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Evolution of fracture and fault-controlled fluid pathways in carbonates of the Albanides fold-thrust belt
Abstract
The process of fracture and fault formation in carbonates of the Albanides fold-thrust belt has been systematically documented using hierarchical development of structural elements from hand sample, outcrop, and geologic-map scales. The function of fractures and faults in fluid migration was elucidated using calcite cement and bitumen in these structures as a paleoflow indicator. Two prefolding pressure-solution and vein assemblages were identified: an overburden assemblage and a remote tectonic stress assemblage. Sheared layer-parallel pressure-solution surfaces of the overburden assemblage define mechanical layers. Shearing of mechanical layers associated with folding resulted in the formation of a series of folding assemblage fractures at different orientations, depending on the slip direction of individual mechanical layers. Prefolding- and folding-related fracture assemblages together formed fragmentation zones in mechanical layers and are the sites of incipient fault localization. Further deformation along these sites was accommodated by rotation and translation of fragmented rock, which formed breccia and facilitated fault offset across multiple mechanical layers. Strike-slip faults formed by this process are organized in two sets in an apparent conjugate pattern. Calcite cement and bitumen that accumulated along fractures and faults are evidence of localized fluid flow along fault zones. By systematic identification of fractures and faults, their evolution, and their fluid and bitumen contents, along with subsurface core and well-log data, we identify northeast-southwest-trending strike-slip faults and the associated structures as dominant fluid pathways in the Albanides fold-thrust belt. Copyright © 2006. The American Association of Petroleum Geologists. All rights reserved.
... In fact, although the External Hellenides gained a renewed interest for hydrocarbon exploration in the last two decades (Grigoriou et al., 2019;Karakitsios & Rigakis, 2007), with several plays discovered, no studies on fault-related fluid circulation have been performed so far. In contrast, many studies regarding fault-related fluid circulation were performed in the nearby Albanides fold-and-thrust belt (De Graaf et al., 2019;Graham Wall et al., 2006;Roure et al., 2010;Swennen et al., 2000;Van Geet et al., 2002;Vilasi et al., 2006Vilasi et al., , 2009, where important hydrocarbon reservoirs were discovered (Bega & Soto, 2017;. ...
... We combine the geological and geochemical data to propose a spatio-temporal tectonic model of regional-scale fluid circulation in the Ionian zone of the External Hellenides fold-and-thrust belt, Greece. Since previous studies on paleo fluid flow in this belt are not available, we compare our results with those obtained for the Albanides (De Graaf et al., 2019;Graham Wall et al., 2006;Swennen et al., 2000;Van Geet et al., 2002;Vilasi et al., 2006Vilasi et al., , 2009, and others for some fold-and-thrust belts worldwide: ...
... 1. Bed-perpendicular, both hairline and non stratabound calcite veins occur in two orthogonal sets striking respectively WNW-NW and NNE-NE (Figure 4). These strike directions are similar to those documented for the veins and joints in the Ionian zone of the external Hellenides fold-and-thrust belt, a few tens of km northwest of the Araxos area (Tavani et al., 2019), and in the Albanides fold-and-thrust belt, a few hundreds of km northwest of the Araxos area (De Graaf et al., 2019;Graham Wall et al., 2006;Lacombe et al., 2009). These authors attributed the formation of bed-perpendicular veins/joints and bed-parallel stylolites to the overburden stress during burial and/or in response to the foreland flexuring of the lithosphere, which occurred since late Eocene-early Oligocene (Figure 11a) (Sotiropoulos et al., 2008), ahead of the advancing thrust sheets, as observed in other fold and thrust belts worldwide (e.g., Ahmadhadi et al., 2008;Beaudoin et al., 2012;Branellec et al., 2015;Carminati et al., 2014;Hernández & Franzese, 2017;La Bruna et al., 2018;Tavani et al., 2018). ...
We combined field mapping, structural and microstructural analyses, stable‐clumped isotope geochemistry, and U‐Pb dating of calcite veins and syn‐tectonic slickenfibres, to assess the regional scale fault‐related fluid flow during the evolution of the External Hellenides fold‐and‐thrust belt. We show that fluid circulation during forebulge uplift was characterized by cold meteoric water‐derived fluids, from which calcite precipitated and sealed bed‐perpendicular joints. Fluid circulation during foreland flexuring and early layer‐parallel shortening was characterized by warm fluids buffered by the carbonate host rock, which circulated through normal faults and bed‐parallel veins. Mixing with meteoric‐derived fluids also occurred at this stage of tectonic evolution. Fluid circulation during the late stage of thrust wedge accretion and post‐orogenic extension at 1.6 ± 1 Ma was characterized by increasing dominance of cold meteoric water circulating in strike‐slip and normal faults. The ingress of meteoric‐derived fluids was controlled by throughgoing fault conduits, while host rock‐buffered fluids were confined in isolated structures such as minor faults and veins. We developed a conceptual model of fault‐related fluid circulation, which invokes a transition from an open fluid system during forebulge uplift, to a semi‐closed fluid system during foreland flexuring and early layer‐parallel shortening, and to an open system during late thrust wedge accretion and post‐orogenic extension. This type of fluid circulation may have impacted fluid migration/leakage, including hydrocarbons, into or outside potential reservoirs in the highly prospective Hellenides‐Albanides fold‐and‐thrust belt, a renovated frontier for hydrocarbon exploration in the Mediterranean area.
... The carbonate rocks are mainly composed of calciclastic gravity flows and mass transport deposits (Rubert et al., 2012;Le Goff et al., 2015). Events of fracturing provided discrete pathways facilitating the episodic migration of fluids and governing the emplacement of abundant hydrocarbon accumulations in the area (Graham Wall et al., 2006). Previous veinbased fluid-flow reconstructions in the Ionian zone focused on the areas of Kremenara (Van Geet et al., 2002;Swennen et al., 2003), Shpiragu (Graham Wall et al., 2006), Kelçyrë , and Saranda . ...
... Events of fracturing provided discrete pathways facilitating the episodic migration of fluids and governing the emplacement of abundant hydrocarbon accumulations in the area (Graham Wall et al., 2006). Previous veinbased fluid-flow reconstructions in the Ionian zone focused on the areas of Kremenara (Van Geet et al., 2002;Swennen et al., 2003), Shpiragu (Graham Wall et al., 2006), Kelçyrë , and Saranda . These previous studies inferred multiple fluid-flow episodes throughout the evolution of the foreland fold-thrust belt. ...
... Red lines depict major thrust faults and decollements. circulation of fluids in the Cretaceous-Eocene carbonates (Graham Wall et al., 2006). Episodes of fluid flow occurred before and during the main phase of folding and thrusting (Swennen et al., 2000). ...
... Due to their impact on the evolution of reservoirs, the links between fracture development and fluid circulation in foreland fold and thrust belts has been the subject of active research in the last decades (Evans & Battles, 1999;Ramsey & Onasch, 1999;Ferket et al., 2000Ferket et al., , 2002Kirkwood et al., 2000;Machel et al., 2000;Douglas et al., 2003;Evans & Hobbs, 2003;Bellahsen et al., 2006;Katz et al., 2006;Wall et al., 2006;Breesch et al., 2009;Fischer et al., 2009;Amrouch et al., 2010aAmrouch et al., ,b, 2011Callot et al., 2010;Beaudoin et al., 2011Beaudoin et al., , 2014bBarbier et al., 2012a,b;Evans & Fischer, 2012;. In the south Pyrenean fold and thrust belt, where the impact of thrust emplacement on fluid flow has been extensively studied (e.g. ...
... This evolution from a closed and stratified, to a more open hydrological system during thrust propagation and associated fracturing has been also described in other fold and thrust belts (Evans & Battles, 1999;Swennen et al., 2000;Van Geet et al., 2002;Evans & Hobbs, 2003;Schneider, 2003;Wall et al., 2006;Vilasi et al., 2009;Evans, 2010;. For example, in the Appalachian province, fluid flow was mainly restricted to permeable layers bounded by low-permeability marls and shales, but hydrothermal waters of metamorphic or connate parentage also migrated along active decollement levels and fracture networks (Evans & Battles, 1999;Evans & Hobbs, 2003;Evans, 2010;. ...
... For example, in the Appalachian province, fluid flow was mainly restricted to permeable layers bounded by low-permeability marls and shales, but hydrothermal waters of metamorphic or connate parentage also migrated along active decollement levels and fracture networks (Evans & Battles, 1999;Evans & Hobbs, 2003;Evans, 2010;. A very close paleohydrological behaviour was described in the Albanides, which recorded the transition from a closed system to the active migration of hydrothermal water having interacted with underlying evaporites (Swennen et al., 2000;Van Geet et al., 2002;Wall et al., 2006;Vilasi et al., 2009), and in the Venezuelan foreland basin system, where large-scale thrusting favoured connection between several productive reservoirs (Schneider, 2003). However, contrary to the South Pyrenean Foreland Basin, these fold and thrust belts exhibit the wide scale remobilization of deeply infiltrated warm meteoric water toward the entire foreland basin, which had a crucial impact on their thermal and diagenetic evolution. ...
This study aims at understanding the origin and nature of syn-orogenic fluid flow in the Jaca basin
from the South Pyrenean fold and thrust-belt, as recorded in calcite and quartz veins of the Sierras
Interiores (Spain) and the turbiditic basin, which cover upper Cretaceous to Late Eocene syntectonic
deposits. The fracture network consists of a classical pattern of transverse and longitudinal fractures
related to Layer Parallel Shortening (LPS) and folding respectively. Veins filled equally about the
third of fractures in the carbonate shelf and turbidites. Carbon and oxygen isotopes of calcite veins
mostly indicate precipitation from isotopically buffered water, consistent with high water-rock interaction.
