoric diagenesis and occasionally appear filled with iron oxides released by calcitization. This example demonstrates how the same type of stylolites (bedding-parallel) can act as barriers/inhibitors and/or conduits for different types of diagenetic reactions through time, and how important it is to consider their collective role when they form networks.
... The standard explanation has been that dedolomitisation is related to the uplift and dissolution of evaporitic deposits by meteoric waters to supply excess calcium. An alternative mode of dedolomitisation was presented by Schoenherr et al. (2018) for the Permian Zechstein Basin. These authors propose that excess calcium from gypsum-to-anhydrite conversion and pressure solution within over and underlying anhydrite strata at burial depths of 900-1400 m were the dominant factors. ...
Over the past decades, the burial realm, the most prolonged and arguably the least well understood diagenetic environment, has received significant research attention. Despite remarkable progress driven by exploratory drilling, outcrop analogue studies and experimental work, the scientific theories defining the burial sub‐domains are inconsistently presented in the literature. This paper reviews the concepts, processes and products that characterise the burial realm from the viewpoint of the carbonate geoscientist. Typical features of carbonate burial (fluid types, porosity evolution, diagenetic fabrics, patterns in isotope geochemistry) in epicontinental and marine basins are presented and discussed. A step towards an improved conceptual delimitation and a subdivision of the carbonate burial realm is taken, and an intuitive terminology is proposed. The very shallow limit of the burial realm is placed within the upper sediment column (redox boundary; centimetres to tens of metres). In the shallow (marine) burial domain (down to depths of many hundreds of metres), carbonate dissolution and reprecipitation, sediment dewatering and grain reorganisation take place. Interstitial waters are mainly marine (and subordinate meteoric) in origin and the system is fluid‐dominated. Under ongoing burial, physical and chemical compaction reduces pore space. At burial depths of ca 750 m, initial sediment porosities (40‐80 %) are reduced to about 30 %. The intermediate burial domain (hundreds of metres to about 2 km; T < 100 °C) is characterised by the transition from fluid‐buffered to rock‐buffered diagenesis. In the deep burial domain (ca 2‐12 km; T > 100 °C), marine formation fluids are increasingly modified by rock‐fluid interaction and replaced by saline brines. The transition from the deep burial to the very‐low grade metamorphic domain is placed at depths of 12‐15 km (T > 250°C). Here, carbonates undergo recrystallisation into metacarbonate and equigranular marble fabrics.
... A close look at fronts at the microscale indicates that calcitization is a progressive process that can present variable intensity (Fig. 7). The presence of stylolites acting as barriers to calcitization fluids has also been reported from studies of mudstones of the Zechstein Ca2 Stassfurt carbonate of the Southern Permian Basin (NW Germany) (Koehn et al., 2016;Schoenherr et al., 2018;Humphrey et al., 2019). Although the extent and distribution of dolomite recrystallization and calcitization products are difficult to quantify, the Benicàssim case study demonstrates that stylolites can constrain these diagenetic reactions acting as barriers for the fluids that caused them. ...
There is an ongoing debate on whether stylolites act as barriers or conduits for fluids, or even play no role in terms of fluid transport. This problem can be tackled by examining the spatial and temporal relationships between stylolites and other diagenetic products at multiple scales. Using the well-known Lower Cretaceous Benicàssim case study (Maestrat Basin, E Spain), we provide new field and petrographic observations of how bedding-parallel stylolites can influence different diagenetic processes during the geological evolution of a basin. The results reveal that stylolites can serve as baffles or inhibitors for different carbonate diagenetic reactions, and act as fronts for dolomitization, dolomite recrystallization and calcitization processes. Anastomosing stylolites, which pre-date burial dolomitization, likely acted as a collective baffle for dolomitization fluids in the study area, resulting in stratabound replacement geometries at the metre-to-kilometre scale. The dolomitization front weaves up and down following consecutive anastomosing stylolites, which are typical of mud-dominated facies that characterize limestone-dolostone transition zones. Contrarily, dolostone bodies tend to correspond to grain-dominated facies characterized by parallel (non-anastomosing) stylolites. The same stylolites subsequently acted as fluid flow conduits and barriers again when the burial and stress conditions changed. Stylolites within dolostones close to faults are found corroded and filled with saddle dolomite riming the stylolite pore, and high-temperature blocky calcite cements filling the remaining porosity. The fluids responsible for these reactions were likely released from below at high pressure, causing hydraulic brecciation, and were channelised through stylolites, which acted as fluid conduits. Stylolites are also found acting as baffles for subsequent calcitization reactions and occasionally appear filled with iron oxides released by calcitization. This example demonstrates how the same type of stylolites can act as barriers/inhibitors and/or conduits for different types of diagenetic reactions through time, and how important it is to consider their collective role when they form networks.
... Diagenetic facies (Diagenetic Carbonate Subunit), as here described, are characteristic products of meteoric diagenesis in near-surface environments [34,[61][62][63][64][65], whose formation may be related to water table fluctuations [66] and/or major changes in basin paleogeography [35,62,67]. Additionally, several features demonstrate the subaerial and pre-burial nature of the Intra-Vallesian paleokarst such as (i) breccia deposits that include pseudospar and speleothem fragments, together with fluvial deposits capping the karst surface; and (ii) cave-fill deposits (clays, subarkoses and flowstones) within the subsurface karst system. ...
An intra-Vallesian (Upper Miocene) paleokarst developed at the top of the Intermediate Miocene Unit in the continental intracratonic Madrid Basin is recognized. This paleokarst is an early shallow, tabular-shaped karst that shows a marked control by the depositional facies pattern and lithologies. By integrating morphological, petrological, and geochemical data, three hydrogeological zones were established throughout the paleokarstic profiles: (i) a paleo-vadose zone, characterized by vertically elongated caves and vadose cementation; (ii) a 3–7 m thick paleo-epiphreatic zone (paleo-water table fringe), with development of stratiform breccia bodies, the superimposition of both vadose and phreatic features, and the lowest Fe and Mn contents in host-rock carbonates; and (iii) a paleo-phreatic zone characterized by an increase in δ13C values and the predominance of phreatic cementation. The paleogeographic reconstruction for the intra-Vallesian paleokarst using profiles revealed relative topographic highs to the north and topographic lows to the south, drawing the paleokarst landscape. Immediately overlaying the paleokarst surface are fluvio-lacustrine facies belonging to the Miocene Upper Unit (Late Vallesian to Late Turolian). Their lowermost deposits consist of fluvial terrigenous facies deposited by approximately N–S fluvial streams, and pass upward into fluvio-lacustrine fresh-water limestones. This paleokarstic surface represents a major change in the evolution of sedimentary patterns of basin, from endorheic to exorheic conditions, as the result of a change from compressive to extensional conditions in the tectonic regime.
... The mineral assembly associated with quartz, observed in the veins and hydraulic breccias and disseminated in the host rocks, encompasses a hydrothermal paragenesis of K-feldspar, hyalophane (barium feldspar), barite, apatite, and Fe-oxides/hydroxides. Similar mineral paragenesis has been recognized in hydrothermal systems affecting carbonate units elsewhere (e.g., Cazarin et al., 2019;Dristas et al., 2017;Menezes et al., 2019;Schoenherr et al., 2018;Ś rodoń et al., 2018). ...
Hydrothermal silicification generates secondary porosity and permeability and could play an important role in carbonate reservoirs. We investigated the Cristal Cave, which is hosted in carbonate units of the Caboclo Formation, São Francisco Craton, Brazil, to assess the role of sedimentary facies and stratigraphy in hydrothermal
silicification. Our results indicate that in the cave area, the carbonate units are composed of (1) ooidal grainstones, (2) intraclastic grainstones and rudstones, (3) heterolites, (4) marls, (5) stromatolites, (6) oncolithicintraclastic grainstones and rudstones, and (7) hydraulic breccias. QEMSCAN and energy dispersive X-ray spectroscopy (EDS) analyses show that hydrothermal silicification is the most common diagenetic process in these rocks and reveal a mineral paragenesis composed of quartz, chalcedony, K-feldspar, barite, hyalophane (Bafeldspar), talc, and chlorite. The marls (Unit 4) are impermeable rocks that represent a maximum flooding surface and divide the sedimentary succession into a bottom and an upper interval. They behaved as a sealing unit that prevented the upward flow of silica, channeling hydrothermal silicification in the bottom stratigraphic
interval. Within the silicified interval, units 1, 2, and 3 show great differences in the degree of silicification since the ooidal grainstones (Unit 1) concentrated the hydrothermal fluids, reaching the highest degree of silicification in the stratigraphic column. We conclude that the stratigraphic framework can control the vertical distribution of
hydrothermal fluids and the development of layer-parallel fluid flow conduits. The Cristal cave serves as an analog, which could contribute to the understanding of silicified carbonate reservoirs.
... The mineral assembly associated with quartz, observed in the veins and hydraulic breccias and disseminated in the host rocks, encompasses a hydrothermal paragenesis of K-feldspar, hyalophane (barium feldspar), barite, apatite, and Fe-oxides/hydroxides. Similar mineral paragenesis has been recognized in hydrothermal systems affecting carbonate units elsewhere (e.g., Cazarin et al., 2019;Dristas et al., 2017;Menezes et al., 2019;Schoenherr et al., 2018;Ś rodoń et al., 2018). ...
