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Fault-controlled carbonate cementation: A case study from Eocene turbidite-delta sandstones (Dongying Depression) and implication for hydrocarbon migration
Abstract
The subsurface distribution of carbonate cements in sandstones is cornerstone in understanding the fluid regime and origin of cements to shed the light on hydrocarbon migration. Petrography, cuttings, cores, well logs, and seismic data were integrated to better understand the distribution of carbonate cements in the Eocene turbidite-delta sandstones of the Dongying Depression. Petrographic examinations (including cathodoluminescence, CL) reveal that the carbonate cements are mainly calcite with minor ankerite that precipitated in deep burial settings. The significant abundance of carbonate cements near fault zones implies migration of fluids from deep settings through late fractures developed by compaction under deep burial conditions. The extent of carbonate-cemented zones may reach hundreds of meters away from the faults, while scatter carbonate cements are much more extensive. The relationship of pervasive and scattered calcites with oil-bearing sandstones suggests that the pervasive calcite cementation started as soon as the hydrocarbon and brine mixed fluid were displaced while the scatter calcite cements came after. The occurrence of abundant carbonate-cemented zones in the delta front oil-free sandstone intervals suggests possible hydrocarbon migration to the overlying sandstone units.
... The sediments of Mbr 3 and Mbr 4 have undergone intensive diagenesis including dissolution and precipitation (Luan et al., 2019;Yang, 2019;Luan et al., 2021;Luan et al., 2022), indicating active components exchanging between the water and the ambient solids. Kaolinite, albite, quartz overgrowth, ankerite, and post-ankerite calcite are the most abundant authigenic minerals in this study area (Yang, 2019;Luan et al., 2022). ...
... Multistage feldspar dissolution has significantly contributed to the porosity of the clastic reservoirs (Yang, 2019;Luan et al., 2022). The origin and distribution of the authigenic minerals in the source rock and the clastic reservoirs have been widely discussed (e.g., Luan et al., 2019;Yang, 2019;Luan et al., 2021;Luan et al., 2022), whereas the hydrogeochemistry and the origin of the formation water are still unclear. ...
... The composition of interstitial water likely reflects the water-rock interaction equilibrium that the reservoirs established. Sandstones in the Shahejie Fm (Niuzhuang Sag), particularly those near fault zones, are pervasively cemented by carbonates (Fig. 3D, F, G, and I; Luan et al., 2021). An internally buffered carbonate system, where increased/decreased pCO 2 would cause carbonate dissolution/precipitation (Equations (5) and (6)), cannot explain the carbonate equilibrium without an increase in HCO 3 − along with the increasing Ca 2+ (Fig. 12A). ...
The composition of formation waters in the Dongying Depression provide important clues about regional water-rock interactions. Chemical analysis (181 samples) of formation waters form the Shahejie Fm (58 wells) in the Niuzhuang oilfield suggest two main geochemical facies, the chloride-magnesium (Cl–Mg) and bicarbonate-sodium (HCO3–Na) facies (pH = 7.8 ± 0.9) that have low salinity (TDS = 7.3 ± 7.8 g/L) and occurs within source rocks, and the Cl–Ca facies (pH = 6.5 ± 0.7) that has high salinity (TDS = 79.1 ± 79.0 g/L) and occurs within clastic reservoirs.
For the water within the clastic reservoirs, salinities and dissolved chloride concentrations (47.8 ± 48.8 g/L) are attributed to halite dissolution. The broad systematic increase in dissolved Mg²⁺ and Ca²⁺, and decrease in pH with increasing salinity (Na⁺ concentration) strongly suggest that the formation water has been thermodynamically buffered by silicate-carbonate mineral assemblages. The Caexcess increase with Nadeficit suggests that the albitization of detrital plagioclase (anorthite) might have provided a internal source for Ca²⁺. The pH was mainly buffered by silicate minerals, which is indicated by the increase of acidity with Na⁺. This is also consistent with the externally buffered carbonate system indicated by the negative correlation of log(HCO3⁻) with log(Ca²⁺). The increasing acidity associated with the addition of Na⁺ might have been responsible for the abundant kaolinite and secondary porosity developed by K-feldspar dissolution.
The contribution from halite dissolution to formation waters within the source rocks was relatively restricted. The positive correlation of log(HCO3⁻) with log(Ca²⁺) suggests an internally buffered carbonate system. The formation water might have been partly buffered by illitization, which is consistent with the negative correlation of Nadeficit with Caexcess.
... Based on the oxygen isotope fractionation equations for calcite water [39], the calculated precipitation temperatures of low-Fe calcite ranged from 54.1-66.7 • C, with an average value of 60.4 • C ( Table 2). 1000 lnα Ca−H2O = 2.78 × 10 6 /T 2 − 2.89 (1) Carbonate cements have various sources, including internal, external, and mixed sources [40,41]. The internal sources are mainly derived from dissolution of detrital and biogenetic carbonate fragments [42,43], while the external sources are a result of the diffusive transport of ions from adjacent mudstones [40,44]. ...
The diagenetic evolution of sandstone is very complicated under the conditions of high temperatures and pressures in deep-water, deep-buried regimes, which have great influence on reservoir quality. This study investigates the typical reservoir target of Neogene deep-water, submarine-fan sandstones under high-temperature, high-pressure regimes in the Qiongdongnan Basin, South China Sea. Utilizing a thin section, scanning electron microscope (SEM), mineral geochemistry combined with burial history evolution, complex diagenetic events, and main controlling factors of the sandstone in the Neogene Meishan Formation were determined. The results show that the evolution of sandstone reservoirs is initially controlled by depositional framework compositions and subsequently modified by eogenetic and mesogenetic alterations during progressive burial. Eogenetic alterations mainly include the following: (1) mechanical compaction; (2) dissolution of feldspar; (3) low-Fe calcite cementation. Mesogenetic events were identified as the following: (1) dissolution of feldspar; (2) ferroan calcite and ankerite formation; (3) precipitation of quartz and clay mineral. Mechanical compaction is greatly influenced by the original depositional framework composition, and sandstone samples enriched in high contents of detrital clay matrix always experienced extensive mechanical compaction. Different phases of carbonate cement during different diagenetic regimes lead to continuous destruction on reservoir porosity. The dissolution of unstable feldspar minerals during eogenetic and mesogenetic environments leads to the development of secondary porosities and would enhance the quality of the reservoir. Overpressure formation is pervasively developed owing to early disequilibrium compaction and subsequent natural gas charging. Only well-sorted sandstones with low contents of detrital clay matrix could resist early mechanical compaction, lead to ample residual original porosities, and then undergo extensive mineral dissolution to generate sufficient secondary porosities. Subsequently, these porosities would be effectively protected by overpressure formation. Poor-sorted sandstones with high contents of detrital clay matrix would experience strong mechanical compaction and extensive destruction of original porosities. Thus, these sandstones are difficult to have significant dissolution and are unable to be effectively protected by overpressure formation. Therefore, the interplay between the original framework composition and the corresponding diagenetic pathways coupled with overpressure formation would result in strong reservoir heterogeneity for the deep-buried sandstones during progressive burial.
... Around the contacts of faults and reservoir sequences, chemical diagenesis could occur but may be diverse based on various depositional backgrounds (Busch et al., 2017;Ferrill et al., 2019;Luan et al., 2021;Vincent et al., 2018). For instance, acidic fluid from coal seams may result in extensive dickite growth and feldspar dissolution in the inner zone of faults, and wide illitization in the outer zone of faults for hundreds of meters. ...
Many extraordinary cements, including mosaic dolomite, poikilitic celestine–barite, anhydrite and chemically zoned saddle-formed dolomite–ankerite, were observed in a reservoir sandstone unit underlying an evaporite sequence in the northern Bonan Sag, which is a saline lacustrine rift basin in the Jiyang Depression of Bohai Bay Basin, China. A suite of petrologic and chemical methods, including optical observation with cathode luminescence, fluid inclusion with laser Raman spectroscopy, X-ray fluorescence, and stable isotopes of carbon, oxygen and sulfur, were carried out. The results suggest that these extraordinary cements are mainly distributed near a fault zone and related to episodes of faulting events. Fault rupturing at shallow burial depths resulted in the dislocation of the evaporites and sandstones and made it possible for an influx of alkaline fluid from evaporites into the fault zone, where mosaic dolomite precipitated, and near fault reservoir units, such as where celestine–barite precipitated. Within the ‘oil window’, the reactivated normal faults, which induced a pressure drop in the reservoir unit, may have resulted in gas influx (e.g., CO2 and H2S) and hydrocarbon charging, enhancing K-feldspar dissolution and cementation of assemblages of quartz, albite and illite. The cyclic fault rupturing and sealing in this setting probably resulted in repeated ‘openness’ of the diagenetic system, chemical perturbation of the pore fluids (e.g., pH dropping and returning), and cyclic dissolution–regrowth within the silicate–sulfate–carbonate diagenetic system. This study may highlight the awareness of diagenetic complexity in fault zones and shed light on reservoir evaluation in rift settings.