In the Sierras Interiores, petrographical observations and fluid inclusion microthermometry
are consistent with two distinct stages of precipitation. The first stage is characterized by relatively
low Th and low salinities (155–205 °C and 0.5–3.2 wt% eq. NaCl). The second stage, which was
characterized both by the formation of mode-I joints and by mode-I reactivation of pre-existing
veins, shows higher Th and salinities (215–270 °C and 2.2–5.7 wt% eq. NaCl). Waters recorded in
the second stage are interpreted to have interacted with underlying Triassic evaporites and flowed
along major thrusts before vein precipitation, which are locally in thermal disequilibrium with hostrocks.
We suggest the transition from a rather closed hydrological system during the first stage of
vein formation, interpreted to have occurred during Eaux-chaudes thrusting (upper Lutetian-Bartonian),
to a more open hydrological system during the second stage, which likely occurred during
Gavarnie thrusting (Priabonian-early Rupelian). Finally, we also document the migration in space
and time of hydrothermal pulses along the South Pyrenean Foreland Basin, related to the westward
propagation of major thrusts during the Pyrenean orogeny.
... A new play has been recently explored by the industry in the central and southern part of the Ionian zone. Some of the recent discoveries have occurred in deep footwall-ramp anticlines, where the Mesozoic carbonate sequence sealed by Oligocene shales is thrust and folded beneath the outcropping thrust sheets (e.g., Curi, 1993;Picha, 1996;Graham, Girbacea, Mesonjesi, & Aydin, 2006). The Delvina gas-condensate discovery (Sejdini et al., 1994;Velaj et al., 1999;Velaj, 2001Velaj, , 2015aVelaj, ,2015b, for example, and the Shpiragu 1 light oil discovery (Kotenev, 2015;Velaj, 2015aVelaj, ,2015bBarbullushi, 2016) confirmed that the buried thrust-related folds constitute a valuable target for hydrocarbon exploration in the Ionian FTB. ...
... This structure could be generated as an out-of-sequence thrust in relation to the backlimb of the buried Patos-Verbas thrust. Location of the section is shown in The Mesozoic reservoirs comprise buried hanging-wall thrust ramps, involving thick (several hundred meters) carbonate sequences with a low porosity matrix (0.7%-4.2%), although the total permeability is also controlled by an important secondary porosity in relation to open fractures and joints (Roure et al., 1995;Graham et al., 2006). Upper Cretaceous carbonates represent the main reservoir, where thick layers with slumps and slope mobilized sediments can be found, which significantly modifies the reservoir characteristics. ...
... A new play has been intensively explored by the industry in Albania during the last few years, namely the Mesozoic hanging-wall thrust ramps under the exposed thrusts in the northern and southern Ionian Zone. The gas-condensate discovery of Delvina, as well as the light oil discovery of Shpiragu 1, confirmed the effective sealing performed by the lowporosity reservoirs of the Oligocene shales (Graham et al., 2006;Velaj, 2011;Kotenev, 2015). ...
The Ionian fold-and-thrust belt (FTB) is formed by a piggyback sequence of W-directed thrusts that were emplaced over the Apulian Platform during the late Oligocene to the latest Miocene. From bottom to top, the Cika, Kurveleshi, and Berati belts constitute the three main thrust sheets of the Ionian FTB. All of them contain a rather similar thick sequence (~ 2–3 km) of basinal, pelagic carbonates of latest Triassic to Eocene age. The Late Triassic evaporite sequence (in Keuper facies) acts as a regional decoupling level for thrusting during the Alpine contraction. The structure of three major structures of Triassic evaporites has been studied in detail in central and southern Albania. They document different styles of salt tectonics in the precursor Ionian basin and different initial conditions for the structural style during the subsequent Alpine contraction. The Dumre diapir is interpreted as a thick salt plug, which has been squeezed and displaced toward the west, promoting a marked change in the geometry of the overlying thrust that passes from a frontal to a lateral hanging-wall ramp. In southern Albania, a variation of the Triassic salt structures was reconstructed toward the W and SW, possibly representing a downslope evolution from salt pillows (Delvina and Picari-Kardhiq) to massive salt walls (Butrinti-Xarra).
... These deposits dip eastward with angles of about 35° and are covered by Oligocene clay-claystone flysch filling the Vila Syncline. In accordance with the tectonic style of the Kruja zone, the Makaresh structure was affected by westward longitudinal thrusting during Eocene and late Oligocene to Miocene times [27,40,41]. ...
... Karst reservoirs are characterized by very low matrix porosity and by high porosity related to fissures and karst, developed more intensively at the crests of anticlinal structures [42,67]. Transverse trend (NW-SE) breakdowns in the external tectonic areas of Albania are predominant as fluid movement routes, as well as these are favoured by presence of open gaps, calcium fillings and bitumen [41]. In conditions of difficult groundwater circulation in deep structures, their enrichment with different chemical components is mainly related to two processes. ...
... These deposits dip eastward with angles of about 35° and are covered by Oligocene clay-claystone flysch filling the Vila Syncline. In accordance with the tectonic style of the Kruja zone, the Makaresh structure was affected by westward longitudinal thrusting during Eocene and late Oligocene to Miocene times [27,40,41]. ...
... Karst reservoirs are characterized by very low matrix porosity and by high porosity related to fissures and karst, developed more intensively at the crests of anticlinal structures [42,67]. Transverse trend (NW-SE) breakdowns in the external tectonic areas of Albania are predominant as fluid movement routes, as well as these are favoured by presence of open gaps, calcium fillings and bitumen [41]. In conditions of difficult groundwater circulation in deep structures, their enrichment with different chemical components is mainly related to two processes. ...
... These deposits dip eastward with angles of about 35 • and are covered by Oligocene clay-claystone flysch that fills the Vila syncline ( Figure 2a). In accordance with the tectonic style of the Kruja zone, the Makaresh structure was affected by westward longitudinal thrusts during Eocene and late Oligocene to Miocene times [27,40,41]. ...
... Karst reservoirs are typically characterized by high porosity related to fissures and karst conduits that have developed more intensively at the crests of anticlinal structures [42,67]. Transverse trend (NW-SE) breakdowns in the external tectonic areas of Albania are predominant routes for the movement of fluid, and these are favored by the presence of open gaps, calcium fillings, and bitumen [41]. In conditions of difficult groundwater circulation in deep structures, their enrichment with different chemical components is mainly related to two processes. ...
Carbonate rocks cover about 23% of Albania, with exploitable karst water resources estimated at 2.84 × 109 m3/year (about 65% of the total exploitable groundwater resources in the country). The Kruja tectonic zone is characterized by the presence of SE–NW-oriented carbonate structures, rich in fresh and thermal groundwaters. More than 80% of the thermal springs in Albania are present in this tectonic zone. One of its most interesting carbonate structures, with the presence of both cold and thermal waters, is the small karst structure of Makaresh, with a surface of 22 km2. The purpose of this article is to describe the hydrogeological characteristics of this massif; based on the physico-chemical characteristics, groundwaters of the study area are classified as cold waters (belonging to the local flow system) and thermal waters (originating in intermediate/deep flow systems). The former are mainly of HCO3-Ca or HCO3-Ca-Mg type (electrical conductivity 580–650 μS/cm, Temperature 13.9–16.6 °C). Thermal waters are mainly of the Cl-Na-Ca type (EC 7200–7800 μS/cm, T 18.5–22.5 °C); they are further characterized by high hydrogen sulfide concentration, up to about 350 mg/L. The presence of two groundwater types in the Makaresh massif is connected to the presence of two groundwater circulation systems. The main factors of the groundwater physico-chemical quality are the dissolution of rocks and minerals contained therein, the presence of hypogenic speleogenesis, and the mixing of the groundwater of the two systems. The hydrogeological studies proved that karst rocks contain considerable freshwater resources, partly used for water supply. Thermal waters are not currently exploited due to their temperature, but they are potentially suitable for thermal uses by drilling boreholes to a depth of about 1000 m.
... These deposits dip eastward with angles of about 35° and are covered by Oligocene clay-claystone flysch filling the Vila Syncline. In accordance with the tectonic style of the Kruja zone, the Makaresh structure was affected by westward longitudinal thrusting during Eocene and late Oligocene to Miocene times [27,40,41]. ...
... Karst reservoirs are characterized by very low matrix porosity and by high porosity related to fissures and karst, developed more intensively at the crests of anticlinal structures [42,67]. Transverse trend (NW-SE) breakdowns in the external tectonic areas of Albania are predominant as fluid movement routes, as well as these are favoured by presence of open gaps, calcium fillings and bitumen [41]. In conditions of difficult groundwater circulation in deep structures, their enrichment with different chemical components is mainly related to two processes. ...