Hydrothermal silicification generates secondary porosity and permeability and could play an important role in carbonate reservoirs. We investigated the Cristal Cave, which is hosted in carbonate units of the Caboclo Formation, São Francisco Craton, Brazil, to assess the role of sedimentary facies and stratigraphy in hydrothermal silicification. Our results indicate that in the cave area, the carbonate units are composed of (1) ooidal grainstones, (2) intraclastic grainstones and rudstones, (3) heterolites, (4) marls, (5) stromatolites, (6) oncolithicintraclastic grainstones and rudstones, and (7) hydraulic breccias. QEMSCAN and energy dispersive X-ray spectroscopy (EDS) analyses show that hydrothermal silicification is the most common diagenetic process in these rocks and reveal a mineral paragenesis composed of quartz, chalcedony, K-feldspar, barite, hyalophane (Bafeldspar), talc, and chlorite. The marls (Unit 4) are impermeable rocks that represent a maximum flooding surface and divide the sedimentary succession into a bottom and an upper interval. They behaved as a sealing unit that prevented the upward flow of silica, channeling hydrothermal silicification in the bottom stratigraphic
interval. Within the silicified interval, units 1, 2, and 3 show great differences in the degree of silicification since the ooidal grainstones (Unit 1) concentrated the hydrothermal fluids, reaching the highest degree of silicification in the stratigraphic column. We conclude that the stratigraphic framework can control the vertical distribution of hydrothermal fluids and the development of layer-parallel fluid flow conduits. The Cristal cave serves as an
analog, which could contribute to the understanding of silicified carbonate reservoirs.
... The floor and roof anhydrites originally were deposited as gypsum [17]. Up to 40 vol% of structural bound water is expelled during recrystallization from gypsum to anhydrite, which likely takes place in burial depth of less than 250 m [49,50]. The escaping water is oversaturated with respect to CaSO 4 [51] and might have led to a second pulse of anhydrite cementation and replacement during shallow burial. ...
... If dolomite is the source of Mg, then the volume of burial dolomite would have to be closely matched by a similar volume of calcite formed by dedolomitization (or calcitization). Dedolomitization typically involves meteoric waters with high Ca/Mg ratios and high SO4 concentrations (from anhydrite dissolution; Fu et al., 2008;Schoenherr et al., 2018). On this basis we reject dolomite as a source of Mg. ...
Dolomite is commonly associated with evaporite deposits in the Williston Basin, and brine reflux is a frequently proposed genetic model. However, dolomite directly underlying the Middle Devonian Prairie Evaporite exhibits 87Sr/86Sr ratios that are higher (average of 0.7082, from 35 samples) than Middle Devonian seawater (0.7079). This prompted others to suggest that dolomitization of the Lower Member Winnipegosis Formation in the Saskatoon area was by ascending fluid flow during a late Paleozoic heating event. Herein we present a study of magnesium (Mg) isotopes that strengthens this interpretation and broadens its significance. The same dolomite body records a basin-wide gradient in δ26Mg values from the centre (–2.03‰) to the edge (–0.96‰) of the Williston Basin. We also find a basin-wide gradient in 87Sr/86Sr. Three mechanisms involving ascending migration of Mg- and 87Sr-bearing fluids are evaluated to explain the origin of these gradients, including: 1) Rayleigh distillation, 2) temperature, and 3) mixing. The Rayleigh model predicts the correct orientation of the δ26Mg gradient, but is rejected on the basis that dolomite abundance does not also decrease in the direction of increasing δ26Mg. Temperature is rejected because the δ26Mg gradient cannot be formed by changing the dolomite (temperature-dependent) fractionation in a way that aligns with the geothermal gradient. We propose instead that the δ26Mg gradient formed when an older Type 1 proto-dolomite (formed by brine reflux in the Middle Devonian) was altered to an 87Sr-enriched Type 2 dolomite by younger, ascending, Mg-bearing fluids at progressively decreasing water-rock ratios away from the deep centre of the Williston Basin. Isotopic mapping of the Winnipegosis and earlier-studied Late Ordovician Red River burrow-dolomite together point to the underlying crystalline basement as the source of the Mg- and 87Sr-bearing fluids. We propose that seismic pumping drove crustal fluids into the bottom of the Williston Basin during a Late Devonian/Early Carboniferous heating event using down to-the-basement faults as fluid conduits, pressurizing deep aquifers like the Winnipegosis and Red River, and initiating up dip fluid flow directed toward the edges of the basin. The contour maps of dolomite δ26Mg and 87Sr/86Sr reveal the subsurface pathways taken by the migrating fluids, and as such may be of value for locating co-migrated petroleum and Mississippi Valley–type ore deposits. One prominent pathway extends from the centre of the basin in North Dakota into southeastern Saskatchewan and Manitoba. Most of the light and medium crude oil produced in southeastern Saskatchewan intersects this hypothesized flow path. Furthermore, Pb-Zn mineralization in Manitoba occurs along an extension of the flow path.
... Dedolomitization can be used to identify the unconformity. Although dedolomitization can occur in early diagenetic stages (Arenas et al. 1999), epidiagenetic environments and burial diagenetic environments (Schoenherr et al. 2018), dedolomitization can be considered unconformity-controlled if it is laterally traceable (Zhang et al. 2012a, b), and meteoric fluids are vital diagenetic fluids for transforming dolomite to calcite (Zeeh et al. 2000). Figure 8 presents typical dedolomitization features under the microscope. ...
The Permian Fengcheng Formation is a set of complex sedimentary strata in the Mahu Sag, northwestern Junggar Basin, where various types of carbonate minerals occur. Coarse crystalline calcite assemblages and other carbonate minerals, such as dolomite, shortite and sodium carbonates, are also common in the Fengcheng Formation. Their dissolution by meteoric freshwater leaching at tectonic high positions, which was controlled by an unconformity, developed uncommonly high-quality secondary porosity and contributed to unconventional oil and gas exploration. Based on core analyses, thin section studies under optical and cathodoluminescence (CL) microscopes, geological interpretations of seismic profiles, and carbon and oxygen isotope composition analyses from coarse crystalline calcite assemblages and comparisons to other carbonate minerals, this study focuses on the origin of coarse crystalline calcite assemblages and the mechanism of unconformity-related dissolution to form hydrocarbon reservoir spaces. Coarse crystalline calcite assemblages are the products of multiple stages of recrystallization of other carbonate minerals in nearby host rocks. Hydrolysis of feldspar and volcanic sediments provided additional Ca2+ for calcite precipitation. The dissolution and reprecipitation of carbonate minerals were associated with organic maturation and hydrocarbon expulsion and migration. The crystallization process was slow, which allowed the crystal assemblages to grow to large sizes. Organic carbon was involved in the formation of the coarse crystalline calcite assemblages, which resulted in much more negative carbon isotope compositions than those of other carbonate minerals via diluting the carbon isotope compositions of preexisting carbonate minerals. Later tectonic uplift caused a regional unconformity and formed dissolution reservoirs in tectonically high positions. Furthermore, faults and fractures provided paths for the solutes to transfer to lower positions and reprecipitate when they became saturated.
... TSR-related processes and/or CaF 2 bearing hydrothermal fluids (Fig. 6A) provided Ca 2+ for calcitization of dolomite. According to Schoenherr et al. (2018), fabrics that indicate dedolomitization in the Dengying Formation include: (i) partial replacement of saddle dolomite rhombohedra with sparry calcite; and (ii) poikilotopic inclusions in calcite cements of partially-replaced saddle dolomite (Fig. 6C). Homogenization temperatures of calcite cements, burial temperature of the Dengying Formation (Fig. 3), and the paragenetic sequence ( Fig. 9), suggest that TSR and dedolomitization likely occurred during the Jurassic to the Cretaceous. ...
The deeply buried (>7 km) upper Ediacaran (Sinian) Dengying Formation (ca. 551.1–541 Ma) in the Sichuan Basin, China, is the largest Precambrian dolostone gas reservoir worldwide. Gas exploration from the Dengying Formation, however, is hampered by a limited understanding of its complex dolomitization history and porosity evolution. New petrological, geochemical and petrophysical analyses were performed and are discussed here to develop a better understanding of the formation’s complexities. Microbialite, dolo-grainstone, and crystalline dolostone lithologies from the platform margin have high primary porosities relative to dolo-mudstone and less common microbialite lithologies from the low-energy platform interior. Spatially-variable primary porosity was subsequently overprinted by meteoric dissolution and dolomitization, often inducing secondary porosity. The various forms of dolomitization (sabkha, reflux, and burial dolomitization, and cementation) increased the resistivity of these rocks to chemical and physical compaction and porosity destruction. In the deep burial domain, medium- to coarse-crystalline dolomite cements and saddle dolomite precipitated at fluid temperatures of 120–160 °C and 160–220 °C, respectively. The significance of deep burial dissolution is critically discussed and constrained by: (i) corrosion of late diagenetic minerals, (ii) pores cross-cutting (or forming around) stylolites, and (iii) the occurrence of solid bitumen in the center of secondary pores. Vuggy, inter- and intracrystalline pores developed during a late and deep burial stage, and comprise ca. 20% of the overall Dengying reservoir porosity. Mechanisms which induced corrosion at this burial stage include hydrothermal pulses and thermochemical sulfate reduction. Similar to many carbonate reservoirs in the Phanerozoic, this study documents that the platform margin has better porosities and higher gas production compared to the platform interior due to its specific carbonate rock properties and diagenesis. Data shown here document the complex multiphase dolomitization history and high potential for gas production from deeply buried Precambrian dolostone reservoirs.
... Stage I gas migration was associated with progressing calcitization and bedding-parallel stylolitization (calcite I-IV cements), which postdated early dolomitization and dolomite dissolution (Figs. 6, 5C-F). In the PB these processes took place during intermediate burial starting in Early Triassic (Schoenherr et al., 2018;Humphrey et al., 2019). Migration of CH 4 -CO 2 ± N 2 gas is recorded by type 1a inclusions hosted in calcite I-II cements (Figs. ...
Upper Permian Zechstein carbonate Ca2 gas reservoirs in the southern part of the Pompeckj Block in the North German Basin locally contain up to 36 vol% hydrogen sulfide (H2S) produced by thermochemical sulfate reduction (TSR). TSR was triggered by migration of dry to extremely dry coal gas from Upper Carboniferous into the Zechstein carbonate reservoirs. Methane reacted with dissolved sulfate at temperatures of <150 °C, as inferred from fluid inclusions hosted in fracture-filling minerals and cements in the carbonate reservoir rocks. Such low temperatures for methane-dominated TSR are unique and were not observed so far, as it was widely believed that alteration of super dry methane requires much higher temperatures. Here we present detailed compositional and carbon isotope data of reservoir gases as well as those of gases trapped in fluid inclusions hosted in cements and fracture-filling minerals in Zechstein Ca2 carbonate reservoir rocks. We constrained the P-T conditions of gas entrapment, hydrocarbon reactivity and the lower temperature limit for TSR.