... Carbonates are generally the predominant cements in sandstones (e. g., Xiong et al., 2016;Luan et al., 2021Luan et al., , 2022a and understanding their origin is a cornerstone in the evaluation of reservoir quality (Morad, 1998;Worden et al., 2019). In shallow marine sandstones, calcite cementation may occur as pervasively calcite-cemented concretions, and occupy substantial proportions of intergranular porosity (e.g., Olanipekun and Azmy, 2022). ...
Inversion tectonics are of paramount importance for forming a range of hydrocarbon traps and reservoirs for petroleum exploration. Also, the uplift and exhumation of strata, the activation and creation of faults and leakage or breaching of hydrocarbon traps are often attributed to multiple inversion. Changes in fluid activity and the geochemical system are frequently observed during such tectonic inversions. However, the impacts of tectonic inversion on fluid flow and potential diagenetic alterations within deeply buried sandstone reservoirs are not fully understood, posing challenges to accurately assessing reservoir quality and furthering exploration efforts. The Oligocene fluvial sandstones in the Yuquan anticline of the Xihu Depression were investigated by XRD, optical microscope and SEM-EDS-CL along with and petrophysical data to illustrate the impact of tectonic inversion on the evolution of diagenesis and reservoir quality. The two representative exploration wells, C-1 and C-2, originating from the same provenance and sedimentary background in Yuquan anticline, exhibit significant diagenetic alterations attributed to varying degrees of tectonic inversion. Notably, well C-1 has experienced more intense alterations and dissolution of detrital grains compared to well C-2 (average secondary pore, well C-1: 4.8%, well C-2: 2.9%), particularly the complete dissolution of plagioclase grains. Moreover, the content of authigenic kaolinite and quartz cement of well C-1 is higher than that of well C-2, averaging 7.2%, 4.5% and 5%, 1.7% respectively. This diagenetic difference is attributed to the fact that the activity of early NE-NNE oriented faults and the development of late nearly E-W oriented faults are stronger in the northern well C-1 field than in the central well C-2 field. The structural characteristics resulting from tectonic inversion led to enhanced efficiency in both organic acid supply from underlying source rocks in well C-1 field and export of dissolution products within the diagenetic system when compared with Well C-2 field. Regional tectonic inversion plays a predominant role in improving reservoir quality than sedimentary factors in the study area. While the well C-1 has a higher porosity than well C-1 (well C-1: average 18%, well C-2: average 13.1%), the permeability of well C-1 is worse than that of well C-2, with an average of 14.7 mD and 27.9 mD respectively. The alteration and dissolution associated with tectonic inversion significantly enhance the porosity of sandstone reservoirs. However, the formation of secondary pores and dissolution products can complicate the pore system of reservoirs, thereby reducing the reservoir permeability. Our research aims to provide a better understanding of fluid-rock interactions and genetic mechanisms in gas-bearing sandstone reservoirs under varying degrees of tectonic inversion. This will support pre-drill evaluations of reservoir quality in deeply buried formations as well as other analogous targets.
This study aims to determine the interdependence of deposition, tectonics and diagenesis in a prolific hydrocarbon reservoir. This has been achieved through an integrated field study, petrography, scanning electron microscope (SEM), zircon microscopy and X-ray diffraction (XRD) in the fluvio-deltaic sandstone of the Lower Cambrian Khewra Sandstone. Sandstones are subarkose, sublitharenite and feldspathic litharenite subjecting to compaction, cementation, dissolution and replacement. The pore types are intergranular, vuggy, and fractures. The paragenetic sequence suggests a multistage diagenetic history for the sandstone under eogenetic, mesogenetic and telogenetic regimes. The telogenetic alterations are triggered by Himalayan orogeny. Both deposition and tectonics control the spatial and temporal distribution of diagenetic facies and sweet spots. The bounding unconformities and variation of compositional and textural patterns determine the reservoir quality. The dynamic interaction among deposition, tectonics and diagenesis yields a comprehensive understanding of reservoir potential on a global scale.
The Ben Nevis sandstones in the Jeanne d’Arc Basin in offshore eastern Newfoundland have abundant carbonate cements, and are the primary hydrocarbon reservoirs of the White Rose Oilfield. A multiapproach of petrographic, carbon and oxygen isotopic, and in situ geochemical analyses were applied to investigate the origin of the carbonate cements in the Ben Nevis sandstones (quartzose to litho-quartzose, Well F-04). Petrographic examinations reveal five generations of cements that occur in concretions, which are (from earliest to latest), siderite, early calcite cement (C1), recrystallized fossil (Crf), sparry fossil-filling calcite (Cf), and late calcite cement (C2). The comparable carbon (3.6‰ VPDB) and oxygen isotopic (−1.3‰ VPDB) compositions of siderite to those of the bivalve shells (δ¹³C = 3.6 ± 0.6‰ VPDB and δ¹⁸O = −1.7 ± 0.5‰ VPDB, respectively) indicate an early precipitation from suboxic water near the sediment-seawater interface. The well-preserved intergranular volumes (IGVs up to 41%) and low δ¹⁸O (−3.3 ± 1.1‰ VPDB) as well as comparable δ¹³C values (3.6 ± 0.6‰ VPDB) of the early poikilotopic calcite (C1) argue for a pre-compaction precipitation at a shallow burial setting with carbon derived mainly from the dissolution of bioclasts. The later calcite cement (C2) precipitated during progressive burial with involvement of organic carbon as indicated by its depleted δ¹⁸O (−6.7 ± 2.0‰ VPDB) and δ¹³C (1.8 ± 1.3‰ VPDB) values, and the reduced IGVs (28.2%), as well as the consistent depletion of Sr with enrichment of Mn. The dissolution of aragonitic bioclasts, was likely the main source of Ca for calcite, which is consistent with the high Sr content of C1 (1369 ± 597 ppm), Crf (2005 ± 472 ppm), Cf (1354 ± 48 ppm) and C2 (1831 ± 696 ppm). The parallel shale normalized (REESN) patterns of the cements and the way they mimic, to some extent, that both of seawater and meteoric water, suggest that the diagenetic fluid is bracketed by those of the early Cretaceous marine and meteoric waters, as reflected by the estimated δ¹⁸OSW composition of parent fluid for C1. The consistent Eu anomalies of carbonate cements and the increasing δ¹⁸OSW composition of pore water during C2 precipitation reflect the evolution of the diagenetic fluids in siliciclastic intervals during progressive burial.
The secondary porosity development as the main contributor to the present reservoir porosity seem to be closely related to the dissolution of calcite cements, especially in the lower part of the Ben Nevis interval. The meteoric water influx from the overlying unconformities may have induced the dissolution of calcite concretions.
Clarifying the precipitation and dissolution processes of carbonate cement is of great significance for reconstructing the history of reservoir diagenesis, quantitatively evaluating reservoir quality, and enhancing acidification-related oil recovery. In this study, comprehensive experiments were performed, including thin section observation, cathodoluminescence, scanning electron microscopy, electron probe, QEMSCAN, carbon and oxygen isotopes, and fluid inclusions, to investigate the carbonate cements from the lower Huangliu Formation in the XD 10 block of the Yinggehai Basin in the South China Sea. The types, distributions, paragenetic framework, and formation mechanisms of carbonate cements were studied systematically to further reveal their impacts on clastic reservoir quality. The results were as follows: (1) The average absolute content of carbonate cement was 7.5%, which accounted for 84.3% of the total authigenic minerals and showed a negative correlation with reservoir properties. The early-stage calcite and dolomite being controlled by sedimentation with the δ¹⁸OPDB values ranging from −12.28‰ to −5.28‰, filled in the intergranular pores with poikilotopic crystals, which resulted in almost 90% of the primary porosity to be lost; the late-stage ferrocalcite and ankerite that were characterized by δ¹⁸OPDB values varying from −17.0‰ to −9.02‰ filled up approximately 30% of the secondary pores caused by minerals dissolution. (2) The overpressure of deeply buried clastic reservoirs resulted from CO2-rich fluid charging, water saturation, and their distributions, which directly affected the stability of the reservoir carbonate cements. For a gas layer containing bound water, spot-like selective dissolution developed because the film-type bound water contained only a small amount of CO2. For the water layer, the carbonate cements suffered from strong dissolution owing to the long-term sufficient CO2 supply. (3) In situ dissolution experiments of carbonate cement, periodically monitoring the relationship between injection pressure and dissolution time, indicated that the intrusion pressure represented a periodic leaping change. This is attributed to the periodic leap between the “throat blocking” resulted from the precipitation of dissolved carbonate grains and the “throat opening” caused by the further dissolution of carbonate grains. (4) Analysis of 3D petrophysical properties and 2D images showed that the early-stage carbonate cements dissolution enhanced the porosity of the reservoir from 3% to 28%, which increased the permeability by 2000–4000 times. The dissolution of late-stage carbonate cement improved the porosity from 8% to 16% and resulted in a permeability increase of 5–100 times. The mean radius of 2D pores were extended from 68.4 to 266.3 μm, the mean radius of 2D throat were increased from 14.9 to 35.1 μm. Systematic analysis of the influence of carbonate cements in deep tight reservoirs with high temperature and overpressure (HTOP) on the rock quality provided some insights into the analysis of quality control factors and the later development plan of carbonate cement-rich deep tight reservoirs.