Carbonate rocks occupy about 6490 km2 in Albania (23% of the country) and the total exploitable karst water resources are estimated in 2.84*109 m3/year (about 65% of the total exploitable groundwater resources in the country, and about 70% of those used for water supply). In addition, karst massifs are very rich in thermal waters, related to the ascending fluids which are essential for the development of hypogenic karstification in the outer Albanides. This type of karst facilitates the circulation regime of deep fluids, which are revealed at the surface through both numerous phenomena of hypogene karst and hot springs. The relatively small Kruja Tectonic Zone is characterized by the presence of SE-NW oriented carbonate structures, rich in thermal sources, and contains more than the 80% of Albania thermal springs. One of its most interesting carbonate structures, for the presence of both cold and thermal waters, is the small karst structure of Makaresh, with a surface of 22 km2. The purpose of this article is to describe the characteristics and factors that affect the quantitative and qualitative formation of cold and thermal groundwater in this massif. Based on the physico-chemical characteristics, groundwaters of the study area are classified in cold and thermal waters. The former are mainly of HCO3-Ca or HCO3-Ca-Mg type (Electrical Conductivity 580-650 μS/cm, Temperature 13.9-16.6° C). Thermal waters are mainly of the Cl-Na-Ca type (EC 7200-7800 μS/cm, T 18.5-22.5° C); further, they are characterized by high hydrogen sulphide concentration, up to about 350 mg/l. The presence of two groundwater types in the Makaresh massif is connected to the presence of two groundwater circulation systems. The main factors of the groundwater physic-chemical quality are the solution of rocks and minerals therein contained, the presence of hypogenic speleogenesis, and the mixing of the groundwater of the two systems. The hydrogeological studies proved that karst rocks contain considerable fresh water resources; the capacity of water wells used for the drinking water supply reaches up to about 50 l/s. Thermal waters are not currently exploited due to their temperature, but they are potentially suitable for thermal uses, by drilling boreholes at depth of about 1000 m.
... The Korab, Mirdita, and Gashi zones form the Internal Albanides, dominated by metamorphic rocks and ophiolites. The Albanian Alps, Krasta-Cukali, Kruja, Ionian and Sazani zones belong to the External Albanides (Fig. 1A), a part of the S. Adriatic sedimentary basin highly affected by the westward thrusting in the Eocene and the late Oligocene-Miocene (WALL et al., 2006 and references therein). Besides a system of longitudinal faults, the Albanides are broken by a series of transversal (NE-SW) faults on many scales, of which the basement-related Scutari-Pec and the Vlora-Elbasan-Diber faults are the major ones ( VAN GEET et al., 2002). ...
... IV -Karstologia Mémoires n°24 -SYMPOSIUM 04 -Geomorphology and Speleogenesis _____________________________________________________________________________________________________ folding in the Late Oligocene-Aquitanian, reflecting largescale fluxes of external CO 2 -charged fluids derived from the basement or evaporitic décollement horizons ( VAN GEET et al., 2002). This was enabled by the creation of vertical throughgoing discontinuities, of which those trending transversely (NE-SW) were the dominant pathways for carrying and storing fluids, based on widespread occurrence of open voids, calcite fill, and bitumen accumulation along them (WALL et al., 2006). During uplift and denudation of the flysh cover during the Late Miocene -Pleistocene, upward flow from deep aquifers focused upon anticlines, particularly in localities where carbonate formations were being brought to the shallow subsurface and locally exhumed. ...
The structural fabric of Albania is defined by the NNE-SSE-trending tectonic zones. Besides longitudinal faults, the Albanides are broken by transversal faults at various scales. In the Kruja and Ionian zones of the External Albanides, carbonates of Cretaceous-Eocene age are widely spread although only locally exposed. Wells tap carbonate thermal aquifers and hydrocarbon reservoirs at depths up to 3,700 m. The deep aquifers/reservoirs have low matrix porosity but substantial and locally high fracture and karstic porosity. We investigated three sites of expressed hypogene karst (Holta, Lengarica, and Leskovik), scattered along the Kruja zone but possessing important common characteristics: (1) canyons crossing NE-SW the massifs of carbonates exhumed from beneath the flysh at the crests of anticlines; (2) rising thermal springs discharging H 2 S-enriched waters, which are variable mixes of the deep flow with shallow groundwater; (3) abundant hypogene karst features, mainly associated with water-table-related SAS, although sub-vertical features controlling major canyon segments could have been formed in the deep setting by other processes. During uplift and denudation throughout Late Miocene-Pleistocene, discharge of deep groundwater focused upon the tops of emerging anticlines and caused intense hypogene karstification, which was the primary control in the formation of the antecedent canyons by karst piracy.
... The External Albanides are home to a thick succession of homogeneous and fractured calciturbidites, wherein the vein system, its diagenetic properties and tectonic history have been well studied due to the presence of hydrocarbons in the region (Velaj et al., 1999;Nieuwland et al., 2001;Van Geet et al., 2002;Roure et al., 2004;Graham Wall et al., 2006;Vilasi et al., 2006;Le Goff et al., 2015;Le Goff et al., 2019). The calciturbidite succession is of Cretaceous to Paleocene age, and was buried to depths of 4 ± 1 km, at temperatures up to 100°C (Van Geet et al., 2002;Roure et al., 2004;Vilasi, 2009;Lacombe et al., 2009). ...
... The paragenetic sequence of vein formation was established by analysing the crosscutting relations between veins and stylolites, both in the field and petrographically. The vein populations were matched to the vein populations previously defined in studies performed by Graham Wall et al. (2006), Vilasi et al. (2006) and Vilasi (2009), and were categorized as V1, V2 or V3 veins, which link to different deformational phases that affected the Ionian Zone (de Graaf et al., 2019). We have selected clean (i.e. ...
The distribution of oxygen isotopes between calcite and fluid inclusions has demonstrated utility for reconstructing near-surface calcite precipitation temperatures. For calcite that formed at depth, however, the resilience of this paleothermometer to diagenetic oxygen isotope alteration is poorly constrained. Clumped isotopes also document calcite precipitation temperatures and are similarly vulnerable to diagenetic alteration. Post-entrapment isotope exchange between calcite and fluid-inclusions could alter the calcite-fluid oxygen isotope distribution (αc-fi), as well as the clumped isotope composition (Δ47) of calcite, and therefore these two seemingly independent paleo-thermometers are potentially linked via the same alteration process. Using closed-system batch fractionation equations, we have modelled various scenarios of oxygen isotope exchange between water and host-rock during burial, as well as internal oxygen isotope exchange between calcite and fluid inclusions during exhumation. Assuming both paleo-thermometers record concordant temperatures at the time of vein formation, our models predict that if a fraction of calcite is available for isotopic interaction with fluid inclusions, the fluid inclusion and clumped isotope-derived paleothermometers yield discrepant temperature estimates after exhumation. We show that the fluid inclusion thermometer is more sensitive to isotopic alteration than the clumped isotope thermometer and that the mass balance of oxygen between calcite and fluid inclusions determines the sensitivity of both paleothermometers as well as the vulnerability of fluid inclusions (δ¹⁸Ofi) to diagenetic overprinting. We applied coupled clumped isotope and fluid inclusion measurements on calcite veins from the External Albanides (Albania), which were formed at depth and subsequently exhumed, in order to compare natural samples to our isotope exchange model. These veins show strongly discrepant calcite-water equilibrium temperatures and clumped isotope temperatures, suggesting the fraction of calcite available for isotope exchange with internal fluids may indeed be a key parameter of diagenetic alteration during exhumation. Even though the clumped isotope temperatures of our samples appear to be insensitive towards internal oxygen isotope exchange, our model predicts that at low burial temperatures, the carbonate clumped isotope thermometer may be susceptible to alteration by diagenetic isotope exchange with fluid inclusions under certain conditions.
... Graham Wall et al. (2006) and Lacombe et al. (2009) ...
... Based on crosscutting relationships, we suggest that early-orogenic extensional structures predate the layer-parallel shortening pattern, as documented also in the Albanides (Graham Wall et al., 2006;Lacombe et al., 2009), where the shortening direction is well clustered and oriented almost perpendicular to the strike of the belt (Lacombe et al., 2009). In the Ionian zone, where we carried out our meso-structural study, solution seams are organised in different sets, oblique to each other, and, together with reverse faults, define multiple layer-parallel shortening directions. ...
In this work we report on early-orogenic fracture patterns affecting a Cretaceous to Eocene sedimentary succession exposed in western Greece. These rocks belong to the Cenozoic External Hellenides, which form the western portion of the Aegean orocline. The analysis of fracture type, orientation, and crosscutting relationships provides constraints on the stress and strain patterns during the early stages of orocline formation. The fracture patterns in the study area includes early-orogenic extensional fractures arranged into two mutually orthogonal sets. These developed during progressive burial in the forebulge-foredeep system, ahead of the advancing compressive front. Tectonic solution seams at a high angle to bedding postdate these extensional structures. Solution seams are arranged in different sets, oblique to each other, and developed in the early stages of thrusting and folding. Extensional structures and solution seams are oblique to each other. Their orientation and crosscutting relationships provide evidence for clockwise vertical axis rotation of stress directions with time. This is consistent with the progressive arching of the forebulge-thrust belt system during progressive slab retreat.
... In addition to fracturing which is a fundamental factor for tight carbonate reservoirs (Agosta et al., 2010;Graham Wall et al., 2006), there are broad pathways with complicated internal architectures, and complex fluid-rock interactions in the fault zone due to different structural fabrics and deformation mechanisms (Brogi and Novellino, 2015;Faulkner et al., 2010;Hausegger et al., 2010). The petrophysical properties, especially permeability in the fault zone, show strong heterogeneity across-and along-fault zones, which may be fluid conduits or barriers in different segments and evolutionary stages (Agosta et al., 2010;Faulkner et al., 2010;Jeanne et al., 2012;Rotevatn and Bastesen, 2012). ...