The results of this study decipher three major stages of gas migration in the Pompeckj Block. Stage I commenced in the Late Triassic during burial when Zechstein Ca2 reservoirs were charged with dry CH4-CO2 ± N2 gas sourced from mature Upper Carboniferous coals. Burial continued through the Jurassic and caused alteration of Ca2 reservoir gas by sulfate reduction reactions due to increasing temperatures. Entrapment of CH4-H2S-CO2-N2 gases in fluid inclusions, hosted in cements and fracture-filling minerals, occurred at temperatures between 100 and 152 °C and was related to Stage II uplift in the Early Lower Cretaceous. In the Late Cretaceous (Stage III) deep burial of the Pompeckj Block led to charge of the Zechstein Ca2 carbonate reservoirs with Upper Carboniferous-derived CH4-CO2 ± N2 ± C2+ coal gas and/or dilution of existing reservoir gas at temperatures of 144–167 °C. Highly variable δ13CCH4 values from −18.7 to −8.7‰ and very negative δ13CCO2 values (−22.4 to −18.9‰) of H2S-rich fluid inclusion gases as well as negative δ13C values (−10.4 to −4.6‰) of host calcites reveal compelling evidence for participation of methane in TSR. Fluid inclusions imply that CH4-dominated TSR proceeded at Tmin of 135 °C in the presence of catalyzers such as H2S and dissolved Mg2+.
This study demonstrates that fluid inclusions serve as an excellent and accurate tool for tracing H2S concentrations in hydrocarbon gases through time and space, which is not possible using the present-day compositions of natural reservoir gases. It also contributes to the understanding of carbonate reservoir-hosted hydrocarbon-bearing fluid systems and processes that significantly control the quality of reservoir gases.
... The floor and roof anhydrites originally were deposited as gypsum [17]. Up to 40 vol% of structural bound water is expelled during recrystallization from gypsum to anhydrite, which likely takes place in burial depth of less than 250 m [49,50]. The escaping water is oversaturated with respect to CaSO 4 [51] and might have led to a second pulse of anhydrite cementation and replacement during shallow burial. ...
A common problem in dolomite reservoirs is the heterogeneous distribution of porosity-reducing diagenetic phases. The intrasalt carbonates of the Ediacaran-Early Cambrian Ara Group in the South Oman Salt Basin represent a self-sourcing petroleum system. Depositional facies and carbonate/evaporite platform architecture are well understood, but original reservoir properties have been modified by diagenesis. Some of the carbonate reservoirs failed to produce hydrocarbons at acceptable rates, which triggered this study. The extent of primary porosity reduction by diagenetic phases was quantified using point counting. To visualize the distribution of diagenetic phases on a field scale, we constructed 2D interpolation diagenesis maps to identify patterns in cementation. The relative timing of diagenetic events was constrained based on thin-section observations and stable isotope analyses. Near-surface diagenesis is dominated by reflux-related processes, leading to porosity inversion in initial highly porous facies and a patchy distribution of early cements. This strong diagenetic overprint of primary and early diagenetic porosity by reflux-related cements leads to a reduction of stratigraphic and facies control on porosity. Calcite was identified as a burial-related cement phase that leads to an almost complete loss of intercrystalline porosity and permeability. Bitumen is an important pore-occluding phase and time marker of the deep-burial realm. The stratigraphic position of the dolomite reservoirs embedded at the base of a salt diapir had a strong impact on its diagenetic development. The salt isolated the dolomites from external fluids, leading to a closed system diagenesis and the buildup of near lithostatic fluid pressures. In combination, these processes decreased the impact of further burial diagenetic processes. The study highlights that cement distribution in salt-encased carbonate reservoirs is mainly related to early diagenetic processes but can be very heterogeneous on a field scale. Further work is needed to implement these heterogeneities in an integrated numerical reservoir model.
The Utsira High is a prominent intrabasinal basement structure in which the eroded remnants of a widespread, Late Permian, Zechstein carbonate shelf are preserved locally. The western margin of the central Utsira High is mainly characterized by weathered basement rocks with a thin Mesozoic cover. However, the recently discovered Symra Field forms an isolated sedimentary inlier basin of the area, where deeply eroded Zechstein shelf carbonates are preserved within a half-graben. The Zechstein carbonates consist of mainly ZS2 marginal marine carbonate facies and are similar to those described elsewhere from the Zechstein Basin. They are unconformably overlain by Paleogene chalks. Based on detailed facies analysis of two recently drilled cores combined with detailed petrographic and stable isotopic analysis and supplemented by age dating of selected carbonate phases using U-Pb geochronology, we show that the Zechstein carbonates have been subjected to several phases of near-surface diagenetic alterations. Volumetrically the most dominant diagenetic product comprises nonplanar dolomites interpreted to have formed by recrystallization of a precursor reflux-type dolomite phase. The recrystallized dolomites retain enhanced reservoir quality in comparison to stratigraphic equivalent ZS2 reflux-type dolomites found elsewhere on the Utsira High. Based on U-Pb-derived age constraints, the recrystallization took place during a long-lived Late Triassic exposure event of the Utsira High, and a near-surface origin for the recrystallized dolomites is proposed. The porosity enhancement occurred contemporaneously with the dolomite recrystallization process and was facies controlled. Reservoir modifications associated with the exposure led to the dissolution of CaSO4 cement, and metastable dolomite phases, in addition to the enlargement of existing pores which had created zones of weakness prone to further dissolution. Overall, the Zechstein carbonates preserved on the Utsira High illustrate the complexity of the diagenetic alterations resulting from the uplift of a carbonate shelf, and its importance for reservoir quality.
The Central High Atlas of Morocco is a double verging major diapiric province encompassing SW-NE trending salt-related ridges and Lower to Middle Jurassic deposits over synclines (minibasins) in between them. Typically, Pliensbachian and Bajocian platform carbonates flanking diapirs exhibit partial dolomitization in the vicinity of the diapir wall. The interpretation of the diagenetic products and diagenetic evolution of these dolomitized carbonates may be complex due to different fracture patterns, brecciation, fluid pathways, heterogeneities, localized uplift associated with diapiric activity, among others. The present work focuses on the dolomitization affecting Late Aalenian-Bajocian platform carbonates flanking the Tazoult salt wall, localized in the center of the Central High Atlas. Analytical work includes standard microscopy, cathodoluminescence and 𝛅13C–𝛅18O isotopic analyses. A paragenetic sequence is discussed to reconstruct the diagenetic evolution of the flanking platform carbonates. The results highlight the major influence of diapirism and igneous intrusions on the fluid circulation. Moreover, the present work complements a previous work related to the Liassic successions flanking the Tazoult salt wall.
This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cuttings data from 25 wells which were integrated with observations based on existing and new 3D seismic.
Based on thin-section petrography of cuttings and core from the wells studied, it is evident that Hauptdolomit microfacies are distributed in a relatively predictable way, and well-defined platform interior, platform margin, slope and basin settings can be distinguished. Platform margins are typically characterised by the development of ooid shoals and, to a lesser-extent, by microbial build-ups. High-energy back-shoal settings are characterised by a
more complex combination of peloid grainstones, thrombolitic and microbial build-ups, and fine crystalline dolomites. Lower energy lagoons which developed further behind the platform margin are characterised by a variety of microfacies types; fine crystalline dolomites are common in this setting as well as peloidal facies and local microbial build-ups. Intertidal and supratidal settings are typified by increased proportions of anhydrite and the development of laminated microbial bindstones (stromatolites). Platform margins are in general relatively steep and pass into slope and basinal settings. Only a few wells have penetrated Hauptdolomit successions deposited in a slope setting, and these successions are characterised by a range of resedimented shallow-water facies together with low-energy laminated dolomicrites and fine crystalline dolomites. Slope zones in the study area are interpreted from seismic data to be typically 1-1.5 km in width. Basinal Hauptdolomit deposits have been strongly affected by post-depositional diagenesis and are dedolomitised to variable degrees. The original depositional facies are rarely preserved.
Diagenetic studies show that dolomitisation has affected almost the entire Hauptdolomit Formation throughout the study area in both basinal and platform settings. The dolomite is considered to result from seepage-reflux processes and is an early diagenetic phase. Mouldic porosity is present in many facies types as a result of dissolution, especially in ooid grainstones, thrombolitic build-ups and peloidal facies. The dissolution cannot be associated with any one diagenetic phase but was most likely a result of the dolomitisation process itself. Stable isotope analyses indicate that all dolomites were precipitated from Permian marine derived
pore fluids. Fluid inclusion analyses of dolomite cements indicate that cementation continued into the burial realm. Anhydrite cementation occurs in two phases: early anhydrite precipitation was associated with dolomitisation, and can be distinguished from a later, pore-filling cement which is highly detrimental to
reservoir quality.
The Hauptdolomit succession in basinal wells (and in some slope wells) in the study area has undergone significant dedolomitisation. Dedolomitisation was a shallow burial process which affected precursor dolomites, whereby excess calcium from the transition of gypsum to anhydrite during burial combined with CO2 and organic acids derived from basinal sediments. The process was triggered by excess calcium reacting with excess
carbonate ions from dissolution.
3D seismic volumes supplemented by numerous 2D lines were available in the study area and allowed an interpretation to be made of Hauptdolomit gross depositional settings; platform margins and base of-slope polygons were mapped, with the greatest confidence in areas of 3D seismic. The basin, slope
and platform settings were distinguished using seismic data integrated with the results of micro-facies analysis and incorporating seismic-to-well ties. The data shows that large parts of the study area are characterised by the presence of polyhalites within the overlying (Z2) Stassfurt Halite Formation, which may create particular seismic geometries at the Hauptdolomit slope. These are interpreted to be intra-Stassfurt Halite features, providing an alternative model to the thickened,
prograded Hauptdolomit which has been suggested in previous publications.