Diagenetic carbonate cements occur throughout Eocene turbidite lithic arkose in Niuzhuang sag, eastern China, displaying as sporad-ical siderite, dolomite, calcite 1, and massive ankerite and calcite 2. Petrographic and geochemical investigations suggest that the non-ferroan calcite 1 (d 13 C carbonate (carb) +3.1‰ to +4.7‰ Vienna Pee-dee belemnite [VPDB]; d 18 O carb À12.5‰ to À10.5‰ VPDB) was the earliest carbonate cement that was followed by Ak (d 13 C carb +0.2‰ to +5.1‰ VPDB; d 18 O carb À12.8‰ to À10‰ VPDB) and ferroan calcite 2 (d 13 C carb +2‰ to +3.1‰ VPDB; d 18 O carb À12.6‰ to À15.8‰ VPDB). The homogenization temperatures (74.3°C-105.8°C, 105.6°C-130.4°C, and 119.7°C-144.6°C, respectively) reflect the pattern of increasing temperature with progressive burial. The d 13 C carb values suggest that calcite 1 and cal-cite 2 were mainly derived from dissolution of carbonates in calcar-eous shales (d 13 C carb +3.5‰ to +6.3‰VPDB), with minor contributions from organic matter. The d 13 C carb values of ankerite documented some contributions from magmatic carbon.Ankerite and calcite 2 were restricted under the top seal (geochemical barrier) of the overpressure compartment, and the highly cemented zones occur mainly along faults. Carbonate cementa-tion seems to negatively impact the reservoir quality when it exceeds 10%.
Some pockmarks on the Norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms. Detailed examination of a sandstone sample from a pockmark in Norwegian Block 25/7 in the North Sea has revealed the presence of Mg calcite and aragonite cements, some of the latter forming botryoids. All cement types are characterized by extremely light carbon isotopic compositions, with a mean 13C value of -56.1 PDB, which shows that the cements contain carbonate produced by oxidation of biogenic methane. Oxidation occurred in both the oxic and anoxic diagenetic zones. Trace amounts of interstitial methane (mean 13C = -40.8%) and higher hydrocarbon gases (up to C5) with a C1/Cn ratio of 0.855-0.874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas.
The quality of the reservoirs of the Lower Cretaceous Pendencia Formation of the Potiguar basin, northeastern Brazil, is directly controlled by depositional facies-related carbonate cementation and compaction. The study of the interplay of these processes in the reservoirs offers an opportunity to unravel the diagenetic patterns of clastic sequences in interior rifts and, in particular, the role of carbonate cementation in poorly understood continental systems. The Pendencia Formation is a thick sequence of fan-deltaic, fluvial-deltaic, turbiditic, and lacustrine sandstones, conglomerates, and shales deposited during the rift stage of the basin. The sandstones are predominantly arkoses (average Q49F40L11), with subordinate plutonic and volcaniclastic feldspathic litharenites. Compaction and cementation had similar importance in the destruction of porosity, with a dominance of cementation in the turbidites and of compaction in the fluvial deposits. Carbonate cementation in Pendencia reservoirs increases progressively with depth and is facies controlled. Eodiagenetic, nonferroan calcite I (δ18OPDB -10.7 to 4.8‰; δ13CPDB -17.5 to +8.5‰), mesodiagenetic rhombohedral ferroan dolomite/ankerite (δ18OPDB -9.3 to -3.9‰; δ13CPDB, -1.7 to +1.1‰), and ferroan calcite II (δ18OPDB -17.2 to -6.8‰; δ13CPDB - 13.6 to +2.3‰) were precipitated at three distinct temperature intervals calculated from the δ18O values: 21 to 58°C, 70 to 79°C, and 85 to 150°C, respectively. According to the δ13C values, dissolved carbonate for calcite I was derived from oxidation and methanogenic fermentation of organic matter and from methane oxidation. Ferroan mesogenetic cements were derived from thermal decarboxylation of organic matter. The shales were a major source of dissolved carbonate, as indicated by the isotopic similarity between their calcite (δ13CPDB -0.2 to +1.8‰; 87Sr/86SR ≈ 0.719) and most cements in the sandstones and by the peripheral cementation along the contacts of interbedded sandstones. As a result of this cementation pattern, thin turbiditic and deltaic sandstone beds are pervasively cemented. The best reservoir quality potential is encountered in the partially cemented fluvial sandstones at moderate depths. Deltaic and turbiditic sandstones are more pervasively cemented by carbonate derived from the interbedded shales. Alluvial-fan conglomerates and sandstones were flushed by telogenetic meteoric waters close to the borders of the basin and to the proximity to the postrift unconformity. However, porosity enhancement was very limited, due to the precipitation of kaolinite and the intense compaction related to the compositionally immature detrital framework.
The Dongying depression in the Bohai Bay Basin is a young, prolific oil-producing basin in China. The gray to black mud-stones, calcareous mudstones, and oil shales of the Eocene Shahejie Formation (Es3 and Es4) are the major source rocks that are primarily dominated by type I kerogens with a high total organic carbon of up to 18.6%. The Es3 interval is char-acterized by a sedimentation rate of up to 500 m/m.y. Wide-spread overpressures are present in the Eocene Es3 and Es4 intervals in the depression, with pressure coefficients up to 1.99 from drillstem tests. Among the sonic, resistivity, and density logs, only the sonic-log displays an obvious response to the overpressure from which the top of the overpressure can be clearly identified. Acoustic traveltime versus effective ver-tical stress analysis of more than 300 wells in the Dongying depression suggests that the acoustic traveltime of the nor-mally pressured and overpressured mudstones is reduced with increasing vertical effective stress. Pore pressures are accu-rately predicted in the Dongying depression using an Eaton (1972) exponent of 2.0 by comparing the predicted pressure coefficients with measured pressure coefficients. Disequilibrium compaction has been previously proposed as the sole cause of the high-magnitude overpressures in the Eocene strata of the Dongying depression because of rapid de-position of fine-grained sediments. However, our data indicate that the overpressures are caused by oil generation from the source rocks within the Es3 and Es4 intervals. The over-pressured sediments display a normal compaction as evidenced Xiaowen Guo received his B.S. degree in petro-leum geology from the China University of Geo-sciences in 2006. He is now a doctoral student of petroleum geology in the China University of Geosciences. His recent research interest is the overpressure mechanism and evolution in pet-roliferous basins. (Wuhan). His re-search interests include overpressure, petroleum system modeling, and petroleum geochemistry. Keyu Liu is a principal research scientist and re-search leader of the Fluid History Analysis Team at CSIRO. His principal research areas are hydro-carbon migration and charge history of petroleum reservoirs and enhanced oil recovery. Liu has a B.Sc. degree from China Ocean University, an M.Sc. degree from the University of Sydney, and a Ph.D. from the Australian National University. from the overpressured mudstones exhibiting no anomalously low density, the apparent lack of correlation between mudstone densities and effective vertical stress, and the overpressured reservoir sandstones showing no anomalous high-matrix po-rosities or anomalous geothermal gradient. The depths to the top of the overpressure intervals range from 2000 to 3000 m (6562–9843 ft) following closely source rock depths. All the overpressured reservoirs and source rocks have a minimum temperature of approximately 87°C, and overpressured source rocks generally have vitrinite reflectance (R o) values of 0.6% or higher. Overpressures are not found in the strata within which the R o values are less than 0.5%. The overpressured reservoirs in the Es3 and Es4 intervals are predominantly oil saturated or oil bearing. Organic-rich source rocks with overpressures are capable of generating hydrocarbons and thus can maintain an abnormally high pressure. The calcite precipitation in the cal-careous mudstones may have caused a significant reduction in porosity and permeability to form an effective pressure seal. The presence of widespread microfractures in the source rocks may relate to episodic expulsion of hydrocarbons or over-pressure dissipation. Overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks through active faulting and fracturing. INTRODUCTION
Seismic diagenetic facies is an important control on reservoir quality. This study investigated the feasibility of predicting sandstone diagenetic facies using conventional 3D seismic data by seismic sedimentology and calibrated by laboratory core-analysis data in the Qijia area of the Songliao Basin. Three core issues are related to seismic characterization of diagenetic facies, including how to correlate stratigraphic and diagenetic units from seismic data, how to evaluate the relationship between core-based diagenetic facies and seismic attributes, and how to find an effective way of mapping seismic diagenetic facies. Well- and seismic-based, high-resolution sequence analysis and seismic stratal slicing provide reservoir-scale (20 m) seismic representations of diagenetic units. Core-based analyses of sandstone diagenetic processes and diagenetic sequences reveal the kind of diagenesis that most influences reservoir quality. An investigation of reservoir parameters and acoustic rock properties further reveals the link between diagenetic facies and impedance, leading to a recognition of calcite cementation as the process that can be detected by seismic data. A seismic-based lithology cube (e.g., 90°-phased seismic volume) provides the amplitude (impedance) signal for detection of diagenetic facies. Stratal slices made from the seismic-based lithology cube are then used to interpret depositional facies and systems. Eventually a seismic diagenetic-facies map is generated through analysis of the relationships between depositional facies, impedance, and diagenetic facies. A case study of clay- and calcite-cemented sandstone in the Qijia area shows that although still in its infancy, seismic detection of sandstone diagenetic facies using conventional seismic data is definitely feasible.