... The petrophysical properties, especially permeability in the fault zone, show strong heterogeneity across-and along-fault zones, which may be fluid conduits or barriers in different segments and evolutionary stages (Agosta et al., 2010;Faulkner et al., 2010;Jeanne et al., 2012;Rotevatn and Bastesen, 2012). Indeed, studies have demonstrated that fault zones exert primary controls on carbonate reservoir development, as well as fluid migration and/or accumulation (Aydin, 2000;Graham Wall et al., 2006;Hausegger et al., 2010). Unfortunately, we have not yet achieved full understanding of the impact of fault segmentation on the subsurface reservoirs because of the limited data and the complicated elements constraining the fault zone, its geometry and properties. ...
The largest condensate field of the Ordovician reef-shoal reservoirs, in China, was discovered along the N°1 fault zone in the Tarim Basin. However, current drilling activities are hindered by the low and unstable production from these carbonate reservoirs. A detailed structural analysis of 3-D seismic data identified three segments along the N°1 fault zone characterized by differences in structural geometry and evolution. Segmentation of the N°1 fault zone has resulted in the variation of characteristics and distribution of the Upper Ordovician carbonate reservoirs. Due to fault induced differential uplift before the Late Ordovician, a narrow and thick-rimmed platform margin rich in reefs has developed in the eastern segment, whereas wider and thinner shoals developed in the central and western segments. We clearly identified a fault block uplift related erosional unconformity, and associated karstification at the top of the Upper Ordovician carbonates in the eastern segment. In this segment, the karst caves resulted in the high and stable hydrocarbon production. In the central segment, good matrix reservoirs exist, but due to the lack of fault-related enhancement of the reservoirs quality, only relatively low production is observed. In the western segment, where fracture-vug reservoirs occurred in relation to later fault damage zones, production is much greater but unstable. Large-scale fracture-caves reservoirs related to fault activity, rather than the good unfaulted matrix reservoirs, lead to the high production of oil and gas. We classify these reef-shoal reservoirs as faulting modified reservoirs. Further exploitation should focus more on the fault damage zone beyond the platform margin rather than the microfacies in the Lower Paleozoic carbonates.
... The fractures generated through tectonism, along with their associated features are crucial to the formation and transformation of carbonate rock fracture karst reservoir. The fractures transform the reservoir by enhancing fluid activity, thus improving the reservoir permeability of carbonate rock reservoir (Graham WBR et al., 2006;Pöppelreiter M et al., 2005;Larsen B et al., 2010;Su J et al., 2010), further controlling the hydrocarbon accumulation and enrichment. While predecessors have examined the types, stages, causes, and relationships between fractures and karst reservoir (Gao JX et al., 2012;Wu GH et al., 2019), there remains a dearth of comprehensive anatomical analysis and statistical evaluation. ...
... Fracture stratigraphy subdivides rock into fracture units according to extent, intensity, or some other observed fracture attribute (Cooke, 1997;Corbett et al., 1997;Hanks et al., 1997;Fischer and Jackson, 1999;Di Naccio et al., 2005;Peacock and Mann, 2005;Bertotti et al., 2007;Laubach et al., 2009). The understanding of the geometry and distribution of the fracture systems in fractured reservoirs is crucial in the oil industry with respect to hydrocarbon accumulation and fluid flow, as well as optimal well design (e.g., Nelson, 1985;Aydin, 2000;Graham Wall et al., 2006;Evans and Fisher, 2012;Tavani et al., 2014). ...
A good understanding of the fracture distribution and of the main
mechanisms controlling fault and fracture impedance across
different lithologies is key to estimating both flow properties and
recovery of geofluids in fractured reservoirs. This multidisciplinary
study of the Cretaceous carbonate platform of the Maiella
Mountains focused on the occurrence and impedance of two
main mechanical boundaries for large-scale fractured zones. Additionally, a workflow for applying the outcrop findings to the
subsurface was developed. The two identified mechanical boundary types are related to well-sorted bioclastic packstone - grainstone units and a dolomitized bed, respectively. The first type occurs within an Upper Cretaceous succession originally deposited in an open platform
environment; this mechanical boundary is associated with marine
flooding surfaces that occur at the bases of 50- to80-m-thick depositional cycles. The second type is found within a Lower Cretaceous succession deposited in a more restricted environment.
Through well-log analyses, both mechanical boundary types are
then identified in the subsurface and used to implement the reservoir
static model through the generation of vertical transmissibility
multiplier arrays for large-scale faults and fractures. The latter is an
essential parameter controlling vertical fluid flow in a dynamic
simulation grid of fractured reservoirs.
... Diagenetic studies indicate that after an expulsion/over-pressurized regime with rock buffered (closed system) fluids, open system fluid advection became involved after folding in the Late Oligocene-Aquitanian, reflecting large-scale fluxes of external CO 2 -charged fluids derived from the basement or evaporitic décollement horizons ( VAN GEET et al., 2002). This was enabled by the creation of vertical throughgoing discontinuities, of which those trending transversely (NE-SW) were the dominant pathways for carrying and storing fluids, based on widespread occurrence of open voids, calcite fill, and bitumen accumulation along them (WALL et al., 2006). During uplift and denudation of the flysh cover during the Late Miocene -Pleistocene, upward flow from deep aquifers focused upon of anticlines, particularly in localities where carbonate formations were being brought to the shallow subsurface and locally exhumed. ...
The structural fabric of Albania is defined by the NNE-SSE-trending tectonic zones. Besides longitudinal faults, the Albanides are broken by transversal faults at various scales. In the Kruja and Ionian zones of the External Albanides, carbonates of Cretaceous-Eocene are widely spread although only locally exposed. Wells tap carbonate thermal aquifers and hydrocarbon reservoirs at depths up to 3,700 m. The deep aquifers/reservoirs have low matrix porosity but substantial and locally high fracture and karstic porosity. We investigated three sites of expressed hypogene karst (Holta, Lengarica, and Leskovik), scattered along the Kruja zone but possessing important common characteristics: (1) canyons crossing NE-SW the massifs of carbonates exhumed from beneath the flysh at the crests of anticlines; (2) rising thermal springs discharging H 2 S-enriched waters, which are variable mixes of the deep flow with shallow groundwater; (3) abundant hypogene karst features, mainly associated with water-table-related SAS, although sub-vertical features controlling major canyon segments could have been formed in the deep setting by other processes. During uplift and denudation through Late Miocene-Pleistocene, discharge of deep groundwater focused upon the tops of emerging anticlines and caused intense hypogene karstification, which was the primary control in the formation of the antecedent canyons by karst piracy.
... The results obtained will be added to the several studies of world analogs that have contributed to the understanding of the formation and evolution processes of the reservoir rocks and their main structures (Wall et al., 2006;Smith et al., 2006;Jiang et al., 2015Jiang et al., , 2016Wang et al., 2016;Lima et al., 2019). Obviously, the choice of the Salitre Formation was mainly motivated by its carbonate composition. ...
Carbonates of the Salitre Formation, Irecê Basin, Brazil, have been the focus of extensive stratigraphic and structural studies for several decades. Besides hosting phosphorites deposits, these rocks are analogues to fractured and certified oil reservoirs, such as those of the Cretaceous pre-salt oil system offshore Brazil. The present study presents an integration of X-Ray Diffraction (XRD), Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMScan), Optical Petrography, Cathodoluminescence (CL), and Charge Contrast Imaging (CCI) data that allows a high-resolution characterization of the mineralogy within the carbonate units. In addition, we also present carbon and oxygen stable isotope data to identify possible origins of the associated fluids. The mineral characterization revealed that calcite (50–70%), quartz (20–30%) and dolomite (10–20%) are the main post-sedimentary mineral phases found in veins, vesicle structures and cement of the carbonate units. The mineral phases of veins were formed by the interaction with hydrothermal fluids, which had an important role in the dissolution, karstification, and remobilization of carbonate units. Isotopic analysis shows that δ13C and δ18O are more negative in veins and vesicles when compared to host rock, indicating that there is no effective isotopic exchange between the host rock and the filled structures and that these structures were filled with hydrothermal fluids from an external source.
... Micro-and macro-fractures and stylolites have been identified at all scales ranging from core samples to outcrops. The different fractures sets originated during the pre-orogenic compactional mesogenetic to late synorogenic stages (Van Geet et al., 2002;Graham Wall et al., 2006;Vilasi et al., 2006;de Graaf et al., 2019;Nooitgedacht et al., 2021). Fracture permeability operates at different scales from plug to bed-sets and is a fundamental component in the reservoir performance of the dominantly low matrix-porosity carbonates of Ionian Zone in Albania. ...
Understanding acoustic velocities of sedimentary deposits is crucial for realistic subsurface imaging. The velocities rely on mineralogy, density, as well as pore volume (porosity) and pore structure. In carbonate rocks, both depositional and diagenetic histories exert a significant control on these characteristics. In order to determine this control, petrographic analysis (thin sections) associated with the analysis of the carbonate content, grain density, permeability, porosity, and acoustic measurements were performed on samples from Cretaceous to Eocene carbonate platform-to-basin series of Albania. Rock facies showed two depositional associations and four pore-type associations. Very low average permeability (< 1 mD) and variable porosity values (0 – 14%) characterize the samples. Pore type and diagenetic alteration steered the porosity-permeability distribution. Modelling of the velocity-porosity relationship, using the differential effective medium theory (DEM), revealed that variations in the acoustic velocity were mainly driven by variations in porosity and pore type, while facies and mineralogy were of minor importance.