Because few wells drilled in the study area had the Hauptdolomit as the primary target, cores were limited but significant data was obtained from cuttings analyses. More than 400 thin sections were evaluated, allowing depositional models based on microfacies observations to be developed, verifying the seismic-scale observations.
Karst processes may be critical for developing secondary porosity and permeability within carbonate reservoirs
and aquifers. Karstification can significantly influence reservoir storage capacity and subsurface fluid flow. This
study investigates the interplay between fluid-flow pathways and diagenesis in an epigenic karst setting in Cretaceous
shallow-water carbonate rocks, Potiguar Basin, Brazil. The results indicate that dissolution occurred at all
diagenetic stages, but the last stage is the most important because it is related to the most recent subaerial exposure.
Dissolution produced during the last karst stage occurred in the vadose and phreatic environments, leading
to the precipitation of isopachous, blocky and syntaxial cements. Dissolution affected the original pore network
within the matrix and localized on preexisting discontinuities such as stylolites, fractures, and bedding surfaces.
The resulting karst cavities are classified as vugs, channels and caverns, varying from a few millimeters to hundreds
of meters in size. In epigenic karst systems, high porosity percentages are commonly due to moldic, intragrain/
intrafossil and intercrystal porosities. The results of our study show how such porosities can be connected
to fractures, veins and stylolites at the microscale. Karst dissolution can further increase porosity and the overall
rock permeability due to aragonite and calcite cement dissolution during eodiagenesis, microfractures and stylolites
dissolution during mesodiagenesis and all cements, fractures and stylolites during telodiagenesis.
Karst caves formed by meteoric water leaching are important oil/gas storage spaces in carbonate reservoirs. However, carbonate reservoirs are transformed by atmospheric freshwater due to long-term cumulative effects that take place over millions of years, and laboratory physical simulations are therefore difficult to conduct. The mechanism by which meteoric water leaching transforms the carbonate sequence stratigraphy is therefore unclear, which has hindered the exploration and development of oil/gas resources. In this study, a large set of geological test data was utilized to develop a solute transport numerical simulation model based on multi-phase flow and multi-component reaction. This was then applied to construct a conceptual model of a grade III sequence stratigraphy for a reservoir in the Yingshan Formation in the central Tarim Basin, China. The multi-stage leaching process of meteoric water during the periodic exposure of the sequence stratigraphy was simulated to explore the effects of the diagenetic transformation of meteoric water on sequence stratigraphy and the associated physical responses. Results revealed that the composition of carbonate rocks affects the degree of reservoir development under the sequence boundary. Calcite dissolution plays a strong role in meteoric water leaching. Carbonate strata with a high calcite content forms secondary pores relatively easily and dolomite can be preserved. Under open geological conditions, the transformation of reservoirs by meteoric water in different periods has the characteristics of continuity, indirectness, superposition, and heterogeneity. In addition to the mineral dissolution caused by the unsaturated solution of the underlying strata, the transformation was also affected by the superposition and indirect influence of the migration and infiltration of the overlying strata fluid. Under heterogeneous conditions, meteoric water preferentially flowed along the dominant channels, which could improve reservoir performance locally, and may form oil and gas migration channels or large karst caves. This investigation of the control of meteoric water during reservoir development provides a theoretical basis for the genetic mechanism by which a high-quality reservoir can develop under the carbonate sequence boundary.
Massive dolomitization is common in carbonate platforms but determining the causes of dolomitization remains challenging. A particular difficulty lies in identifying cases where petrographic and geochemical attributes of dolostone related to one mechanism could be obscured by a later, different one. To better understand whether traditional approaches are sufficient to unravel the origins of dolostone resulting from successive, different mechanisms, this study investigates the formation mechanism(s) of dolostone along a platform-to-basin transect of a Permian-Triassic isolated platform in the Nanpanjiang Basin. The dolostone in the Lower Triassic succession comprises three dolomite phases that can be distinguished through field relationships, petrography, ⁸⁷Sr/⁸⁶Sr ratios, and microthermometry. Dolomite type 1 formed due to the reflux of platform-top evaporated seawater that flowed through the platform interior in the Early Triassic. Dolomite types 2 and 3 are interpreted to have formed at elevated temperatures during or after Late Triassic platform burial and to have played a secondary role in forming the dolostone. The dolomitizing fluids that resulted in the formation of dolomite types 2 and 3 were derived from Early Triassic seawater-like fluid that was expelled from the Lower Triassic basinal carbonate sediments and moved updip to the platform interior. Dolomitized clasts in partially or non-dolomitized slope breccias demonstrate pre-burial timing of dolomite type 1, and distinguish the earlier dolomitization from later, post-burial dolomitization represented by dolomite types 2 and 3. Dolomite type 1 retains its Early Triassic seawater δ¹³C and ⁸⁷Sr/⁸⁶Sr signatures, whereas overlapping geochemical fields of the three types of dolomite (trace element concentration, δ¹⁸O) imply that burial dolomitizing fluids locally reset the geochemistry of dolomite type 1. This finding suggests that the same dolomite archive may retain well-preserved or altered data depending on the specific geochemical proxy and that identifying individual dolomitization mechanisms using geochemical proxies is possible only in some cases.
Dissolution is the main process responsible for the development of the specific geomorphic and hydrologic features found in karst terrains. Most karst rocks are essentially monomineralic, hence the problem of their karstification mainly concerns the dissolution of the dominant constituent mineral. This chapter is focused on the dissolution of carbonate rocks in shallow meteoric environments, where CO 2 from the atmosphere and especially from the soil is the main source of water acidity. Dissolution of carbonate rocks and cave development by sulfuric acid are known since the first half of the twentieth century. The chapter describes chemical and physical processes that affect the solubility and saturation state of soluble minerals in water, and therefore influence their dissolution and precipitation. There is a large gap between dissolution processes investigated in laboratory experiments and numerical models, and those that operate in real karst systems over geological time scales, as the comparison of some results with field data reveals.
In this chapter, the authors deal with the physical, organic, and chemical deposits found in the dark or semi‐dark areas of caves, excluding those found in cave entrances or rock shelters. Clastic sediments in caves, excluding those found at entrances, have been the subject of numerous studies, mainly during the last 60 years. Phosphorite is a chemical deposit that can be deposited in caves. Speleothems are secondary mineral deposits that form in caves by flowing, dripping, ponded, or seeping water and take on a typical shape. They are mostly composed of minerals such as calcite, aragonite, or gypsum, but other minerals can also form entirely or partially speleothems. Speleothem texture and fabrics are increasingly used to support the interpretation of the geochemical signals (stable isotopes and trace elements) in the paleo‐environmental and paleoclimatic reconstructions based on speleothem archives.
The oldest Jurassic (Kimmeridgian) Plattenkalk occurs in Wattendorf on the northern Franconian Alb (southern Germany). It is a 15 m thick alternation of laminated dolomite and limestone, interbedded with carbonate debris layers in a depression ~2 km across and a few tens of metres deeper than the surrounding microbial-sponge reefs. The Plattenkalk overlies a few tens of metres of microbial-sponge biostrome facies and bedded, micritic basinal limestone. The bulk-rock stable isotopes of the micritic basinal facies gradually change from normal marine (δ13C ~ +2‰, δ18O ~ –2‰ VPDB) to lower values (δ13C ~ 0‰, δ18O ~ –6‰) in a ~ 40 m thick interval including Plattenkalk and suggest ageing of the bottom waters. The surrounding reefs are isotopically nearly invariant (δ13C ~ +2‰, δ18O ~ –2‰ VPDB). An isotope anomaly (δ13C of > ~ –9‰) is restricted to the basinal facies and is most pronounced in the biostrome facies. This indicates methanogenesis, which is documented in negative δ13C in dedolomite, calcite-cemented dolomite and calcite concretions and occurred probably mainly below seabed. The Konservat-Lagerstätte was probably deposited near an oxygen minimum zone in a water column with low productivity of organic material. Dolomite is in isotopic equilibrium with Plattenkalk and was probably deposited as protodolomite from chemically modified, aged seawater. 87Sr/86Sr ratios of bulk carbonate are often slightly radiogenic, probably due to random analytical sample contamination by clay minerals. Belemnite and some matrix 87Sr/86Sr is slightly lower than that of Kimmeridgian seawater, either caused by basin restriction or by fluids derived from the diagenesis of Oxfordian rocks below. An equivalent Upper Kimmeridgian depression ~23 km distant and a somewhat younger Konservat-Lagerstätte in Poland show a δ13C isotope anomaly below the main fossil beds. Isotopic evidence for saline bottom waters, the current interpretation, is lacking. This study also shows that micritic carbonates can preserve their early diagenetic, marine δ18O signal, which is correlatable over tens of kilometres.
Polycrystalline calcite pseudomorphs after dolomite can exhibit mosaic or drusy fabric caused by replacement of the dolomite by calcite (one-step dedolomitization) or dolomite dissolution followed by calcite cementation (two-step dedolomitization), respectively. This study discusses the differences between the two types and interprets the characteristics of the fluid causing them. Conclusions drawn here are founded on a microscopic and petrophysical examination of an outcrop of Tuwaiq Mountain Formation, central Arabian Peninsula. Petrographic examination shows that dedolomite has distinct microfabrics in each part of the section. The lower part of the section is not affected by dolomite calcitization, the middle part is dominated by replacement fabrics, and the upper part is dominated by dissolution and/or cementation fabrics. Petrophysical examination shows a wider range of porosity and permeability values for the upper part of the section, relative to the middle and lower parts. One-step dedolomitization is characterized by preservation of the external morphology of the parent dolomite, confinement to the volume previously occupied by the dolomite, development of intracrystalline porosity and permeability that allows the fluid to maintain contact with the reaction front, and no significant effect on porosity of the original rock. Two-step dedolomitization is characterized by corrosion of the external morphology of the parent dolomite, association with other forms of calcite cementation, development without intracrystalline porosity and permeability, and positive or negative effects on the porosity. While one-step dedolomitization appears to occur within a stagnant zone of fluid circulation, the two-step dedolomitization is considered to reflect an active fluid-flow system.