Analysis of well log data from Cenozoic basin fill of the Deer Lodge Valley, southwestern Montana, provides evidence for identifying paleosols and paleosol stacks in the subsurface. The paleosol stacks are continental sequence boundary markers and appear as several relatively thin, high-velocity/high-density zones within basin fill. Zone thickness ranges from 1 to 1.5 m; zones are stacked to thicknesses of up to 15 m. Density varies within the zones by as much as 0.6 g/cm³, and differs by as much as 0.9 g/cm³ from material immediately above these zones. Velocity differs by as much as 10 ft/ms from the overlying material and causes bright reflections on seismic sections. Synthetic seismograms are used to tie well log and seismic data. Basing our interpretation upon well log data and well cuttings analyses, we determined the high-velocity/high-density zones to be limestone. The pedogenic origin of the limestone is shown by (1) well cutting chips from the high-velocity/high-density zones that exhibit pedogenic features associated with calcic paleosols, (2) paleosol horizonation interpreted from well log analysis, (3) the absence of minerals normally associated with lacustrine deposits, and (4) comparison with surface paleosol exposures.
Calcite cementation within shallow marine sandstones typically occurs as (1) continuously cemented layers, (2) layers of concretions, and (3) concretions scattered throughout the sandstone. This paper presents a model for the nucleation and diffusion-controlled growth of calcite cement which explains how these three types of calcite cementation arise by diagenetic redistribution of carbonate fossils originally contained within the sandstones. The model predicts that nucleation points in a system where clastic carbonate is concentrated in layers, will be confined to these layers, and a continuously cemented layer or a layer of stratabound concretions will form from the carbonate fossil-rich layer depending upon the original amount of clastic carbonate present within the layer. The semi-regular distance observed between stratabound concretions is a result of each nucleation point lowering the concentration of dissolved calcite in the surrounding sandstone and thereby inhibiting nucleation in close proximity to earlier formed nuclei. In a system with a homogeneous distribution of clastic carbonate, no preferred levels of nucleation are present, and this explains how a uniform three-dimensional distribution of concretions may arise. Flattening of concretions parallel to bedding is a natural consequence of diffusion-controlled growth in a system where the clastic carbonate is concentrated in layers, and need not have anything to do with permeability anisotropy. Formation of continuous calcite cemented layers by merging of concretions explains why closely spaced vertical geochemical profiles through a calcite cemented layer often are widely different.
The volume of precipitated quartz cement and the resulting porosity loss in a quartzose sandstone can be calculated from the temperature history of the sandstone based on an equation relating the quartz precipitation rate per unit surface area and per unit time to temperature. In addition to temperature and time, the quartz surface area available for quartz cement precipitation will control the progress of quartz cementation within a given sandstone. Grain size, detrital grain mineralogy and abundance of grain coatings, factors which are controlled by provenance and depositional environment, are therefore also essential input parameters for modelling of quartz cementation.Computed quartz cement volumes and porosities were compared with measured values for Brent Group sandstone samples from two wells in the northern North Sea. Porosities and quartz cement volumes in these sandstones currently vary from 8 to 19% and from 6 to 28%, respectively, due to large variations in grain size, grain coating abundance and quartz clast content. Despite these compositional and textural variations, modelled and measured values for both quartz cement and porosity in most cases differ by less than a few percent. Mean measured porosity and quartz cement volume differ from mean modelled porosity and quartz cement volume by less than one percent in both wells.
The lacustrine deep-water gravity-flow sandstone reservoirs in the third member of the Shahejie Formation are the main exploration target for hydrocarbons in the Dongying Sag, Eastern China. Carbonate cementation is responsible for much of the porosity and permeability reduction in the lacustrine deep-water gravity-flow sandstone reservoirs. The sandstones are mainly lithic arkose with an average framework composition of Q43F33L24. The carbonate cements are dominated by calcite, ferroan calcite, ankerite and a small amount of dolomite. The calcite and ferroan calcite are mainly poikilotopic blocky crystals, while the dolomite and ankerite are mainly euhedral rhombohedra crystals filling intergranular and intragranular pores. The relatively positive δ¹³C values (−2‰ to +3.9‰) of the carbonate cements in the sandstone reflect a mainly inorganically sourced carbon. From 32 Ma to 25 Ma, the pore water was rich in bicarbonate and Ca²⁺ due to carbonate dissolution in mudstone, and which were transported with the pore water from mudstone to sandstone via advection and precipitated calcite cementation in thinly bedded sandstones and some high permeability zones in the middle of medium-to-thick sandstone beds. From 12 Ma to present, abundant Ca²⁺, Fe³⁺, Fe²⁺, Mg²⁺ and bicarbonate had been transported from mudstone to sandstone via diffusion to form tight ferroan calcite cementation in the upper and lower parts of the medium-to-thick bedded sandstones. Ankerite is mainly distributed in the reservoirs associated with oil migration or charge, because change of Fe³⁺ to Fe²⁺ from oil charge may supply sufficient Fe²⁺ for ankerite precipitation. The center of sandstone beds (>0.6 m) is with potential of high-quality reservoirs in the research area. Carbonate cementation appears to be an important factor that controls the accumulation of oil in deep-water gravity-flow sandstone reservoirs in the study area.
A comprehensive analysis was conducted to evaluate the diagenesis and property evolution of Es3z in Niuzhuang sag, by combining core observation, thin section identification, image analysis, fluid inclusion analysis, carbon and oxygen isotope analysis and physical property test. The burial, thermal and hydrocarbon charging history of the reservoirs were also taken into consideration. The results show that the turbidite reservoirs of Es3z are characterized with low porosity and permeability, and may have experienced diagenetic environment changes through the sequence of weak alkaline, acidic, alkaline, and weak acidic conditions. With various diagenesis, the main dissolving-cementation sequence was: early siderite and calcite cementation; feldspar dissolution/quartz overgrowth/authigenic kaolinite precipitation; first hydrocarbon charging; ferrocalcite/ankerite cementation/quartz dissolution; second hydrocarbon charging; a small amount of feldspar dissolution/quartz overgrowth/pyrite cementation, and finally compaction existing throughout the whole burial history. According to the matching relation of reservoir physical properties and hydrocarbon accumulation, both conventional reservoirs and tight reservoirs coexisted in the turbidites, and the tight reservoirs could be further divided into two types, one became tight reservoirs during hydrocarbon accumulation and the other became tight before hydrocarbon accumulation. It is concluded that the best exploration targets are the conventional reservoirs followed by reservoirs becoming tight during hydrocarbon accumulation, and then the reservoirs becoming tight before hydrocarbon accumulation. © 2017, Periodical Office of China University of Petroleum. All right reserved.
Analysis of well log data from Cenozoic basin fill of the Deer Lodge Valley, southwestern Montana, provides evidence for identifying paleosols and paleosol stacks in the subsurface. The paleosol stacks are continental sequence boundary markers and appear as several relatively thin, high-velocity/high-density zones within basin fill. Zone thickness ranges from 1 to 1.5 m; zones are stacked to thicknesses of up to 15 m. Density varies within the zones by as much as 0.6 g/cm3, and differs by as much as 0.9 g/cm3 from material immediately above these zones. Velocity differs by as much as 10 ft/ms from the overlying material and causes bright reflections on seismic sections. Synthetic seismograms are used to tie well log and seismic data. Basing our interpretation upon well log data and well cuttings analyses, we determined the high-velocity/high-density zones to be limestone. The pedogenic origin of the limestone is shown by (1) well cutting chips from the high-velocity/high-density zones that exhibit pedogenic features associated with calcic paleosols, (2) paleosol horizonation interpreted from well log analysis, (3) the absence of minerals normally associated with lacustrine deposits, and (4) comparison with surface paleosol exposures.
Dongying Depression is a typical petroliferous lacustric basin in Bohai Bay Basin and has entered a high-exploration stage. There are two hydrocarbon source rocks and two petroleum systems including Es3-Es2/Es3 (!) and Es4-Es4/Es2 (!) in the Tertiary of Dongying Depression. The vertical hydrocarbon migration is intensively active in this area because of faulting. The two petroleum systems in Dongying Depression share some geological elements for reservoir formation and have the characteristics of some complex petroleum system. Dongying Depression has multiple centers of hydrocarbon generation and expulsion. The petroleum generation and accumulation systems in every sub-depression are independent and can be divided into four sub-petroleum systems according to the hydrocarbon generation centers and distance of petroleum migration. Based on the division of petroleum system, eight hydrocarbon migration and accumulation units are divided in Dongying Depression. The petroliferous prospects of those sub-petroleum systems and hydrocarbon accumulation units have been evaluated.