The Albanian dataset was compared with petrophysical data retrieved from time- and deep-water facies equivalent carbonate series of the Gargano Promontory in Italy. The comparison demonstrates that the variation in acoustic velocity is limited within the Albanian samples due to the pronounced infill of primary and secondary pores. Acoustic velocity data, on the other hand, are more scattered for the Gargano and this is essentially related to the higher porosity due to the preserved primary macro-intergranular and micro-intercrystalline pores, and/or to significant development of secondary mouldic pores.
This study contributes to the understanding of heterogeneities and spatial distribution of pore types resulting in different petrophysical signatures of Cretaceous platform to deep-water carbonates. The data provide a better insight into possible variations in reservoir quality and seismic signature of these types of carbonates in subsurface reservoirs.
... Large oil and gas reserves have been discovered in the Ordovician carbonate reservoirs in the Central Uplift of Tarim Basin (Fig. 1b, Jia, 1997;Du, 2010;Wu et al., 2016a;Yang et al., 2020). Previous studies demonstrated that fault zones played a dominant role in the development of carbonate reservoirs by enhancing fluid infiltration and migration (Aydin, 2000;Graham Wall et al., 2006;Hausegger et al., 2010). The high and stable hydrocarbon production in the Central Uplift of the Tarim Basin is considered to benefit from faulting-related enhancement of reservoir quality (Wu et al., 2016a). ...
Direct evidence for fault reactivation is crucial for understanding long-term fault behavior and reconstruction of fluid circulation and hydrocarbon migration and accumulation during active tectonics. However, absolute dating of fault reactivations and fluid circulation is still challenging due to dissolution and recrystallization of fault zone materials and obliteration of previous events by the latest deformation/hydrothermal event. The lower Ordovician limestone strata at 3.8 km depth in the Central Uplift of the Tarim Basin, northwest China experienced multi-stage regional tectonic activities that developed a series of NW–SE striking reverse faults and NE–SW oriented strike-slip faults. In this study, we performed in situ laser-ablation ICP-MS U–Pb dating and trace element characterization of calcites collected from borehole cores within one of the main fault zones in the Central Uplift of the Tarim Basin. A new in-house calcite standard AHX-1A (209.8 ± 1.3 Ma), collected from the Tarim Basin, calibrated against well-calibrated WC-1 and ASH-15 calcite standards, is used for mass-bias correction. We obtained in situ U–Pb ages of 9 calcite samples from two wells within a Paleozoic fault zone in the Tarim Basin. They are 456 ± 11 Ma, 454.7 ± 7.2 Ma, 450.4 ± 6.2 Ma, 435.2 ± 9.7 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma from well TZ2, and 371 ± 18 Ma, 390.6 ± 6.0 Ma and 395 ± 14 Ma from well TZ4 (all reported uncertainties are 2σ). Combined with petrographic evidence, these results define at least six generations of secondary calcite precipitations in the fault zone, that occurred during 456 ± 11 to 450.4 ± 6.2 Ma (weighted mean 452.8 ± 4.3 Ma), 435.2 ± 9.7 Ma, 395 ± 14 to 390.6 ± 6.0 Ma (weighted mean 391.3 ± 5.5 Ma), 371 ± 18 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma. Chondrite-normalized rare earth element (REE) + yttrium (Y) patterns of these calcite samples show prominent positive Gd and Y anomalies, lack of negative Ce anomaly, and enrichment in light REE but depletion in heavy REE, similar to the shallow marine platform host-rock limestone. The results suggest multi-stage fluid circulation under shallow burial, low temperature, and non-oxidizing environments along the fault zone, with the secondary calcite precipitating from a similar fluid source that originated from dissolution of host-rock carbonate or early-stage diagenesis. The detail petrographical and mineralogical analysis of this study, combined with the new interpretation of high-resolution seismic data, constrains two main episodes of faulting and related uplift during 456–435 Ma and 395–371 Ma, as well as a younger faulting episode with weaker intensity during 328–308 Ma. This study demonstrates that the central fault zone of the Tarim Basin experienced ~150 Ma of episodical reactivation during the Paleozoic, which has important implications for hydrocarbon exploration.
... Closing mode fractures are typically tighter than the surrounding matrix. However, if they are mechanically or chemically reactivated, then they can become hydraulically conductive (Graham Wall et al. 2006). Stylolites are typically formed during the compaction of carbonates by localized pressure solution and have a characteristic wavy or sawtooth appearance. ...
The characterisation of fractured reservoirs and fractured geothermal resources requires a thorough understanding of the geological processes that are involved during fracturing and the host rock rheological properties. The presence or absence of mechanical layering within the rock and the mode of failure substantially control the organization and scaling of the fracture system; subsequent chemical alteration and mineralization can both increase or decrease porosity and permeability. An integration of this understanding using information from outcrop analogues, together with static and dynamic subsurface data,can improve our ability to predict the behaviour of fractured reservoirs across a range of scales.
Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs
... Katz et al. (2006) identified these as hydrothermal breccias (minimum temperature for the calcite cements of 120-140°C based on fluid inclusions), and envisage seismic pumping from thrusts during the Laramide Orogeny. Similar breccias from contractional tectonic regimes have also been reported by Graham Wall, Girbacea, Mesonjesi, and Aydin (2006). brittle rocks (e.g. ...
A mechanism is presented for the pulses of high fluid pressure (PF) necessary for fluid‐assisted brecciation. Establishment of hydraulic‐ or pneumatic‐connectivity between rock masses with different PF can cause overpressure in the higher rocks because the PF gradient is parallel to the hydrostatic gradient (the centroid effect). PF can become high enough to create a fracture network, with an influx of fluids and mineralisation occurring as fluids migrate to areas of lower PF. Changes in PF caused by the centroid effect can cause other structures and seismicity.
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... Dissolution-related cleavage associated with stress concentration ahead of advancing fault tips was identified by Salvini et al. (1999) and several authors documented the occurrence of outcrop scale faults initiated by shearing of pre-existing pressure solution seams (e.g. Graham et al., 2003;Daniel et al., 2004;Wall et al., 2006;Agosta and Aydin, 2006). ...
The structure, frictional properties and permeability of faults within carbonate rocks exhibit a dynamic interplay that controls both seismicity and the exchange of fluid between different crustal levels. Here we review field and experimental studies focused on the characterization of fault zones in carbonate rocks with the aim of identifying the microstructural indicators of rupture nucleation and seismic slip. We highlight results from experimental research linked to observations on exhumed fault zones in carbonate rocks. From the analysis of these accumulated results we identify the meso and microstructural deformation styles in carbonates rocks and link them to the lithology of the protolith and their potential as seismic indicators. Although there has been significant success in the laboratory reproduction of deformation structures observed in the field, the range of slip rates and dynamic friction under which most of the potential seismic indicators is formed in the laboratory urges caution when using them as a diagnostic for seismic slip. We finally outline what we think are key topics for future research that would lead to a more in-depth understanding of the record of seismic slip in carbonate rocks.
... He applies a multidisciplinary approach combining tectonics, sedimentology, geophysics, and numerical modeling to foldthrust belts and related foreland basins, to rift systems, and to salt tectonics at different scales. Wall et al., 2006;De Keijzer et al., 2007;Ahmadhadi et al., 2008;Mynatt et al., 2009;Casini et al., 2011;Lacombe et al., 2011;Tavani et al., 2011Tavani et al., , 2015Navabpour and Barrier, 2012;Quintà and Tavani, 2012;Su et al., 2014;Branellec et al., 2015). ...
Fieldwork and remote sensing data from the Siah Kuh anticline, simply folded belt, Zagros, Iran indicate that specific structures and fracture systems formed during the development of its sigmoidal shape, and that conceptual fracture models developed for cylindrical folds are inadequate for the correct evaluation and development of hydrocarbon accumulations in this type of anticline. The sigmoidal shape of the Siah Kuh anticline was achieved in Pliocene time due to vertical axis rotations of an already existing anticline. These rotations promoted the development of two systems of normal faults in the outer arcs of the sigmoidal shape, a low-angle thrust, and the N-S Danan anticline in the inner arc of its easternmost bend. The passive margin to syn-folding structures, typically observed in nearby cylindrical and periclinal anticlines, predated the development of the sigmoidal shape and were passively rotated into the segments of the anticline. The sigmoid-related structures are spatially, geometrically and kinematically related to the bends of the anticline trend, hence they can be predicted and modeled in the sub-surface. The sigmoid-related normal faults have a great potential to preserve porosity, promote localized high flow rates or early water breakthrough. On the other hand, if they cut through thin reservoir and seal units, sigmoid-related thrusts and normal faults might compromise lateral reservoir continuity and seal integrity. The results of this study can help in reducing risks and uncertainty in the evaluation and development of business opportunities in secondary sigmoidal anticlines within the Zagros or any other fold and thrust belt.
... These various platforms to basinal facies carbonates (see Apulian, Ionian and Kruja Zones, Chapter 24) are the primary reservoirs in the region, even though they have very small primary porosity (Velaj, 2002(Velaj, , 2011(Velaj, , 2015. However, intense fracturing due to thrusting along multiple salt detachment levels does provide fracture porosity (Wall, Girbacea, Mesonjesi, & Aydin, 2006) large enough for sustainable flow rates. As an important current example, the Shpirag-2 well was drilled by Shell in 2013 targeting a subthrust trap (Fig. 7), within less than a kilometer from where Occidental drilled Shpirag-1 in 2001. ...