The Zechstein-2-Carbonates represent one of the most prolific hydrocarbon systems of Central Europe. Carbonate reservoir quality is primarily controlled by mineralogy, with dolomite representing moderate-to-good porosities and calcite commonly representing low porosities. Current models suggest that this calcite is the result of a basin-wide phase of dedolomitization. The calcium (Ca) source for the dedolomites is thought to be derived from the fluids liberated during gypsum-to-anhydrite conversion. We present an easy-to-use and generally applicable template to estimate the dedolomitization potential of these fluids. Depending on reaction stoichiometry, salinity, and temperature, we estimate that between 2.8 * 10 −3 m 3 and 6.2 * 10 −3 m 3 of calcite may replace dolomite for each m 3 of anhydrite created. Within the constraints dictated by the environment of the late Permian Zechstein basin, we estimate that about 5 * 10 −3 m 3 of dedolomite is created for each m 3 of anhydrite. Mass balance constraints indicate that fluids derived from gypsum-to-anhydrite conversion account for less than 1% of the observed dedolomite in most of the studied industry wells from northern Germany.
The role of deep-burial dissolution in the creation of porosity in carbonates has been discussed controversially in the recent past. We present a case study from the Upper Permian Zechstein 2 carbonate reservoirs of the Lower Saxony Basin in northwest Germany. These reservoirs are locally characterized by high amounts of carbon dioxide (CO2) and variable amounts of hydrogen sulfide (H2S), which are derived from thermochemical sulfate reduction (TSR) and inorganic sources. To study the contribution of these effects on porosity development, we combine petrography, stable isotope, and rare earth and yttrium(REY) analyses of fracture cements with Raman spectroscopy and δ¹³C analyses of fluid inclusions. It is shown that fluid migration along deep fault zones created and redistributed porosity. Fluid inclusion analyses of vein cements demonstrate that hydrothermal fluids transported inorganic CO2 into the reservoir, where it mixed with minor amounts of TSR-derived organic CO2. The likely source of inorganic CO2 is the thermal decomposition of deeply buried Devonian carbonates. The REY distribution patterns support a hydrothermal origin of ascending iron- and CO2-rich fluids causing dolomitization of calcite and increasing porosity by 10%-16% along fractures. This porosity increase results from hydrothermal dolomitization and dissolution by acids generated from the reaction of Fe²⁺ with H2S to precipitate pyrite. In contrast, hydrothermal dolomite cements reduced early diagenetic porosity in dolomitic intervals by approximately 17%. However, the carbonate dissolution in the predominantly calcitic host rock results in a net increase in porosity and permeability in the vicinity of the fracture walls, which has to be considered for modeling reservoir properties and fluid migration pathways. Copyright © 2016. The American Association of Petroleum Geologists. All rights reserved.
The conversion of gypsum to anhydrite upon burial and heating is accompanied by about a 39% volume decrease, a four-fold increase in thermal conductivity, and consumption of heat. Accurate "back-stripping" of an evaporite basin and tracking its thermal evolution requires a knowledge of the depth of the gypsum-anhydrite transition. This depth depends on temperature, fluid pressure, lithostatic pressure, and activity of water in the pore fluid. Using a finite-difference heat-conduction program, we have determined the depths of transition for gypsum in evolving basins beneath commonly associated sediments and in several tectonic environments represented in the program by different basal heat flows and sedimentation rates. The activity of water is kept at 0.93 (precipitation of gypsum from seawater). All physical properties are recalculated at each time step as temperature increases and porosity decreases with burial. (An algorithm for calculating thermal conductivities using parallel-series mixtures is presented.) The modeling results show that overlying lithologies and the tectonic environment are two important factors. Gypsum converts to anhydrite at shallow depths (~400 m) when overlain by poor conductors like shale or gypsum in a rift environment, and at great depths (hypothetically >4 km) when overlain by good conductors like salt in a stable cratonic region. Sedimentation rate and the transient "heat sink" effect of the endothermic reaction have little effect on transition depth.
The Zechstein 2 Carbonate, also called Ca2 or Stassfurt-Carbonate, is Germany’s most prolific carbonate gas play. An important target for exploration in Germany for more than half a century, the Ca2 has been penetrated by several hundreds of wells.
Calcitization of dolomite in sedimentary carbonate sequences is often interpreted as a near-surface process reflecting either an erosional unconformity within a sequence or late, postburial weathering. In the Madison Group of the Western Overthrust Belt there is evidence of widespread dedolomitization. Petrographic, isotopic, and trace element analyses of Madison dedolomite indicate that multiple phases of calcitization have occurred in several different diagenetic environments.-from Authors
Resulting in dense lithology and poor physical properties, dedolomitization has detrimental effects on reservoir characteristics, so there has been little systematic research reported on the dedolomitization in the Carboniferous Huanglong Formation from the Eastern Sichuan basin. Through well drilling, well logging, thin-section authentication, and geochemistry tests, we studied for the first time the growth characteristics, regularities of distribution, genesis, and geological significance of dedolomitization in rock with typical fabric. From this study, we arrived at three conclusions: (1) the microstructure type divisions of dedolomite are as follows: A. a large micrite core and clear calcite rim, B. dolomite crystal, C. gypsum crystal, and D. rhombohedral pores; (2) rocks had low porosity and high density after dedolomitization in Eastern Sichuan, which is detrimental to reservoir development; and (3) micrite–microlite crystal dolomite and micrite–microlite crystal gypsum dolomite are the protoliths before dedolomitization. Sabkha is the primary sedimentary environment, in which the sabkha brine was kept within the layer and would become the main fluid of dolomitization in the high-quality dolomite reservoir from Eastern Sichuan. Analyzing the instances of dedolomitization has important geological significance in some ways, such as recovering the protolith, recovering the ancient environment, and discussing the relationships between diagenetic fluid properties and the reservoir’s development.
A multidisciplinary sequence stratigraphic approach has been used to better predict reservoir distribution using seismic data in the Zechstein 2 Carbonate of Northwest Germany. The Upper Permian Zechstein carbonate-evaporite cycles (Z1 to Z8 ?) of North Germany are assigned to eight depositional sequences (ZS1 to ZS8). The gasbearing Zechstein 2 Carbonate (Stassfurt Carbonate, or Ca2) encompasses both transgressive and highstand systems tracts of Zechstein sequence ZS3, and lowstand systems tract (lowstand wedge) of Zechstein sequence ZS4. By means of twenty-six subfacies types, the Ca2 carbonates can be subdivided into platform, upper slope, middle slope, lower slope, and basinal facies. Each Ca2 facies represents a predictable palaeogeographic position on the carbonate shelf. Besides facies and subfacies, Ca2 reservoir character is mainly influenced by multistage diagenetic processes. Predominantly dedolomitization (calcitization) of dolomitic Ca2-slope deposits deteriorated the reservoir quality. In general, the Ca2 is a fair to excellent reservoir where dolomite, but a non-reservoir where calcite. In order to better predict Ca2 reservoir facies, the carbonates are subdivided into seven parasequences bounded by parasequence boundaries, and a Ca2 maximum flooding surface as well as intra-Ca2 sequence boundary ZSB4 have been defined. Palynofacies analyses crucially support the sedimentologically-derived Ca2 sequence stratigraphic framework. Using an adequate a priori geological and sequence stratigraphic input model, forward and inverse seismic modeling make the interpretation of intra-Ca2 seismic reflections locally possible. Detailed well to seismic ties, and the results of seismic modeling, show that platform-ward-dipping intra-Ca2 seismic reflections follow facies/lithology boundaries of the Ca2-slope environment, onlapping the underlying slope of the Werra Anhydrite (A1). Also the intra-Ca2 sequence boundary as well as the porous platform facies of the overlying lowstand wedge can locally be imaged on the seismic. The good correlation between Ca2 facies and the position of the slopes of the underlying Werra Anhydrite (A1) and the sealing Basal Anhydrite (A2) allows the prediction of Ca2 reservoir facies from seismic data. Furthermore, the interpretation of seismic attributes, most critical in bimineral carbonate reservoirs (tight calcite versus porous dolomite) can now be done more reliably.
Due to excellent preservation, the Werra Anhydrite (A1), the upper member of the Upper Permian Zechstein cycle 1 (1st cycle, Z1), is readily studied in terms of the distribution of sulfate facies and sequence stratigraphy that can be interpreted from these facies. In this study cores taken from seven wells in the Southern Zechstein Basin were examined for their sedimentary structures and various petrographic features. Facies interpretation and depositional sequences are based on detailed examination of core material. Four main facies environments have been identified: (I) supratidal, (II) intertidal, (III) shallow subtidal, and (IV) deeper (hypersaline) subtidal. These are further subdivided into 10 subfacies types: (1) karst and (2) sabkha within the supratidal environment (I), (3) algal tidal-flat, (4) tidal flat, and (5) beach deposit within the intertidal environment (II), (6) salina, and (7) sulfate arenites within the shallow subtidal environment (III). The (8) slope subfacies type commonly associated with (9) turbidites and the (10) basin subfacies type subdivide the deeper subtidal environment (IV). Vertical stacking patterns of these facies and subfacies types reveal the sequence stratigraphic development of the sulfate cycles in response to sea-level and salinity fluctuations. The lower Werra Anhydrite (belonging to Zechstein Sequence ZS2) is characterized by a transgressive systems tract (TST) overlying the transgressive surface of Zechstein Sequence ZS2 within the A1-underlying upper Zechstein Limestone (Ca1). The TST of the A1 is several tens of meters thick in platform areas, where it is built up by sulfate arenites and swallow-tail anhydrite-after-gypsum, and thins out to a few meters of thickness toward the condensed basinal section, where laminites ('Linien-Anhydrit') are predominant. Most of the A1 succession consists of three relatively thick parasequences belonging to the highstand systems tract (HST) that shows typical prograding sets. Enhanced platform buildup, including sulfate arenites, salina deposits, intertidal sediments, and sabkha precipitation, as well as turbidite shedding off the platforms produced marginal 'sulfate walls' up to 400 m thick as platform to slope portions of the Werra Anhydrite. Seaward, the A1 thins to a few tens of meters of laminated sulfate basin muds. Increasingly pronounced A1 topography during highstand narrowed the slope subfacies belt parallel to the platform margin. This contrasts with the broad but considerably thinner slope deposits of transgressive times with much shallower slopes. The ensuing sea-level lowstand is reflected by a sequence boundary on top of the karstified A1-platform and a lowstand wedge (Zechstein Sequence ZS3) overlying portions of the slope and basinal subfacies of the A1 highstand systems tract. Beyond the lateral limits of the lowstand wedge, the sequence boundary merges with the transgressive surface of ZS3, shown by the lithologic change from the A1 anhydrites to the overlying carbonates of the Stassfurt Carbonates ('Haupt Dolomit' Main Dolomite, Ca2). The Basal Anhydrite (A2), which overlies and seals the carbonate reservoir of the Ca2, can also be subdivided into systems tracts by means of facies analysis. It is, however, much less complex than the A1 and is comprised almost exclusively of a transgressive systems tract of Zechstein Sequence ZS4.