Wireline log responses, 2D and 3D seismic data, petrographic and isotope results were used to compare major carbonate-cemented zones in Jurassic marine (Angel Field, Carnarvon basin) and fluvial sandstones (Gidgealpa Field, Eromanga basin), Australia. In both fields the carbonate-cemented zones concentrate near the crest of major structures, above areas where regional seals in older formations are breached. In the Angel Field, poikilotopic dolomite-cemented zones with a cumulative thickness of up to 165 m occur in the Upper Jurassic Angel Formation of submarine fan origin, both above and below the present-day gas-water contact. In this field the dolomite cement volume is of the order of 0.6 km3 to 1.4 km3, distributed over an area of 300 km2. At Gidgealpa, poikilotopic calcite-cemented zones with a cumulative thickness of up to 65 m concentrate in the lower portion of the fluvial Namur Sandstone, about 100 m below the present-day oil–water contact. In this field, the calcite-cemented zones extend over an area 7.5 km wide and 20 km long, with the total volume of calcite cement being 0.22–0.37 km3. Both geological areas are characterized by: (i) rapid initial burial; (ii) continuous subsidence; (iii) late-stage Tertiary compression, which triggered structural growth and closure development; (iv) coincidence of timing of peak hydrocarbon generation and migration with the Tertiary compression; and (v) the availability of an effective vertical plumbing system (locally breached regional seals in sequences underlying the carbonate-cemented reservoirs). These observations point towards a migration-related control on carbonate cementation broadly synchronous with hydrocarbon charging into structures. In the Eromanga basin, a statistical correlation exists between major calcite cement occurrence and oil pools in Jurassic reservoirs. The data suggest that seismic (predrill) identification of high-amplitude events related to major carbonate cementation can be useful for highgrading prospects and leads for drilling in clastic petroleum provinces that are characterized by a relatively late-stage compressive tectonic regime.
The present geopressure field in Niuzhuang sag in Dongying depression can be divided into dissimilar zones vertically and different areas horizontally. Overpressure is closely related to both the upper part of the fourth member and the lower part of the third member of Shahejie Formation whose source rocks matured. Large scale faults surrounding Niuzhuang sag limit the horizontal overpressure distribution. Inside the sag it is strongly overpressured while outside it is much less overpressured. The forward modeling indicates that the overpressure evolution in the sag is divided into three stages: original forming, meta declining and latest raising stages. And the main mechanism for generating overpressures evolved from undercompaction into combination of both undercompaction and hydrocarbon generation. Meanwhile, both lateral seal by large scale faults surrounding Niuzhuang sag and vertical seal by the thick and shale enriched bottom layer of the middle part of the third member of Shahejie Formation determined the special distribution of overpressure. The overpressure in Niuzhuang sag provided the petroleum migration with original forces in those two important accumulation periods. Coupling of overpressure evolution and tectonic activities played an active role in petroleum accumulation in the bordering faulted zone near Niuzhuang sag.
Carbonate cements in sandstones are dominated by calcite, dolomite, ankerite and siderite, whereas magnesite and rhodochrosite are rare. The distribution patterns, mineralogy and elemental/isotopic compositions of carbonate cements vary widely, both temporally and spatially. The most important factors controlling these parameters during near-surface eodiagenesis include the depositional setting (e.g. rate of deposition, pore water composition, hydrogeology, climate, latitude and sea-level fluctuation), the organic matter content and the texture and detrital composition of the host sediments. During burial (mesodiagenesis) the important controlling factors include the temperature, residence time, chemistry and flow rates/pattern of subsurface waters, and the distribution patterns of eogenetic carbonate cements. As a result of mass balance constraints, burial carbonates are thought to be formed by the dissolution—reprecipitation (i.e. redistribution) of eogenetic carbonate cements and detrital carbonates. However, cements may also be derived internally from the dissolution of carbonate bioclasts, volcaniclastic material and calcium plagioclase, or externally from associated carbonate rocks, evaporites and mudstones. During uplift and erosion, carbonate cements are subjected to telogenetic alteration and dissolution. The imprints of eogenetic, mesogenetic and telogenetic conditions might be unequivocally reflected in the mineralogy and geochemistry of carbonate cements. However, eogenetic carbonates, particularly calcite and dolomite, may be subjected to recrystallization and resetting of isotopic signatures, fluid inclusion thermometrics and elemental compositions.
Carbonate cement is an important authigenic mineral in sandstone reservoirs of the central uplift belt in the Dongying depression, but no in-depth study has been conducted on its formation mechanisms currently. In this article, carbonate cements of Shahejie Formation of the central uplift belt in the Dongying depression are divided into four periods using petrologic records and their development characteristics are concluded. And then, according to the paragenetic mineral assemblages, carbon and oxygen isotope ratios, chemical elemental composition and other information of carbonate cements, their formation mechanisms were studied. Study results show that carbonates in the study area are mainly affected by the organic acids generated during the process of maturation of organic matter and the dissolution of the carbonate debris during deposition of Es4. In the study area, four periods of carbonate cements developed mostly. The first period of carbonate cements is mainly dolomite. Its formation is related to decomposition effects of methane bacteria on organic matter; the second period of carbonate cements is mainly formed of calcite and there is a layer of chlorite film locating between the second period and the first period of carbonate cements. The formation of cements is related to Ca and HCO3- oversaturation caused by pore fluid condensation; the third period of carbonate cements mainly include calcite, dolomite and ankerite, being characterized by filling dissolution pore of feldspar and primary pore, with their substance derived from the dissolution pore of feldspar and dehydration of mudstone; the fourth period of carbonate cements is characterized by filling the early-stage carbonate-corroded pores. Their substances are derived from the conversion of clay minerals and are usually symbiotic with pyrite particles, and their formation is affected by the hydrocarbon fluid flow.
The Flemish Pass Basin is a deep-water basin located offshore on the continental passive margin of the Grand Banks, eastern Newfoundland, which is currently a hydrocarbon exploration target. The current study investigates the petrographic characteristics and origin of carbonate cements in the Ti-3 Member, a primary clastic reservoir interval of the Bodhrán Formation (Upper Jurassic) in the Flemish Pass Basin.The Ti-3 sandstones with average Q86.0F3.1R10.9 contain various diagenetic minerals, including calcite, pyrite, quartz overgrowth, dolomite and siderite. Based on the volume of calcite cement, the investigated sandstones can be classified into (1) calcite-cemented intervals (>20% calcite), and (2) poorly calcite-cemented intervals (porous). Petrographic analysis shows that the dominant cement is intergranular poikilotopic (300-500 μm) calcite, which stared to form extensively at early diagenesis. The precipitation of calcite occured after feldspar leaching and was followed by corrosion of quartz grains. Intergranular calcite cement hosts all-liquid inclusions mainly in the crystal core, but rare primary two-phase (liquid and vapor) fluid inclusions in the rims ((with mean homogenization temperature (Th) of 70.2 ± 4.9 °C and salinity estimates of 8.8 ± 1.2 eq. wt.% NaCl). The mean δ18O and δ13C isotopic compositions of the intergranular calcite are -8.3 ± 1.2‰, VPDB and -3.0 ± 1.3‰, VPDB, respectively; whereas, fracture-filling calcite has more depleted δ18O but similar δ13C values. The shale normalized rare earth element (REESN) patterns of calcite are generally parallel and exhibit slightly negative Ce anomalies and positive Eu anomalies. Fluid-inclusion gas ratios (CO2/CH4 and N2/Ar) of calcite cement further confirms that diagenetic fluids originated from modified seawater. Combined evidence from petrographic, microthermometric and geochemical analyses suggest that (1) the intergranular calcite cement precipitated from diagenetic fluids of mixed marine and meteoric (riverine) waters in suboxic conditions; (2)the cement was sourced from the oxidation of organic matters and the dissolution of biogenic marine carbonates within sandstone beds or adjacent silty mudstones; and (3) the late phases of the intergranular and fracture-filling calcite cements were deposited from hot circulated basinal fluids.Calcite cementation acts as a main controlling factor on the reservoir quality in the Flemish Pass reservoir sandstones. Over 75% of initial porosity was lost due to the early calcite cementation. The development of secondary porosity (mostly enlarged, moldic pores) and throats by later calcite dissolution due to maturation of organic matters (e.g., hydrocarbon and coals), was the key process in improving the reservoir quality.
Most sedimentary basins contain close to 20% by volume pore water; much of which is of high ionic strength and is classified as brine. Although considerable variation occurs in the composition of basinal waters, some correlation exists between major ion concentrations and total dissolved solids. It is consequently possible to construct geochemical models for basinal waters that have general utility for investigating a variety of subsurface processes.
In this paper, we examine the potential role of brine composition and movement on carbonate mineral mass transport in sedimentary basins up to pressure and temperature conditions of 300 bars and 100°C. Primary emphasis is placed on the impact of vertical and lateral migration of brines and the dispersive mixing of brines with waters containing widely varying concentrations of total dissolved solids. Model results indicate that mixing of subsurface waters may produce fluids which are highly supersaturated and at times slightly undersaturated with respect to calcite. This process may represent a major mechanism for production and destruction of carbonate cements in sediments. It may also offer an explanation as to how basinal scale mass transfer of carbonates can occur in waters that are close to equilibrium with respect to major sedimentary carbonate minerals.