The amount of discovered hydrocarbons associated with Permo-Triassic evaporites in the vast geographic area of Europe, North Africa, and the Atlantic region is enormous. The evaporites have a very wide-ranging impact on the petroleum system elements. The most variable impact appears to be the one affecting the trapping style, which is subdivided into the broad categories of postsalt and presalt plays. The exceptionally large presalt petroleum accumulations in those regions are due to the impermeable character of the evaporites providing the ultimate top seal in almost all cases. The interaction of evaporites with reservoir units is typically underappreciated although it could be critical, especially in the case of carbonate reservoirs. A similarly overlooked aspect generally is the effect of salt on the thermal regime, and therefore on the maturation, of source rocks.
... The observed bedding plane fractures in Lithofacies association F-2 formed as a result of grain-size gradations and internal layering of parallel lamination and depositional fabric [125]. Natural fractures are important features that can help deliver hydrocarbons from tight rock matrix of a formation to the wellbore [126]. ...
Several decades of conventional oil and gas production in Western Canada Sedimentary Basin (WCSB) have resulted in maturity of the basin, and attention is shifting to alternative hydrocarbon reservoir system, such as tight gas reservoir of the Montney Formation, which consists of siltstone with subordinate interlaminated very fine-grained sandstone. The Montney Formation resource play is one of Canada’s prime unconventional hydrocarbon reservoir, with reserve estimate in British Columbia (Natural Gas reserve = 271 TCF), Liquefied Natural Gas (LNG = 12,647 million barrels), and oil reserve (29 million barrels). Based on sedimentological and ichnological criteria, five lithofacies associations were identified in the study interval: Lithofacies F-1 (organic rich, wavy to parallel laminated, black colored siltstone); Lithofacies F-2 (very fine-grained sandstone interbedded with siltstone); Lithofacies F-3A (bioturbated silty-sandstone attributed to the Skolithos ichnofacies); Lithofacies F-3B (bioturbated siltstone attributed to Cruziana ichnofacies); Lithofacies F-4 (dolomitic, very fine-grained sandstone); and Lithofacies F-5 (massive siltstone). The depositional environments interpreted for the Montney Formation in the study area are lower shoreface through proximal offshore to distal offshore settings. Rock-Eval data (hydrogen Index and Oxygen Index) shows that Montney sediments contains mostly gas prone Type III/IV with subordinate Type II kerogen, TOC ranges from 0.39 - 3.54 wt% with a rare spike of 10.9 wt% TOC along the Montney/Doig boundary. Vitrinite reflectance data and Tmax show that thermal maturity of the Montney Formation is in the realm of “peak gas” generation window. Despite the economic significance of the Montney unconventional “resource-play”, however, the location and predictability of the best reservoir interval remain conjectural in part because the lithologic variability of the optimum reservoir lithologies has not been adequately characterized. This study presents lithofacies and ichnofacies analyses of the Montney Formation coupled with Rock-Eval geochemistry to interpret the sedimentology, ichnology, and reservoir potential of the Montney Formation tight gas reservoir in Fort St. John study area (T86N, R23W and T74N, R13W), northeastern British Columbia, western Canada.
... In the process of oil and gas exploration and development, fracture-cavity filling and connectivity in fracture-cavity carbonate reservoir is an important research topic [1][2][3]. The classic techniques commonly used are optical microscopy and X-ray diffraction, and often supplemented by scanning or transmission electron microscope. ...
The characteristics of hole and seepage of carbonate rocks depend on the structural components of carbonate rocks and the combination relationship with these structural components under multi-scale. The quality of filling in reservoir fracture is an important part of fracture description. In this paper, the method of microscopic infrared spectral imaging was used, and the spectra cubic database was established by means of spectral imaging experiment. Each single band image which characterizes the related substances was extracted and the microcosmic characters of the filling in carbonate reservoirs fracture from Wumishan formation in Renqiu oilfield were studied. The spatial distribution maps of different chemical composition and groups in the core sample were obtained. The distribution of different hydrocarbon in the fracture of the reservoir and the connectivity of the pores etc. were better revealed
... The individual rock layers bounded by such interfaces and normally corresponding to single beds or individual bed packages that are characterized by quite homogeneous fracture distribution, due to homogeneous mechanical properties during failure, are named " fracture units " in the terminology of Laubach et al. (2009; Fig. 1). As documented in literature (e.g., Aydin, 2000; Laubach, 2003; Graham-Wall et al., 2006; Agosta et al., 2010; Solano et al., 2011), * * Corresponding author. Email address: andrea.rustichelli@unicam.it (A. ...
This article focuses on field- and laboratory-based characterization of vertically persistent fractures that are part of oblique-slip normal fault zones and crosscut the Cretaceous platform and overlaying ramp carbonates outcropping at Maiella Mountain (central Italy). The achieved results show that: (i) fault damage zones are wider and more densely fractured in the platform carbonates than in the ramp ones; (ii) joints and sheared joints composing the fault damage zones are taller, better connected and less spaced within the former rocks than in the ramp carbonates. The aforementioned structural differences are interpreted to be a consequence of the different mechanical properties of the platform and ramp carbonates during failure. At Maiella Mountain, platform carbonates are, indeed, made up of overall stiffer (higher Uniaxial Compressive Strength values) and less porous rocks, due to more abundant intergranular void-filling cement and presence of matrix.
... Cementation along a fault zone is a record of fluid migration through fractures and pore space (e.g. Sample et al., 1993;Ohtani et al., 2000;Kirschner and Kennedy, 2001;Graham Wall et al., 2006;Balsamo et al., 2012Balsamo et al., , 2013, as well as a record of the source fluid chemistry (e.g. McCaig et al., 1995;Parry, 1998;Chan et al., 2000Chan et al., , 2001Ghisetti et al., 2001;Shipton et al., 2004;Eichhubl et al., 2009). ...
... He applies a multidisciplinary approach combining tectonics, sedimentology, geophysics, and numerical modeling to foldthrust belts and related foreland basins, to rift systems, and to salt tectonics at different scales. Wall et al., 2006;De Keijzer et al., 2007;Ahmadhadi et al., 2008;Mynatt et al., 2009;Casini et al., 2011;Lacombe et al., 2011;Tavani et al., 2011Tavani et al., , 2015Navabpour and Barrier, 2012;Quintà and Tavani, 2012;Su et al., 2014;Branellec et al., 2015). ...
The fracture and fault distribution along a sigmoidal shaped anticline of the Zagros mountain range of Iran was studied integrating field analysis with remote sensing data. Different generations of fracturing/faulting have been recognised, based on cross cutting relationships, which span from pre to post folding.
... After 30 km lateral migration, the petroleum charged into the reservoirs of Lunnan area from source area to northwest. In actual exploration of carbonates, fracture and fault could control the fluid migration pathways (Graham Wall et al., 2006). In the eastern of Tabei area, the high-yield gas wells were mainly distributed in the east area along the deep faults (Fig. 14), demonstrating that natural gas in the east area had come from the deeper sources and the faults were the main migration channels. ...
... Stephenson et al., 2007), and fracture patterns tend to evolve through time and become superimposed, or reactivated with different sense of motion (e.g. Brita et al., 2006;Ahmadhadi et al., 2008). These fractures are important for fluid flow, and particularly in carbonates become partially or completely cemented with time. ...
Differences in fluids origin, creation of overpressure and migration are compared for end member Neogene fold and thrust environments: the deepwater region offshore Brunei (shale detachment), and the onshore, arid Central Basin of Iran (salt detachment). Variations in overpressure mechanism arise from a) the availability of water trapped in pore-space during early burial (deepwater marine environment vs arid, continental environment), and b) the depth/temperature at which mechanical compaction becomes a secondary effect and chemical processes start to dominate overpressure development. Chemical reactions associated with smectite rich mud rocks in Iran occur shallow (similar to 1900 m, smectite to illite transformation) causing load-transfer related (moderate) overpressures, whereas mechanical compaction and inflationary overpressures dominate smectite poor mud rocks offshore Brunei. The basal detachment in deepwater Brunei generally lies below temperatures of about 150 degrees C, where chemical processes and metagenesis are inferred to drive overpressure development. Overall the deepwater Brunei system is very water rich, and multiple opportunities for overpressure generation and fluid leakage have occurred throughout the growth of the anticlines. The result is a wide variety of fluid migration pathways and structures from deep to shallow levels (particularly mud dykes, sills, laccoliths, volcanoes and pipes, fluid escape pipes, crestal normal faults, thrust faults) and widespread inflationary-type overpressure. In the Central Basin the near surface environment is water limited. Mechanical and chemical compaction led to moderate overpressure development above the Upper Red Formation evaporites. Only below thick Early Miocene evaporites have near lithostatic overpressures developed in carbonates and marls affected by a wide range of overpressure mechanisms. Fluid leakage episodes across the evaporites have either been very few or absent in most areas. Locations where leakage can episodically occur (e.g. detaching thrusts, deep normal faults, salt welds) are sparse. However, in both Iran and Brunei crestal normal faults play an important role in the transmission of fluids in the upper regions of folds.