The isotopic and cation chemistry of meteoric waters changes in response to the effects of rock—water interaction, uptake of organically derived CO2, and primary mineralogic differences among carbonate terranes. Moreover, variations in the dominance of these factors produce diverse chemical conditions within the meteoric systems which allow the sub- environments of vadose-phreatic, mixed-water, and spelean diagenesis to be distinguished. Therefore, geochemical patterns within the meteoric water system are examined to provide criteria for recognition of these subenvironments of meteoric diagenesis in ancient carbonate sequences.
Although the Basal Zechstein is classically subdivided into two cycles (Z1 and Z2, lower part) it can, alternatively, be subdivided into three complete third-order depositional sequences (ZS1–ZS3) and the associated lowstand systems tract/transgressive systems tract of a fourth sequence (ZS4). Third- and higher-order sea-level fluctuations influenced the depositional and diagenetic history of the carbonate-evaporite alternations within the Basal Zechstein. The main subject of the present study, the Zechstein 2 Carbonate (Stassfurt Carbonate, or Ca2), is interpreted to encompass both transgressive (predominantly in slope and basinal settings) and highstand systems tracts of Zechstein sequence ZS3 as well as part of the lowstand systems tract of the fourth Zechstein sequence (ZS4).
The Crinkly Bed of the Upper Permian Zechstein Group succession (Roker Formation = Hauptdolomit, Z2C) in northeast England is a distinctive 1.4-meter-thick unit that has been interpreted as a stromatolite of microbial origin, or, alternatively, as the result of purely abiotic precipitation. Close examination of this stratigraphically significant unit shows that it is the result of both physical and microbial processes, and was deposited in hypersaline shallow water immediately following lowstand evaporite deposition within the Zechstein basin. The internal structure of the Crinkly Bed consists of fine, millimeter scale laminae of alternating clotted peloidal or aphanitic micritic layers and granular laminae composed of silt-size peloids, with rare hollow spheroids. The lamination is extremely regular in thickness and laterally persistent for up to 4 meters. A microbial community composed of coccoid and filamentous microorganisms, inferred from fossil evidence, colonized the original sediment surface. It is suggested that this biofilm was responsible for the precipitation of both the micrite laminae and most of the peloids of the granular laminae; the peloids are consequently interpreted as autochthonous. This interpretation of a microbial origin is consistent with the microstructure and geochemical signatures of the micritic dolomite, which constitutes both the continuous micritic laminae and the peloids themselves. Primary dolomite precipitation is hypothesized to have taken place from marine to slightly hypersaline waters, mediated by bacterial metabolism, similar to the process that operates in modern intertidal microbial mats. The Crinkly Bed shows macro-structures that vary along a continuum from symmetrical to asymmetrical, centimeter scale, ripple-like structures, some showing interference patterns, through to domical and conical structures, more typical of stromatolites. The more ripple-like structures have a lamination of microbial origin identical to that within the domal-conical stromatolites; there is no internal cross lamination. Their origin is equivocal: they could be the result of physical (current- or wave-induced) deformation of a surficial biofilm, or their form could be inherited from true mechanically deposited ripples (not observed) that provided a template for biofilm growth. Changes in the nature and strength of the wave and/or current regime, and possibly in the community structure of the microbial mat, produced this array of microbially induced macrostructures (MISS). The latter, plus the absence of desiccation features indicative of intertidal to supratidal deposition, suggest subtidal sedimentation at a depth of several meters. Intergranular gypsum cement precipitated during early diagenesis, and ferroan sparry dolomite precipitated during burial. Tertiary uplift and the consequent exposure to meteoric waters resulted in the precipitation of calcite cements that replaced gypsum and pre-existing dolomite, probably via the biological mediation of heterotrophic bacteria. The interpretation of this distinctive stratigraphic unit illustrates the difficulty in distinguishing between biotic and abiotic processes in the sedimentary record.
Aggressive karstification can take place where dolomite and gypsum are
in contact with the same aquifer. Gypsum dissolution drives the
precipitation of calcite, thus consuming carbonate ions released by
dolomite. Lake Banyoles, in northeastern Spain, is a karst lake supplied
by sublacustrine springs, and karstic collapse is occurring in the
immediate vicinity of the lake. Lake water is dominated by Mg-Ca and
SO4-HCO3, and is supersaturated with calcite that
is actively accumulating in lake sediments. Water chemistry, sulfur
isotope composition, local stratigraphy, and mass-balance modelling
suggest that the primary karst-forming process at at Lake Banyoles is
dedolomitization of basement rocks driven by gypsum dissolution.
Karstification takes place along the subsurface contact between the
gypsiferous Beuda Formation and the dolomitic Perafita Formation. This
process is here recognized for the first time to cause karstification on
a large scale; this is significant because it proceeds without the
addition of soil-generated carbonic acid. Gypsum-driven dedolomitization
may be responsible for other karstic systems heretofore attributed to
soil-generated carbonic acid.
We modeled the geometrical roughening of bedding-parallel, mainly layer-dominated stylolites in order to understand their structural evolution, to present an advanced classification of stylolite shapes and to relate this classification to chemical compaction and permeability variations at stylolites. Stylolites are rough dissolution seams that develop in sedimentary basins during chemical compaction. In the Zechstein 2 carbonate units, an important lean gas reservoir in the southern Permian Zechstein basin in Germany, stylolites influence local fluid flow, mineral replacement reactions and hence the permeability of the reservoir. Our simulations demonstrate that layer-dominated stylolites can grow in three distinct stages: an initial slow nucleation phase, a fast layer-pinning phase and a final freezing phase if the layer is completely dissolved during growth. Dissolution of the pinning layer and thus destruction of the stylolite's compaction tracking capabilities is a function of the background noise in the rock and the dissolution rate of the layer itself. Low background noise needs a slower dissolving layer for pinning to be successful but produces flatter teeth than higher background noise. We present an advanced classification based on our simulations and separate stylolites into four classes: (1) rectangular layer type, (2) seismogram pinning type, (3) suture/sharp peak type and (4) simple wave-like type. Rectangular layer type stylolites are the most appropriate for chemical compaction estimates because they grow linearly and record most of the actual compaction (up to 40 mm in the Zechstein example). Seismogram pinning type stylolites also provide good tracking capabilities, with the largest teeth tracking most of the compaction. Suture/sharp peak type stylolites grow in a non-linear fashion and thus do not record most of the actual compaction. However, when a non-linear growth law is used, the compaction estimates are similar to those making use of the rectangular layer type stylolites. Simple wave-like stylolites are not useful for compaction estimates, since their growth is highly non-linear with a very low growth exponent. In the case where sealing material is collected at the tooth during dissolution, stylolites can act as barriers for local fluid flow as they intensify sealing capabilities of pinning layers. However, the development of teeth and spikes offsets and thus destroys continuous stylolite seams so that the permeability across the stylolite becomes very heterogeneous and they are no continuous barriers. This behavior is best shown in rectangular layer and seismogram pinning type stylolites that develop efficient fluid barriers at teeth tips but destroy sealing capabilities of layers by offsetting them at the flank, leading to a permeability anisotropy along 2-D stylolite planes. Suture/sharp peak stylolites can create fluid barriers if they collect enough sealing material. However, if the collecting material does not seal or if spikes offset the sealing material the stylolite leaks. We propose that our classification can be used to realistically estimate chemical compaction in reservoirs and gives an indication on how heterogeneous the permeability of stylolites can be.
Bedded dolomites in the Permian Basin were formed by the alteration of metastable limestones by hypersaline brines refluxing from evaporate lagoons. The hot, heavy, highly alkaline, carbon dioxide-free, magnesium-supercharged brines displaced connate waters to provide both a chemically favorable environment for magnesium-calcium exchange and a vehicle for removing displaced calcium. Fossil lagoonal brines and fillings of halite and anhydrite in the dolomite pores offer proof of the brine invasion. Sedimentary dolomites in other areas are commonly associated with evaporites, and for these dolomites a similar origin is postulated.