Calcite cement derived intraformationally in seven stratigraphic units of marine origin is distributed heterogeneously at the outcrop scale. Sandstone beds intercalated with calcareous shale older than Pliocene tend to be completely cemented, whereas stacked sandstone beds that lack shale interbeds have calcite cement in the form of tightly cemented concretions that make up only 10-30% of a bed. Patterns of concretions within beds are remarkably varied and include both random and uniform spacing. There is no preference of concretions for shell-rich layers. The lack of strong textural or compositional controls on the localization of calcite cement suggests the preeminence of highly localized hydrologic factors in determining the spatial distribution of authigenic pore-filling calcite. Faults apparently served as fluid conduits and were selectively cemented. In general, only sandstones intercalated with shale are totally cemented. -from Authors
The burial depth of the overpressured top boundary ranges from 2200~2800 m in the Dongying Depression. Interacted by overpressured fluids that flow beneath, the overpressured top boundary is generally characterized by well-developed carbonate mineralization, commonly with carbonate content between 15%~40%. The statistical analysis on in-situ electron-microprobe compositional data of 101 carbonate cements from 53 sandstone samples indicated that carbonate minerals could be basically subdivided into three groups, i.e. penecontemporaneous dolomite, calcite and ankerite. Integrated with X-ray diffraction and cathodoluminescence data, their diagenetic sequence was determined as penecontemporaneous dolomite→calcite→ankerite. On the basis of observation of primary inclusions in carbonate cements, the precipitation of carbonate minerals in the overpressured top seal and adjacent sandstones was accompanied with overpressured fluids, with a minimum paleopressure coefficient ranging between 1.29~1.62 and a precipitation temperature obviously higher than the background temperature, which suggested a significant influence by overpressured thermal fluid invasion. Thus, based on previous study results, we proposed that the precipitation of calcite and ankerite cements might be related to activities of overpressured fluids since the deposition at the terminal Dongying stage and the Minghuazhen stage, respectively. Furthermore, this hypothesis was tested by measured oxygen isotopic values that fluctuate from -16.86‰ to -12.29‰ PDB for calcite and from -12.20‰ to -10.20‰ PDB for ankerite. Further investigations suggested that the calcite should precipitate at 90~120°C for the δ 18O SMOW of its homochronous overpressured fluid is around 0.00‰ and the ankerite should precipitate at 110~135°C for the δ 18O SMOW of its homochronous overpressures fluid is 0.25‰. δ 13C values of carbonate cements formed at the late diagenesis show a positive shift, ranging from -0.9‰ to +3.58‰, which indicates an origin mainly from the dissolution-reprecipitation process of the Es 4 lacustrine carbonate, or an influence of intramolecular carbon isotopic fractionation within carboxylic acids, though the latter seems to play a relatively minor role.
Within very localised areas of the Vulcan Subbasin, the Eocene Grebe Formation sandstones are strongly cemented with carbonate. These cemented sands are recognisable on seismic data as zones of anomalously high velocity, and result in both time 'pull-up' and deterioration of the stack response in the underlying section.To determine the nature and origin of these cemented zones, their isotopic, mineralogical and petrologic compositions have been characterised, their seismic response and areal distribution established, and these observations integrated with ~2,730 km of AGSO water column geochemical ('sniffer-type') data.The carbon isotopic compositions of the carbonate within the cemented Grebe sands are diagnostic of carbonates formed principally via the oxidation of migrating, thermogenic hydrocarbons. Oxidation of the hydrocarbons took place in two stages: an earlier phase led to calcite precipitation, whereas a later phase produced (generally subsidiary) ferroan dolomite/ankerite cementation.Areas of known, present-day hydrocarbon seepage from the seafloor, such as over major faults on the Skua Horst and along the Vulcan Sub-basin/ Londonderry High boundary zone, are invariably associated with zones of highly cemented Eocene sands. Similarly, areas of known Tertiary hydrocarbon seepage, such as those associated with the residual oil columns on the Eider Horst, also contain strongly cemented Eocene sandstones.These observations have established a causal relationship between the presence of these Hydrocarbon-Related Diagenetic Zones (or HRDZs) in the Eocene sandstones and Tertiary-Quaternary hydrocarbon seepage. It is likely that most of the cementation occurred during the Late Miocene/Early Pliocene, when the Grebe Formation sands were at a shallow depth of burial(Recognition of this causal association has allowed several insights to be gained into the exploration potential and reactivation history of structures within the Vulcan Sub-basin. Mapping of the areal distribution of the cemented zones can effectively define hydrocarbon migration pathways. More importantly, however, predictable relationships exist between the seismic expression of the HRDZs, the total amount of hydrocarbons that have leaked from the traps, and the obliquity between the Jurassic and Late Miocene fault trends over the respective structures. A continuum exists between highintegrity accumulations, in which the fault trends are parallel and the HRDZs are small or absent, and breached accumulations, in which a significant obliquity exists between the respective fault trends and the HRDZs are large and seismically-intense.These observations provide a potential predictive tool for evaluating undrilled structures. It may be possible to determine, from the integration of seismic structural mapping and the characterisation of the seismic expression of the HRDZs, not only whether an individual structure is ever likely to have had a hydrocarbon column, but whether that column is likely to be preserved.
Carbonate cements in the fluvio-lacustrine reservoir sandstones of the Albert Formation mainly occur in three phases: pre-compaction calcite, post-compaction calcite and ankerite. The pre-compaction calcite cement (25% to 40%) is characterised by bright luminescence, high concentrations of Fe, Mn and Sr (means of 1.56 wt.%, 1.42 wt.% and 0.25 wt.%, respectively), slightly heavy δ13C and δ18O values (mean = - 1.79‰ PDB, mean = - 14.99‰ PDB, respectively), and its selective occurrence in lacustrine sandstones suggestive of cementation at shallow depths from lacustrine connate water. Dull-luminescence, variable concentrations of Fe, Mn and Sr, variable but slightly heavy δ13C and δ18O values (mean of - 2.97‰ PDB, and - 11.05‰ PDB, respectively) of the post-compaction calcite, as well as its occurrence in alluvial fan sandstones exhibiting high pressure solution of detrital grains, suggest cementation from pore waters modified by water-rock interactions. We suggest that the ankerite cement was formed from upwardly migrating formation water based on the following evidence: (i) ankerite cement filling the secondary pores created in the deep subsurface, (ii) an absence of unconformity during ankerite cementation precluding involvement of meteoric water, (iii) ankerite cementation in brecciated veins synchronous with ankerite precipitation in host sandstones, (iv) occurrences of significant volumes of ankerite (20% to 30%) in the shales indicating that the Fe and Mg ions expelled during smectite to illite transformation reactions were consumed within the shales, and (v) the heavy δ13C and δ18O isotope values of the ankerite cements (means of +2.20‰ PDB and - 11.05‰ PDB, respectively).
Stratigraphic sequences and architectural variability in the Late Eocene lacustrine strata of the Dongying Depression, eastern China, were investigated using the interpretation of 2-D and 3-D high-resolution seismic profiles, analysis of spontaneous potential and resistivity curves, and observation of drill cores. Four third-order sequences controlled by syndepositional faults or fault slope break zones were identified, based on the characteristics of sequence boundaries and sedimentary successions. The architecture of the sequences in the different structural belts of the depression is complicated by the relationship between the rate at which fault-controlled accommodation was created and the rate of sediment supply. At fault margins, the rate of sediment supply exceeded accommodation space. Here, lowstand systems tracts consist of lowstand fan deltas with small progradational to retrogradation stacking patterns controlled by steeply dipping, parallel and cross-shaped syndepositional faults or fault slope-break zones; transgressive systems tracts consist of fan deltas with retrogradational to aggradational stacking patterns; and highstand systems tracts consist of fan deltas with normal regressive or progradational stacking pattern. At hinged margins, the rate of sediment supply was equal to or exceeded accommodation controlled by faults. Lowstand systems tracts at hinged margins consist of incised channel fills deposited on the landward side of gently dipping parallel and broom-shaped syndepositional faults or fault slope break zones and lowstand fans or sublacustrine fans deposited on the shores of lakes. Transgressive systems tracts consist of delta systems and shore to shallow-lake subfacies with retrogradational stacking patterns. Highstand systems tracts consist of braided deltas and fluvial delta systems with progradational or normal regressive and aggradational stacking patterns. Along the axis, the rate of sediment supply far exceeded accommodation. Only the lowstand systems tracts developed, consisting of lowstand deltas deposited on the landward side of the syndepositional faults or fault slope break zones, and lowstand fans or sublacustrine fans deposited on the lakeward side of the zones. Here, transgressive systems tracts consist of thin, deep lacustrine deposits and fluvial delta systems with retrogradational or transgressive stacking patterns; and highstand systems tracts consist of thick fluvial delta systems with a progradational configuration or normal regressive stacking patterns.
The four kinds of syndepositional fault slope-break zones controlled the stratal architecture of sequences and the distribution of lowstand systems tracts. Sand bodies within lowstand systems tracts provide suitable conditions for the formation of hydrocarbon reservoirs when they are overlain by sediments from transgressive systems tracts, and are therefore favorable sites for lithostratigraphic trap exploration.