The structural characterization of fractures is crucial to understand the processes of fluid flow in tight reservoirs. This contribution focuses on the role played by vertical fractures on the permo-porosity properties of the Aptian tight carbonate sequence of the Crato Formation, Araripe Basin (NE Brazil). The study performed a structural analysis in two different scales: reservoir scale (approximately 19.000 m2) and outcrop scale (approximately 125 m2), focusing on the fracture networks in lacustrine laminates, which have been investigated as an analogue of carbonate facies observed within the pre-salt reservoir sequence of the marginal basins in Brazil. This study employed a combination of systematic outcrop-based fracture characterization involving digital outcrop models and mechanical stratigraphy analysis. To evaluate the influence of vertical fracture systems in the fracture porosity and the equivalent permeability we performed DFN (Discrete Fracture Network) models. We focused on vertical calcite-filled fractures, oriented in two principal directions: set 1 NNW-SSE and set 2 NE-SW. Each set shows different vertical linkage patterns due to the influence of the mechanical intervals. Results of the outcrop scale model show similar behaviour for both fracture porosity and equivalent permeability, indicating that storage capacity and fluid flow may not be affected by vertical linkage of fractures. The reservoir scale DFN models show that the fracture porosity was greater in models which consider through-going continuous fractures than in models that consider the segmentation (discontinuity) of vertical fractures. Considering the vertical connectivity of fractures, the equivalent horizontal permeability (Kxx and Kyy) showed similar values in both scale models. This implies that vertical segmentation of fractures does not impact fluid flow in horizontal directions. The calculated values of equivalent vertical fracture permeability (Kzz) at reservoir scale are one order of magnitude higher in the DFNs that consider continuous fractures. Our results suggested that vertically continuous fractures enhance preferential flow pathways allowing greater vertical fluid flow than segmented fracture networks in tight carbonate reservoirs.
L’objectif des travaux de recherche menés dans cette thèse est de mieux comprendre les facteurs sédimentaires, diagénétiques et structuraux à l’origine de l’hétérogénéité des systèmes carbonatés réservoirs et de leur propriétés pétrophysiques en contexte de plateforme et de la transition plateforme-bassin. Pour ce faire, les séries sédimentaires de la plateforme carbonatée du Crétacé Supérieur de Kruja et du Basin Ionien adjacent en Albanie ont été choisis et présentent un bon exemple d’analogue terrain de réservoir carbonaté. Les dépôts carbonatés gravitaires de pente et de bassin de ce système forment des réservoirs pétroliers exploités en Albanie, en Grèce ainsi qu'au large de l'Italie. Les analyses sédimentologiques ont été menées sur trois localités dans le centre et le sud de l’Albanie: (i) massif de Krujë-Dajt, (ii) Çorovodë et (iii) Kremenara, dans lesquelles s’affleurent les successions carbonatées du Crétacé Supérieur de plateforme et de bassin. Douze faciès sédimentaires (5 faciès dans les coupes de Çorovodë; 7 faciès dans le massif de Krujë-Dajt) ont été identifiés le long des successions stratigraphiques de plateforme reflétant des environnements de dépôts représentant des conditions allant de subtidal passant par la zone intertidal jusqu’à l’environnement supratidal avec des périodes d’expositions subaériennes. Les faciès identifiés sont intégrés dans des séquences métriques (small-scale sequences) suggérant une cyclicité conduite par des fluctuations haute-fréquence du niveau marin relatif durant le Crétacé Supérieur. Dans le bassin, la période Campanien est caractérisée par d’épais intervalles de dépôts gravitaires calciturbiditiques. Cette succession de dépôts gravitaires montre une épaisseur de ~ 160 m, constitués essentiellement de séquences calciturbiditiques à basse et à forte énergie intercalées avec des dépôts de débris flow représentant une phase stable de système de sédimentation de plateforme-bassin. Au Maastrichtien, une inversion s’opère de la dynamique de sédimentation représentée par trois intervalles de slump résultants du démantèlement de la bordure de plateforme reflétant une déstabilisation de la pente durant cette période.L'étude diagénétique des faciès de plateforme peu profonds des coupes de Çorovodë a révélé que la succession stratigraphique est impactée par une diagenèse marine représentée par de ciments précoces, micritiques, fibreux, aciculaires et aussi en frange, formés dans la zone phréatique marine. Ces ciments sont responsables de la destruction de la porosité primaire intergranulaire et intraparticulaire dans ces faciès. La majorité des ciments identifiés sont liés à des surfaces spécifiques de type « hardground » qui ont été mis en évidence dans plusieurs intervalles le long de la succession étudiée. En revanche, dans le massif de Krujë-Dajt, des ciments calcitiques d’origine marine, d’origine météorique et des calcites tardives ont été identifiés dans la coupe étudiée.
A variety of tectonic processes spread along the circum‐mediterranean orogenic belts driven by the convergence of major plates, episodes of slab retreat and lateral and vertical mantle flows. Here, we provide an updated view of crustal stress and strain‐rate fields for the Albanides belt in the eastern Adria‐Eurasia convergence boundary. We framed a new geodetic‐based source model for the 2019 Mw6.4 Durrёs earthquake in light of the regional deformation, propending for a transpressional west‐dipping seismogenic fault. Our results highlight a fault‐scale complexity which mirrors the long‐time scale deformation of the Albanides plate boundary, where the rotation induced by the fast Hellenic rollback is accommodated also by transpression on inherited structures.
Ordovician limestone and Cambrian dolomite are the main oil and gas producing layers in eastern Tazhong field, Tarim basin. Industrial oil flow has been obtained from drilling in this zone, proving that the Lower Paleozoic strata is important for oil and gas exploration and development. However, the main controlling factors and distribution of high-quality reservoirs remain unclear. Based on the logging constrained seismic inversion , FMI (Fullbore Formation MicroImager) log interpretation and core analysis of six wells, the main controlling factors of Paleozoic carbonate reservoirs in the study area were investigated. The results reveal that the two buried cuestas control the distribution of the Cambrian and Ordovician strata, the ratio of mudstone in north buried cuesta (NBC) is more than that in south buried cuesta (SBC), and the average porosity of 3xq dolomite formation is more than that of O1−2y limestone formation. FMI logging interpretation results show that fracture density of well TZ-1 is significantly higher than that of well TZ-2, combined with core observation and microscopic analysis we can believe that reconstructed dolomite reservoir has developed dissolution pores and vugs. By the means of seismic impedance reconstruction, the most favorable production areas, so-called sweet-spots, are karst residual mounds that formed independent small anticlinal high points and have been depicted. Finally, total 28 traps in the Cambrian–Ordovician carbonate buried hill are characterized, covering an area of 66.4 km². 17 anticline traps with an area of 26.9 km² in dolomite reservoirs are optimized to be the next exploration targets.
We documented the deformation in the southeastern domain of the Majella anticline (Central Apennines, Italy) to highlight timing and structural characteristics of different fracture sets affecting the outcropping Cretaceous-Miocene ramp carbonates. An isolated and inactive hypogene karst system produced by sulfuric acid (Cavallone-Bove cave system) was studied following a multidisciplinary approach. Our findings suggest that deep-rooted, sub-vertical strike-slip fault zones reaching the H2S source rocks were the main vehicle for ascending acidic fluid flow. Linkage and intersection of these faults by splays in extensional stepovers and pre-orogenic normal faults permitted ascending fluids to reach multiple recharge points (feeders) near the paleo water-table. In proximity to the oxygenated groundwater, where H2SO4 was produced, lateral dissolution focused along bedding planes and zones of localized deformation (fracture clusters) characterizing the hinge of the anticline. We conclude that structural position in the anticline and large-offset, vertically extended strike-slip fault zones control the localization of efficient permeability pathways and represent first order controlling features for fluid flow in the fold-and-thrust belt. This study provides insights into the understanding of time-space evolution, geometry, and pattern of sulfuric hypogene karst systems in folded carbonates, whose prediction is critical for fractured and karstified reservoirs.
To fully characterise the behaviour of carbonate rocks in the subsurface it is important to understand their textural heterogeneity, and how their textures may be modified by faulting. A number of fault zones were investigated in detail, firstly analysing the microstructural, petrophysical as well as mechanical properties of the host rocks. Secondly, describing the fault zone architectures by mapping fault rock distributions and fracture patterns. Lastly, correlating the deformation mechanisms forming the faults to the initial rock properties and the stress conditions during faulting. Moreover, triaxial laboratory deformation was performed on a large number of host rock samples covering all carbonate rock types, as well as the whole range of porosities (<1-52%). Deformation mechanisms that resulted in sample’s failure were studied in order to compare them with the naturally-occurring deformation. Moreover, permeability changes were investigated induced both by natural faulting and laboratory deformation. The results proved to be comparable, and showed that simplified rules may be derived in terms of predicting hydraulic properties of deformed carbonates. For instance, permeability generally seems to decrease due to deformation for carbonates with porosity >10%, and may be either increased or decreased for lower porosity samples. Higher porosity (>10%) carbonates fail due to distributed or localized cataclastic flow or focused damage around the macropores, resulting in porosity reduction. Lower porosity (<10%) carbonates fail in a brittle manner due to brecciation and transitional- or brittle- shearing, leading to porosity increase. Significant reduction in permeability, however, may only be produced by diagenetic processes, such as recrystallization and cementation, or very high-strains, which are able to create fine-grained cataclasites. However, even though these fault rocks gain very low permeability, they become prone to brittle deformation. Therefore, these potentially sealing fault rocks may be cut by open fractures if were subjected to further faulting or uplift, and hence, while creating permeability anisotropy in the reservoir, they may not form good seals. Nevertheless, several fault examples in this study showed fracture blunting at the surface of the fault rocks suggesting that fault sealing is possible both in highly-porous and very tight carbonates.