Carbonate dissolution by acidic CO2-charged fluids is one possible mechanism leading to the creation of burial porosity. The Late Permian carbonates of the Lower Saxony Basin, NW Germany, are a suitable target to study the influence of such CO2-charged fluids on porosity evolution. The so-called Zechstein 2 carbonates (Ca2) consist of alternating dolomitic and calcitic intervals and contain locally very high amounts of CO2 (>80%) and variable amounts of H2S in reservoir gas and vein cement hosted fluid inclusions. Composition and isotope data from fluid inclusions suggest that the CO2 is sourced from the thermal decomposition of underlying Devonian carbonates. Far-reaching fault systems connect the Devonian CO2 source with the Permian Ca2 reservoir, while thick rock salt deposits above the Zechstein 2 carbonates prohibit an influence of surface-derived meteoric fluids. The aim of the study is to evaluate the contribution of CO2 on carbonate dissolution. Seven wells were preferentially sampled along mineral veins, which are interpreted as the preferred migration pathways for the present gas. Classic petrography methods, such as transmitted light microscopy and cathodoluminescence, were combined with fluid inclusion analysis, and porosity data. Petrographic analysis reveals no great impact of the massive CO2 presence on porosity creation, as fluids get quickly saturated with respect to Ca2+. Minor vuggy porosity is associated with fractures in only one of the studied wells, but the dominant porosity types are early diagenetic fabric-selective oomoldic and intercrystal porosity. To evaluate whether the porosity values are higher than usual, a comparison with a global porosity dataset is included, which, however, shows no increase in average porosities compared to global values. The results of this case study suggest that the contribution of CO2 alone on carbonate dissolution and therefore reservoir quality is negligible, as long as other processes, such as late dolomitization or pyrite formation by thermochemical sulphate reduction-derived H2S are absent.
A method for predicting the three-dimensional distribution of reservoir attributes has been developed by integrating geological and statistical models. The general method, applicable to carbonate and siliciclastic reservoirs, has been demonstrated by predicting the distribution of dolomite, calcitized dolomite, porosity, and permeability from regional to field scales in the Permian Zechstein 2 Carbonate of northern Germany. The first step in the prediction process consists of identifying factors potentially responsible for reservoir quality distribution. For the Zechstein 2 Carbonate, the resulting geologic model suggested that paleofaults and related fracture systems controlled the distribution of nonporous calcite (calcitized dolomite) by acting as conduits for calcitizing fluids originating from anhydrites underlying the carbonates. The next step in the prediction process involves determining if the geologic model provides variables that can be used to predict the variable of interest given the predrill data available. If not, then other predictor variables, not necessarily cause-and-effect variables but ones whose values are known predrill, are required. Although a geologic model for Zechstein diagenesis elucidated the probable cause-and-effect relationship regarding the distribution of mineral types, it provided no means for predicting the geographic distribution of mineral types, because data on the distribution of paleofault and paleofracture systems cannot be obtained. For pragmatic purposes, models must both predict the desired parameter at the necessary scale and use predictor variables whose values are known prior to drilling. For the Zechstein 2 Carbonate, linear regression models using faciès and location (x-y coordinates and depth) accomplished practical predictions of mineral distribution. The fact that location provides significant predictions indicates that calcite and dolomite occur in a spatially organized manner, reflecting the geologic processes that caused the calcitization of the dolomite. Because paleostructure presumably controlled calcite distribution, separate models were developed for structurally distinct subareas. The use of structural subdivisions provided a way to account for different types of calcite distribution caused by different types of fault and fracture systems. Although mineralogy is a dominant control on reservoir quality in the Zechstein 2 Carbonate, the porosity and permeability distributions reflect additional factors. Like the mineralogy distribution, however, the porosity and permeability distributions have a dominant nonrandom spatial component, and therefore can be predicted reliably using location information. Because the spatial distribution of porosity and permeability in the Zechstein 2 Carbonate is highly complex, a nonparametric predictive technique (an artificial neural network) was implemented. It produced models that surpassed those of linear regression. Although cast here in terms of a particular application, the methodology is general, and such predictive models can be used to generate maps and cross sections of predicted parameters within any reservoir. In addition, sets of point values generated by the models can be loaded into visualization software to provide three-dimensional representations of the predicted parameters.
Identifies a series of shoals (with thick sulfate deposits) and lows (with thin sulfate and thick halite deposits). Three cores were selected to examine facies variations from a sulfate platform, slope, and basin. The vertical sequence of facies shows a distinct deepening-upward trend starting from the top of the basal nodular units. Deposition in shallow (salina) environments dominated the early history of the Lower Werra Anhydrite basin, although it was probably preceded by transgressive sabkha deposits. Gypsum was the original sedimentary mineral; it was later transformed into anhydrite, during either early or late (burial) diagenesis. -from Authors
The Lower Saxony Basin (LSB) is a highly differentiated graben filled with marine, lacustrine, and hypersaline sedimentary rocks of Late Jurassic and Early Cretaceous age. The basin inverted in Late Cretaceous time and was intruded by large laccoliths. Based on the occurrence of three oil types in close association with two source rocks, two petroleum systems are identified in this hydrocarbon province. The LSB Jurassic(!) petroleum system includes the first and oldest source rock, the Posidonia Shale (lower Toarcian), which is present throughout the entire Lower Saxony basin except for the entire westernmost part. The LSB Lower Cretaceous(!) petroleum system includes the second and younger source rock, the "Wealden' papershale of Berrisaian age, which occurs only in the western half of the basin. Reservoir rocks are sandstones and limestones of Late Triassic (Rhaetian) to Early Cretaceous age. Traps are either facies and unconformity traps of pre-Turonian age or structural traps that originated during the Late Cretaceous basin inversion. Two periods of oil generation are recognized. -from Authors
The structural position of the Southern Permian Basin (SPB) is controlled by the Caledonian and Variscan tectonic framework (Fig. 1). The basin is located between the mid-North Sea High and Ringkobing-Fyn High in the north, the East European Platform in the east and the Variscan tectogene in the south (Fig. 2). In the area of maximum subsidence (i.e. in the Variscan Foreland), the substrate is composed of consolidated Caledonian massifs. This consists of (1) the Netherlands-North German Massif (possibly a terrane), considered now (Hoffmann 1990) as a separate structural unit of Precambrian age which was established finally in Caledonian time, and (2) a part of the Holstein-Rugen-Pomorze Terrane (Caledonian fold zone; Franke 1990) limited by strike-slip faults from the North German-Polish Variscan orogenic belt. The Southern Permian Basin has several narrow connections with adjacent basins (for references, see Sorensen and Martinsen 1987) and possible temporary connections with the Tethys domain via the Polish-Dobrugea trough, along a rift zone (Ziegler 1987) and with small basins on the Inner Variscan domain. These connections and the SPB tectonic framework were induced by pre-Permian tectonics generally described as a subequatorial structural plan and submeridional deep fractures (Franke 1990).
Coarse dolomite crystals in this Limestone contain both calcite and iron oxides in rhombohedral zones and along fractures and cleavage traces. The dolomite crystals are demonstrably nonstoichiometric and are compositionally zoned, ranging from 1.40 to 4.56 mole%FeCO3. The occurrence of nonferroan calcite and ferric oxides in rhombohedral zones in Taum Sauk dolomite suggests that dedolomitization occurred by the oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to Recent weathering.-from Author
Mathematical modeling of the diagenetic behavior of the trace element Sr in conjunction with oxygen isotopic changes during the diagenesis of limestone substantiates recent suggestions that experimentally determined distribution coefficients for calcite do not apply under actual diagenetic conditions. An analogous situation probably exists with respect to dolomite. Most ancient dolomite is trace-element-depleted relative to Holocene analogs. Present, quantitative interpretation of absolute isotopic or trace element values of dolomite is tenuous. Because recrystallization is neither completely open nor completely closed-system, chemical information from early events is incompletely erased by later ones. -from Author
Carbonate rocks from three depositional environments are described: basin facies (Stinkschiefer), slope facies (Stinkkalk/Stinkdolomit) and near-shore and platform facies (Hauptdolomit). In contrast to the thoroughly dolomitized shallow-water carbonates and the partially dolomitized slope- facies carbonates, the primary carbonate sediments of the basin did not suffer dolomitization. They probably originated as high-Mg calcite and, under the influence of pore water with a very low Mg/Ca ratio expelled from the underlying sulphate deposits, were recrystallized to calcite with the release of Mg. Dedolomitization can, on the other hand, be shown to have taken place in platform sediments under the influence of meteoritic water. The organic C concentration varies with the amount of detrital material (excluding quartz) and the concentration of heavy metals. The concentrations of Fe, Zn, Cu, Cr, Pb, V and Ca agree with the average values for carbonate rocks. Sr concentrations range around 2800 p.p.m. and approximately represent the Sr contents of the original sediments. Mn is considerably decreased by diagenetic remobilization under reducing conditions. There is a general upward increase in 13C in all types of carbonate rocks in Zechstein 2; this is thought to be a variation in photosynthesis of the carbonate-producing algae in Zechstein times. Modal, elemental and isotope analyses for 257 samples are presented. -P.Br.
These pinnacle reefs are located in a band between a massive shelf-edge reef complex and deeper water facies. Pinnacles at the shelfward edge are dolomite, those at the basinward edge are predominantly limestone. Two distinct types of dolomite have been observed in the pinnacles. Clear dolomite rhombs are attributed to mixing of meteoric and seawater at the base of a freshwater lens which was lowered down through the pinnacles during their emergence. The second type of dolomite is associated with tidal flat environments which were present over the reefs. This occurs as brownish, anhedral crystals which have usually completely replaced the preexisting limestone. -from Authors Shell Development Company, Houston, Texas 77001.