Up to four calcite-cemented horizons (doggers) form impermeable barriers to fluid flow within the Middle Jurassic Rannoch Formation and are correlatable across the Murchison Field. Calcite precipitated during early diagenesis, within high porosity/permeability sandstones at the top of coarsening (shoaling) upward shoreface cycles. Calcite δ¹³C and δ¹⁸O compositions range from -4.1 to -13.4‰ PDB, and -6.6 to -16.7‰ PDB, respectively. Sr concentrations of up to 1334 ppm are consistent with marine carbonate sources (probably shell fragments), but no viable intraformational carbonate source has been identified in the Murchison Field area. Initial ⁸⁷Sr/⁸⁶Sr compositions (0.71109–0.71266) are higher than Middle Jurassic seawater (0.7073), and consistent with precipitation from modified porewaters containing significant proportions of continentally derived “meteoric” fluids enriched in ⁸⁷Sr as a result of basement weathering, or percolation through hinterland soils/unconsolidated detritus. An internal source of ⁸⁷Sr is not considered viable in view of the high proportion (up to 25‰ clastic constituents) of unaltered detrital alkali feldspar and mica within the Rannoch Formation.
The Murta Member within the Murteree Horst area consists of a very thin, high permeability sand (HPS) within predominantly low permeability, thinly laminated, carbonaceous siltstones collectively referred to as the low permeability region (LPR). This HPS has a thickness of about 1 foot and a large lateral extent of about 10,000 acres.
The HPS and LPR together have been postulated to contain an OOIP of up to 74 million bbls in the Murteree Horst area, however the LPR also acts as a capillary seal to the underlying Mckinlay reservoir. The production behaviour of the Murta reservoirs has remained enigmatic since their discovery.
Under production the HPS initially shows a rapid pressure drop indicative of oil expansion drive in a closed or almost closed system. However eventually a water drive develops which behaves as though there is a very large or possibly infinite aquifer.
Although most of the postulated 74 MMB of oil was thought to be in the LPR it appeared none of the LPR oil was being produced. It was not clear whether this oil could be mobilised. This investigation suggests that oil contained within the LPR is largely immobile and may even be representative of oil migration conduits only.
1. (a) Introduction
The Murta Member oil pools occur in four fields Alwyn, Jena, Ulandi and Limestone Creek/Biala on the Murteree Horst in the Nappacoongee-Murteree Block about 50km south of Moomba (Figure 1) in central Australia.
The Murteree Horst is part of the Nappacoongee Murteree Trend, a northeast-southwest aligned high which separates the Cooper Basin Nappamerri and Tennaperra Troughs to the northwest and southeast respectively. Cooper Basin sediments are absent on the horst with the Eromanga Basin sequence occurring directly on pre-Permian basement.
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This work presents the longest, consistent records of dissolved organic carbon in rivers ever published. Long-term records of carbon flux (as water colour) were constructed for 3 catchments in Northern England for as far back as 1962. Observations show that there have been large increases in DOC concentrations over the period of study with in one case a doubling of the concentration over a period of 29 years. However, in one of the catchments no significant change was observed over a 31-year period. All catch- ments show common inter-annual control on carbon release in response to droughts, but no step increases in DOC concentrations were observed in response to such per- turbations with pre-drought levels being restored within a period 3-4 years. Observed increasing trends do not correlate with changes in river discharge, pH, alkalinity or rainfall, but do coincide with increasing average summer temperatures in the region. The times series of DOC concentration over the period of the record appears station- ary, but the distribution of daily values suggests a change in sources of colour over the increasing trend. The evidence supports a view that increases in carbon release are in equilibrium with temperature increases accentuated by land-use factors. Long-term trends do not support an enzymatic latch mechanism.
Integrated petrographic and burial-history studies of Fall River sandstones from outcrop and the subsurface provide insight into the timing of compaction and quartz cementation, the two main porosity-reducing processes in quartzose sandstones. Petrographic study of 95 thin sections of Fall River fluvial valley-fill sandstones from outcrop, Donkey Creek field at 2 km burial depth, and Buck Draw field at 3.8 km indicates that reservoir quality differs significantly in these three areas. Fall River sandstones at the surface contain an average of 31% intergranular volume (IGV) and 2% quartz cement. In both Donkey Creek and Buck Draw fields, the sandstones average 22% IGV, but quartz-cement volume averages 8% in the shallower field and 12% in the deeper. Geometric mean permeability at the surface is 4,700 md, compared with 42 md at 2 km and 2 md at 3.8 km. Burial history of the Fall River sandstone differs greatly in the three areas. The outcropping sandstones were buried to 2 km and had reached 80 C by the end of the Cretaceous. They were then uplifted and have remained at near-surface temperatures since the Paleocene; the calculated time-temperature index (TTI) of these sandstones is 1. Fall River sandstones at Donkey Creek were also buried to 2 km and had reached 80 C by the end of the Cretaceous but remained at that depth during the Tertiary; TTI is 14. In Buck Draw field, Fall River sandstones were buried to 2.5 km during the Cretaceous and then continued to subside during the Tertiary, reaching depths of 4 km and temperatures of 140 C; TTI is 512.
Carbonate minerals are stained over a set period of time with alizarin red-S and potassium ferricyanide only if they will react with dilute hydrochloric acid solution, with which the stain is prepared. The rates of solution of carbonates in the acid control the intensity of color development. For calcite, the rate of solution varies with the optic orientation of the section. The speed of carbonate solution is changed if the acid concentration is altered, but only at concentrations of about 0.1 N is the optic orientation of calcite differentiated by the stain. Etching reduces thin section thickness and clarifies rock texture. Staining with alizarin red-S differentiates carbonate minerals into two groups. Aragonite, calcite, witherite, and cerussite, which dissolve rapidly in dilute hydrochloric acid, are stained, while dolomite, siderite, magnesite, and rhodochosite, which react much more slowly with the acid, remain unstained. The distribution of ferrous iron, as distinguished by staining with potassium ferricyanide, has proved to be highly significant in the genesis of cements. Ferrous iron can be introduced at any one stage in cementation, or repeatedly, forming zoned patterns. The paragenesis of zoned ferroan cements can be reconstructed after staining. Solution of the more soluble original constituents can sometimes be dated in relation to cementation. Ferroan calcite can be secondary in origin and is usually associated with replacement minerals.
Calcite cement is the dominant control on reservoir quality in turbidite sandstones of the Upper Permian Bell Canyon Formation, Delaware Basin. These well-sorted, very fine-grained arkoses were deposited in a basin-floor setting by channel-levee systems terminating in broad lobes. Calcite cement distribution in the East Ford and Geraldine Ford fields was mapped using core, log, and thin-section data. Calcite is concentrated in tightly cemented zones that are mostly less than 1 ft (0.3 m) thick. Areas that have high percentages of calcite-cemented sandstone (> 20%) occur along the margins of the sandstone in overbank and lobe deposits, where sandstone pinches bodies, out into siltstone. Areas that have the lowest percentage of calcite-cemented sandstone (< 10%) occur where the sandstone is thickest, in the channel facies. Isotopic composition of the calcite (delta C-13=- 1.8 to -3.0 parts per thousand [relative to the Peedee belemnite, PDB] delta O-18 = -4.6 to - 6.3 parts per thousand [PDB]) is consistent with the source of calcium carbonate being from dissolution of detrital carbonate rock fragments and marine skeletal debris. Because internal sources of calcite were apparently insufficient to account for the cement volume, cement components are interpreted as having been transported into the sandstones from organic-rich basinal silt-stones and limestones. Feldspars buffered acidic formation waters near where they entered the sandstone, resulting in calcite concentrated near the sandstone margins. The calcite formed near maximum burial depths of 4 800 ft (1. 5 km) andtemperatures of 104 degrees F (40 degrees C) from marine pore waters with delta O-18 of approximately 0 parts per thousand (relative to standard mean ocean water). Most of the calcite-cemented zones are interpreted as being concretions that extend no more than a few meters laterally. Production data and geophysical log correlations suggest that some cemented zones are laterally continuous at least 1000 ft (300 m) and cause vertical reservoir compartmentalization. Laterally extensive calcite layers may be associated with the base of turbidite deposits.
This study of the geochemistry of formation waters in the Dongying Depression of the Bohaiwan basin, East China, unravels a close relationship between vertical zonality and overpressured fluid flow. The zones have distinctive original pore waters because the sediments formed under different climate conditions. The highest value of the total dissolved solid in the formation water occurs in the ES4 member of Shahejie Formation due to the dissolution of halite and gypsum beds. In the overlying ES3 and ES2 members, low salinity water occurs, except for near syndepositional fault zones where a significant increase in salinity of formation water and a reduction in overpressure were observed. This fact indicates strong overpressured fluid flow where thermal anomalies have been observed, as evidenced from thermal gradient and fluid inclusion homogenization temperatures. The results indicate that the geochemistry of pore water may be used to help identify the main migration pathways for overpressured fluid flow with hydrocarbons.