In the early stages of basin history, pore-water composition was controlled largely by surface conditions existing during sedimentation. The resulting diagenesis included cementation of beach sediments, development of pisoliths in sabkhas, freshwater leaching of carbonates due to subaerial exposure, and pervasive dolomitisation of sediments in response to the reflux of brines from sabkhas, saline lagoons and playas. As burial progressed, the influence of surface-derived pore-waters decreased and the effects of rising temperature and overburden pressure became more important in controlling rock-water interactions. The resulting series of diagenetic events included anhydritisation, pressure solution, leaching and calcitisation of dolomite, halitisation accompanying mobilisation of salt, and redolomitisation of secondary limestones and cementation of leached dolomites.-from Author
Describes a platform carbonate sequence which extends over 100 000 sq. km in the subsurface of Alberta and contains extensive areas of calcitized ('dedolomitized') dolomite reservoir rocks. Environments of deposition range from subtidal and intertidal to supratidal. Early, stratigraphically controlled and later, cross-cutting dolomites are observed. Dissolution fabrics are generally related to the uppermost dolomitized units of the Grosmont Formation, below an unconformity which overlies Paleozoic rocks. Calcitization textures are found in limestone and dolomite units and do not bear any relationship to the unconformity. There is a strong tendency for dolomites from stratigraphic units to be grouped by their isotopic compositions. -from Authors
Carbonates of the second Zechstein cycle, the Stassfurt Carbonate (Ca2), constitute north Germany's most prolific carbonate gas play. The underlying Werra Anhydrite (A1) of the first Zechstein cycle and the overlying Basal Anhydrite (A2) of the second Zechstein cycle enclosed the reservoir. An erosional sequence boundary present at the top of the A1 sulfate platform separates the A1 from the overlying Ca2 carbonate platform. Ca2 thickness ranges from 30 to 80 m on the platform, 40 to 250 m on the slope, and 10 to 40 m in the basin. Following flooding of the underlying A1 sulfate platform (Ca2 maximum flooding), Ca2 shallow-water carbonates prograded basinward. Platform, upper slope, middle slope, and lower slope facies are recognized and are subdivided into 27 subfacies. Systematic trends in facies distribution are apparent on depositional dip-oriented well log cross sections. Trends in vertical succession of subfacies types, along with the good correlation between Ca2 facies and A1 thickness, enable mapping of the updip and downdip limits of the facies. Diagenesis is important in determining Ca2 reservoir quality. Reservoir rock is typically fair to good where it is dolomite. Early calcitization (dedolomitization) of the Ca2 yields poor to non-reservoir rocks. Although calcitization in generalmore » increases basinward, lateral prediction of mineralogy within individual Ca2 facies is difficult. A multivariate statistical investigation has been initiated to determine parameters, which are important in predicting trends in calcite distribution.« less
Fault-controlled hydrothermal dolomitization of the Late Aptian to Early Albian Benassal Fm shallow water carbonates resulted in the seismic-scale stratabound dolostone geobodies that characterize the Benicàssim case study (Maestrat Basin, E Spain). Petrological and geochemical data indicate that dolomite cement (DC1) filling intergranular porosity in grain-dominated facies constituted the initial stage of dolomitization. The bulk of the dolostone is formed by a replacive nonplanar-a to planar-s dolomite (RD1) crystal mosaic with very low porosity and characteristic retentive fabric. Neomorphic recrystallization of RD1 to form replacive dolomite RD2 occurred by successive dolomitizing fluid flow. The replacement sequence DC1-RD1-RD2 is characterized by a depletion in the oxygen isotopic composition (mean δ18O(V-PDB) values from -6.92, to -8.55, to -9.86‰), which is interpreted to result from progressively higher temperature fluids. Clear dolomite overgrowths (overdolomitization) precipitated during the last stage of the replacement. Strontium isotopic composition suggests that the most likely origin of magnesium was Cretaceous seawater-derived brines that were heated and enriched in radiogenic strontium and iron while circulating through the Paleozoic basement and/or Permo-Triassic red beds. Burial curves and analytical data indicate that the replacement took place at burial depths between 500 and 750 m, which correspond to the Late Cretaceous post-rift stage or early Tertiary extension of the Maestrat Basin, and by hydrothermal fluids likely exceeding temperatures of 80°C.
Pore systems in sedimentary carbonates are generally complex in their geometry and genesis, and commonly differ markedly from those of sandstones. Current nomenclature and classifications appear inadequate for concise description or for interpretation of porosity in sedimentary carbonates. Article reviews current nomenclature, proposes several new terms, and presents a classification of porosity which stresses interrelations between porosity and other geologic features. Glossary of porosity terms is included.
Two forms of dedolomitization. surface and near-surface, are found in the dolomitic Carboniferous rocks of the Middle Limestone Group of the northeast coast of Northumberland, England. This work is primarily concerned with the surface form of dedolomitization which is taking place in the Recent and sub-Recent, rusty-looking weathering crusts of the ferroan (iron-rich) dolostones. The rusty colour is caused by the presence of iron hydroxides in the weathering crusts. These iron hydroxides and the associated dedolomites are genetically related. The susceptibility of ferroan dolomites to dedolomitization is apparently caused by their metastability in the surface environment. Circulating sea or fresh water along permeable zones in the carbonates exposed at or near the present erosional surface is responsible for the oxidation and hydration of the ferrous iron content of the metastable ferroan dolomites as well as the dedolomitization of these dolomites. The dedolomitization mechanism has Been found to be a process of great potential in converting highly impermeable and impervious dolomitic rocks into a porous and permeable state. The detection of such a phenomenon in carbonate rocks at depth may, therefore, not only indicate an unconformity, but may help in the prediction of potential oil or gas reservoirs.
Dedolomitization is usually a surface phenomenon related to weathering surfaces and hence is commonly found below unconformities. These unconformities may be ancient ones. Recognition of dedolomite in subsurface may suggest a possible unconformity. Thin sections and stained peels of dedolomite under plane polarized light may reveal a crystalline limestone devoid of ghosts of dolomite rhombs. However, if exactly the same area that has been examined under plane polarized light is viewed under crossed nicols the dolomite ghosts may become evident. These ghosts are diagnostic of dedolomitization. Peels are normally black under crossed nicols because the acetate is isotropic. Etching may bring out solubility differences between calcite crystals making up the matrix of the dedolomite and those in the original rhombs. During peel preparation, the micron-sized crystals occupying the original rhombs may become attached to the surface of the peel and hence show up as an anisotropic substance demarcating the dolomite rhombs as ghosts. Not all peels of dedolomite show dolomite rhombs under crossed nicols.
Hydrocarbon exploration in The Netherlands has a chequered history from
serendipitous oil shows via chance oil/ gas discoveries to finding the
largest continental European oil field in 1943, followed by finding the
largest gas field in the world in 1959. The present contribution traces
the development of moderate to good porosity/permeability trends in
depositional facies of Zechstein Stassfurt carbonates in a ‘gas
play’ intermediate in significance between the above two plays but
all in the northern part of The Netherlands. Various depositional facies
in the Stassfurt carbonates were turned into ‘carbonate fabric
units’ by diagenetic processes creating or occluding the
porosity/permeability. This formed moderate to good gas reservoirs in
barrier-shoal, open-marine shelf and proximal-slope carbonates in the
subsurface of the province of Drenthe in the NE Netherlands. The
diagenetic models forming these carbonate fabric units are linked to the
variety of facies in a depositional model which shows explain and
predicts the reservoir trends. Such depositional/diagenetic facies are
‘translated’ into characteristic petrophysical values
recognisable on wire line logs in uncored wells, and in characteristic
seismic expressions that show these trends in undrilled areas. This
approach has been proven to be effective in delineating porosity trends,
visualised by 3-D seismic in the Collendoornerveen field, and thus
provides a new exploration ‘tool’ in hydrocarbon exploration
.
Limestones are described in which dolomite is replaced by calcite. The petrographie evidence for the replacement is presented and the various textures are described.
The Zechstein Main Dolomite of the Lebe elevation has simple depositional history and, because of shallow burial with consequent relatively minimal depth-related diagenesis, it has been possible to construct a sequential model of the early diagenetic history which can be used as a standard for comparison with other similar intra-evaporitic carbonate sequences. Limited meteoric diagenesis and localized marine cementation governed compaction during subsequent shallow burial. This was followed by anhydritization, dissolution and cementation by dolomite, sulphate and minor halite; processes which locally over printed and obliterated the textures of the initial diagenesis. The early diagenetic imprint was decisive for the later diagenetic history in the Leba elevation and this probably applies to the Main Dolomite over most of the basin. Local geological factors crucially influenced the intensity of the diagenesis in which early meteoric cementation coupled with late diagenetic leaching were crucial in the formation of productive hydrocarbon reservoirs.
Stylolites in evaporites are very uncommon. In the Gorleben Zechstein salt dome, stylolite horizons have been found in the so-called " Banderanhydrit" (z3HA11) at the top of the Zechstein Hauptanhydrit (Main Anhydrite). These pressure-solution features seems to be related to the presence of thin magnesite-clay interlayers. Five principal stylolite morphologies can be distinguished in the core sections studied. The orientation of the sutured seams is generally bedding-parallel with the stylolite axes vertical to the bedding surface. This demonstrates that pressure-solution features clearly predate the formation of the salt dome and that there is no relation between their formation and halokinesis. Solution rates of up to about 26% have been calculated by measuring maximum stylolite amplitudes in certain core intervals. Considering the bedding-parallel occurrence of the stylolites and the fact that the stylolite seams are crosscut by anhydrite crystals that me interpret as pseudomorphs after gypsum, it must be concluded that the stylolites formed before the dehydration of gypsum to anhydrite was completed. The lower parts of the Hauptanhydrit were excluded from these processes because they had already been converted to anhydrite by the overburden of overlying Zechstein units and by ascending compaction fluids of the underlying Zechstein 2 Salt. We assume that the dehydration of gypsum to anhydrite was completed before the end of Zechstein sedimentation and that the formation of stylolites thus must be attributed to the same time interval.
Dolomitization of primarily calcarenite beds of Upper Jurassic age, deposited in the intertidal zone, is recognized in HaMakhtesh HaQatan in southern Israel. Such dolomitization is interpreted as having occurred in the supratidal zone after deposition of the carbonate sediments, and was later followed by partial dedolomitization of the same rocks. Dolomitization is attributed to reflux of hypersaline water, whereas dedolomitization appears related to exposure of the lithified sediment during the development of the Juro-Cretaceous or later unconformities. Small channels are observed arrayed in two systems perpendicular to one another on a carbonate bed. The pattern of channels is interpreted to represent a series of mega mud cracks developed within the supratidal zone and later enlarged by solution.