Pressure seals are economically significant geological phenomena since they play an important role in deep natural gas entrapment. Identified in basins worldwide, they offer a new frontier for exploration of natural gas reservoirs below 10,000 ft. (3000 m). Pressure seals are low-permeability envelopes which enclose abnormally pressured reservoirs. There are three different types of seals-basal, lateral and top. Basal seals, which define the bottom of abnormal pressure compartments, usually follow a stratigraphic horizon. Lateral seals are generally associated with faults. Top planar seals may parallel or cut across time-stratigraphic boundaries; they may be developed in any lithology.The southeastern portion of the Anadarko Basin displays a layered sequence of abnormally pressured fluid compartments at depths ranging from 3,000 to 13,500 ft. (900 to 4150 m). These compartments are separated from each other by pressure seals. In McClain County, a top pressure seal separating two abnormally pressured compartments is located between 10,000 ft. (3000 m) to 12,000 ft. (3650 m) within the Simpson Group (Ordovician). The seal zone is characterized by alternating cemented and porous intervals. Silica and carbonate cements comprise a diagenetic laminated and banded pattern. The compositional and textural heterogeneity within this structure evolved through diagenetic processes active during the seal evolution.
Calcite cement is one of the most volumetrically important diagenetic minerals formed during burial of Frio Formation sandstones from the Corpus Christi area of Texas. Syndepositional calcite is restricted to shore-zone sandstones, whereas later, post-quartz-over-growth, burial-diagenetic calcite is present in both shore-zone and shelf sandstones. The δ¹â¸O of burial-diagenetic calcite becomes depleted with depth. Chemical and textural evidence favors partial dissolution and reprecipitation (recrystallization) with consequent isotopic resetting as being responsible for the change in calcite δ¹â¸O with progressive burial. The change in calcite â¸â·Sr/â¸â¶Sr with depth also supports progressive recrystallization of calcite. There is a strong relationship between the â¸â·Sr/â¸â¶Sr and trace element composition of calcite and formation water from individual growth-fault blocks. Significant differences in strontium isotopic and trace element composition exist between adjacent fault blocks, implying that each fault block has behaved as a chemically separate system since the time of calcite precipitation.
Diagenetic trends near sandstone/shale contacts were studied in 12 cored sequences from four wells between depths of 5200 and 15,700 ft (1585 and 4785 m) to evaluate the heterogeneity of diagenetic processes on a local scale and to evaluate the hypothesis that reactive aqueous fluids and components for cements in sandstones were derived from adjacent shales. The only evidence suggesting that diagenetic components in sandstones were derived from immediately adjacent shales is an increase in chlorite cement in sandstones toward contacts with shale beds for two of three contacts appropriate for study. Secondary pores and cements of quartz, carbonate, and kaolinite do not correlate with proximity to shale beds, but have a preference for sandstones that had relatively high initial porosities and permeabilities. Thus, the flux of formation water and probably long-distance transportation of diagenetic components were more important influences on reservoir quality of sandstones than was the local availability of components. Multiple regression of 22 independent variables indicates that the best predictors of secondary porosity are kaolinite cement and intergranular porosity. Sandstone sequences are extremely heterogeneous in the distribution of total thin section porosity, secondary porosity, and quartz and carbonate cements; in addition, they have significant variations in the abundance of kaolinite and chlorite cements. Mass balance calculations for silica and aluminum indicate silica was imported to and aluminum was exported from the sandstones.
The pore waters in sedimentary basins are ultimately derived from sea water, meteoric water, mineral bound water, or from the underlying basement. Geochemically these waters initially have different signatures; however, the pore water compositions are easily altered by reactions with minerals or by mixing with other fluids.Meteoric water is driven downwards into sedimentary basins by the gravitational potential, which approximately corresponds to the elevation of the ground water table above the sea or lake level. It is difficult to model such flow, however, because the permeability on a large scale is almost impossible to represent realistically. The continuity of confined sandstone and limestone aquifers plays an important role in determining the flux of meteoric water received by rocks in different parts of the basin, but this is again difficult to predict.Pore water flow driven by compaction typically has velocities several orders of magnitude lower than what is commonly found in meteoric water flow regimes. The average rate of upwards flow is lower than the rate of subsidence and the difference is the rate of incorporation of sea water in the topmost layer at the sea floor.The salinity of formation water provides important constraints on fluid flow in sedimentary basins. In the North Sea Basin and the Gulf Coast Basin, these types of data indicate that vertical mixing by compaction-driven flow and convection is limited.Compaction-driven flow obeys Darcy's Law but there are complex interdependencies between pressure, permeability and compaction. Given the low compressibility of water, the flow which can result from pressure release alone is very limited, except on a local scale along permeable faults and fractures. The main flow of pore water during burial is driven by compaction, which is a slow process, even when overpressure is released abruptly with a resulting increase in the net effective stress. In the case of high permeability faults, the flow into the fault from low permeability sedimentary rocks at depth may be rate-limiting. The displacement of pore water in adjacent sediments near the top of the faults may also slow down fluid flow on fractures that do not reach the sea floor. Modelling of fluid flow in sedimentary basins should be based on permeability distributions which are mostly determined by primary facies, diagenesis and the tectonic history of the basin.Extensional tectonics produce a predominantly vertical fracture network which may serve as conduits for pore water flowing deep into the crust. Upward flow of hot fluids in basement fractures must have a high velocity in order to produce large thermal anomalies. Concentrations of dissolved components precipitate at the surface and form deposits of quartz and ore minerals. If basement fractures are overlain by a thick sequence of soft sediments, the rate of flow on the fractures will be reduced because of the low permeability in the sedimentary cover, and conductive heat transport will dominate. Flow of water at high enough velocities to bring hot water to the surface and produce hot springs is most likely to occur in fractured basements rocks or well-cemented sedimentary rocks. Concentrated precipitation of dissolved components will occur near the surface, where the rate of cooling is highest. Also, sediment-hosted ores precipitated from flow of hot water on fractures are therefore likely to form before accumulation of thick overlying sedimentary sequences.
There are numerous suggestions that mass transfer between sandstone and shale is an important factor in the diagenesis of sandstone reservoirs. However, advective transport models for mass transport have not produced results that support these proposals. A model is developed that overcomes the problems of advective transport models by assuming that diffusive mass transfer is the principle transport mechanism. The process occurs when the rate of dissolution of reactive detrital components exceeds the local rate of diffusive transport. At that point in the burial history, units with greater amounts of a reactive component will generate more solute per unit time than units with less reactive component. The process forms chemical gradients that can rapidly transfer mass equivalent to several vol% of rock over distances of at least 5 m. The observed transport of potassium and silica between adjacent sandstone and shale in the North Sea during late-stage illitization was correctly predicted using this conceptual approach. The conceptual framework from the illite model can be extended to other diagenetic reactions, and suggests that the burial of adjacent sandstone–shale units could produce episodic diffusion gradients that transfer mass and form multiple generations of cement in sandstone reservoirs.
Bands of calcite and dolomite cements alternating with zones of nearly carbonate-free sand occur in the Stevens sandston aat North Coles Levee, San Joaquin, Valley, California. Temperatures calculated from O isotopes suggest that the calcite cement bands were emplaced episodically as a result of repeated injections of hot water from deeper in the section. Burial analysis suggests that these cements precipitated from 7 Ma to the present over the temperature range of 45 to ∼95°C.Carbon isotope data suggest that the C in the cements is a mixture derived from two sources, detrital shell material (δ13C(PDB)≈) and CO2 liberated from maturing kerogen (δ13C ≈ −24). Plots of δ13C vs time and depth of crystallization show that the cementation sequence was: (1) dolomite cements, possibly concretionary, precipitated at depths <1–2 km and at temperatures <45°C; (2) calcite cements with δ13C(PDB) values as low as −13, crystallized from depths between 1220 and 1820 m (4000 and 6000 ft) and at temperatures between 45 and 80°C; (3) calcite cements with δ13C(PDB) values approaching zero and calculated temperatures of crystallization up to the present reservoir temperature of 95±3°C.A log of δ13C vs calculated depth of crystallization correlates with the stratigraphic column at North Coles Levee. If the correlation is valid the light δ13 in each cement sample can be tied to its source. A model based on this interpretation suggests that the early, light C was derived from maturing kerogen in the Kreyenhagen Formation (Eocene) as it passed through the oil window between 4 and 5 Ma. The subsequent passage of younger sediments with less organic material produced correspondingly smaller amounts of light CO2 which was reflected in the relatively heavier C isotopes in the later cements.It is suggested that the epidsodic injections of hot water carried dissolved gases and minerals, principally calcite, upward from rocks as deep as 2–3 km below the Stevens sandstone and reprecipitated the calcite in more permeable zones in the rock. Degassing of CO2 from rising pore waters likely triggered the precipitation and accounts for the relatively large volumes of cement. The Sibson model for seismic pumping of pore fluids is considered a likely explanation for the observed cementation.
Geometrical arrangement of calcite cementation within shallow marine sandstones
- Bjørkum
Migration models of hydrocarbon fluids in the Dongying Depression: evidence from boiling fluid inclusions
- Qiu