Outcrops provide crucial information that can be used for the improvement of subsurface geological modeling. The present work provides a great example of such an application from a study on two well-known carbonate reservoirs uniquely exposed in the Kuh-e-Surmeh region, southwest Iran. Surface geology data were collected through extensive field study while the equivalent subsurface geology was reviewed based on bore reports available from the nearby giant gas-fields. Moreover, Schmidt hammer data were collected to investigate the variation in outcrop's uniaxial strength and detailed fracture analysis was carried out using Unmanned Aerial Vehicle (UAV) photography and direct field observations in selected stations. At the end, dolomite and limestone were identified as the dominant lithology of outcrops, greater apparent fracture intensity values were estimated for Kangan formation (P21 = 0.46) and the base of upper Dalan formation (P21 = 0.48). Moreover, fracture density (P20) was shown to be directly related to the formation uniaxial strength and the height of fractures was revealed to be inversely proportional to the thickness of sublayers. At the end, the collected surface and subsurface data were found to be in a general good agreement in terms of the stratigraphic location of fractured zones and mineralogy data, specifically, for the case of upper Dalan formation.
Low-porosity carbonates in San Vito lo Capo underwent two episodes of faulting: extensional faulting in the late Miocene and strike-slip faulting in the Plio-Pleistocene. Limestones and dolostones reacted differently to these faulting events. The first extensional faulting event within the limestone formed localized faults with a single fault core surrounded by a damage zone. The fault rock was either dolomitized as deformation proceeded or resealed by calcite cement after brecciation. The dolomitized fault rocks were reactivated during the following strike-slip faulting, forming polyphase breccias. On the other hand, strain was distributed in dolostones throughout a wider area during the extensional faulting forming a pulverized fault zone. The pulverized rock experienced cataclastic deformation during the following strike-slip faulting forming anastomosing networks of cataclastic shear bands. Fault cores hosted in the limestone appear to have acted as flow conduits until they were cemented or hardened due to evolving cataclasis. The cataclastic shear bands in the dolostone are likely to form baffles to flow, at least on a local scale. The fracture spacing in the damage zone also varies significantly between the lithologies. In particular, damage zones in the limestone have a 5–10 cm fracture spacing whereas fracture spacing is 0.5–3 cm in the dolostone. It is likely that the differing mechanical and chemical properties of the dolostone and limestone were responsible for creating contrasting fault zone architectures.
The distribution pattern of fractures in the Carboniferous volcanic rocks controls the oil and gas distribution in the Xiquan area located in the eastern Junggar Basin. This paper describes the types, characteristics, distribution of fractures in the volcanic rocks, and the evolutionary history of fractures, as well as the influence of fractures on the reservoir distribution in this area, based on analyzing cores, casting thin sections, imaging logging data, the measurement of the physical properties and integrated with the production testing. The lithology of these Carboniferous volcanic rocks is primarily andesite and volcanic breccia. Basalt, dacite, tuff, breccia lava, tuffaceous sandstone, and tuffite were also present. The fracture types are mainly shrinkage fractures, weathered fractures, dissolution fractures, and structural fractures, the majority of which are structural fractures and weathered fractures. The distribution of fractures has obvious zonation characteristics in the vertical direction. Fractures are mainly distributed at four intervals under the top surface of the Carboniferous unconformity: 0–50 m, 65–115 m, 125–155 m, and 170–210 m. There are mostly weathered fractures and dissolution fractures in the 0–50-m interval. The physical properties and oil production of andesite are the highest, and weathering fractures and dissolution fractures play a major role in controlling the secondary pores of volcanic lava and the formation of favorable reservoirs in this interval. The structural fractures were developed in the intervals of 65–115 m, 125–155 m, and 170–210 m. The physical properties and oil production of volcanic breccia are the highest, and therefore, structural fractures mainly control the formation of favorable reservoir in volcanic breccia in these three intervals. The evolution of fractures in the Carboniferous volcanic rocks experienced four phases: the condensation and shrinkage fracture stage, which developed the primary pores in the middle-late Carboniferous period; the weathered fracture and dissolution fracture stage, which greatly increased the porosity in the late Carboniferous period to the early-middle Permian period; the structural fracture and weathered fracture stage, which greatly increased the porosity and permeability of the reservoir from the end of the Triassic to the early Cretaceous period; and the filled fracture and dissolution fracture stage, which can also greatly increase the porosity of the volcanic reservoir and occurred wholly within the late Cretaceous period until the present.
Classification and well-logging evaluation of carbonate reservoir rock is very difficult. On one side, there are many reservoir pore spaces developed in carbonate reservoirs, including large karst caves, dissolved pores, fractures, intergranular dissolved pores, intragranular dissolved pores, and micropores. On the other side, conventional well-logging response characteristics of the various pore systems can be similar, making it difficult to identify the type of pore systems. We have developed a new reservoir rock-type characterization workflow. First, outcrop observations, cores, well logs, and multiscale data were used to clarify the carbonate reservoir types in the Ordovician carbonates of the Tahe Oilfield. Three reservoir rock types were divided based on outcrop, core observation, and thin section analysis. Microscopic and macroscopic characteristics of various rock types and their corresponding well-log responses were evaluated. Second, conventional well-log data were decomposed into multiple band sets of intrinsic mode functions using empirical mode decomposition method. The energy entropy of each log curve was then investigated. Based on the decomposition results, the characteristics of each reservoir type were summarized. Finally, by using the Fisher discriminant, the rock types of the carbonate reservoirs could be identified reliably. Comparing with conventional rock type identification methods based on conventional well-log responses only, the new workflow proposed in this paper can effectively cluster data within each rock types and increase the accuracy of reservoir type-based hydrocarbon production prediction. The workflow was applied to 213 reservoir intervals from 146 wells in the Tahe Oilfield. The results can improve the accuracy of oil-production interval prediction using well logs over conventional methods.
Albania has historically been known to have an active but challenging drilling activity that demands the most innovative technology to develop, predominantly, medium-heavy oil reservoirs. Although recent efforts have been made by the government to stimulate and expand the largest onshore European oilfield, technical and economical obstacles are prevalent. These obstacles make it difficult to fully develop reliable and profitable hydrocarbon bearing zones in a downturn economy, especially since Albanian oil can be costly to produce and refine. Due to these typical issues that affect many local energy sectors, many developed countries diversify their energy production in order to avoid strict dependency on crude oil. An emblematic and modern option that is extensively gaining popularity in Europe focuses on renewable energy from sophisticated recycling programs. All though Albania is a relatively "green" country when it pertains to its electricity production (97% hydropower and 3% fossil fuels), it has yet to develop energy-recycling programs that it can salvage for self-sustainable energy sources. The past years have seen a conscious revitalization and stimulation in the mentality of green economy in Albania. But, in comparison to the rest of "western Europe" that are leading world examples in efficient recycling, it is significantly lagging behind with initial strides just now focusing on aligning national legislations with current EU models. Furthermore, two crucial reasons that should motivate Albania to investigate new applications for energy recycling are (1) alternatives to crude oil and petroleum products that can be supplemental and provide stable access to fossil fuels (2) industrial and municipal recycling via waste management to reprocess waste and produce industrial raw material-spawning the emergence of a "circular economy" to develop the backbone needed to strengthen the industrial and manufacturing markets for a self-sustaining economy. Accordingly, in this paper, the topic that will be addressed, given the recent decrease in oil & gas prices, focuses on the Albanian energy sector's capability to sustain and develop a supplementary recycling program via "waste-to-fuel" (WTF) technology (biofuels and/or inorganic waste). With the intent that it could function cooperatively with Albania's active drilling program in order to mitigate dependency on a single fuel source and produce enough fossil fuel in an effective and sustainable manner.
Yuanba gas field of Sichuan Basin is characterized by weak-tectonic-deformation, and the fractures made significant effect on oil and gas migration. This paper investigated the types, evolution of fractures and their relationship with oil and gas migration of Permian Changxing Formation of the Yuanba gas field according to geological and geochemical analyses. Four genetic types of fractures with different occurrences were distinguished: intercrystal fractures, pressure solution fractures (stylolites), structural fractures, and overpressure fractures. In which, intercrystal fractures present multiple straight edges between the dolomite particles and no fixed direction. Pressure solution fractures are in the form of zigzags. Structural fractures have straight edges and a certain direction and are usually distributed in groups. Overpressure fractures have irregular shapes and irregular directions and occur in multiple quantity. According to the comprehensive analyses, the intercrystal fractures, pressure solution fractures, and structural fractures formed in shallow-intermediate burial environments during the Early Yanshanian period and acted as paleo-oil migration pathways; thus, paleo-oil is widely distributed in the Permian Changxing Formation reservoir, as indicated by the presence of pyrobitumen. The overpressure and structural fractures formed in deep burial environments during the Late Yanshanian and Himalayan periods and served as the dominant pathways for the migration and remigration of cracked gas. This study is significant for understanding the formation of oil and gas fields in the weak-tectonic-deformation zone.
In contrast to faults in clastic reservoirs, rules to predict the exploration and production timescale fault-seal potential in carbonates are lacking. This paper provides a summary of carbonate reservoirs with cross-fault column height differences that represent examples of apparent static fault seal, and a summary of observed examples of dynamic fault seal in carbonate reservoirs and aquifers. These include cross-fault differences in water columns across carbonate–carbonate juxtapositions, cross-fault pressure differences in carbonate aquifers separated by faults, production-induced cross-fault pressure differences in carbonate hydrocarbon reservoirs, sealing behaviour of faults in carbonate reservoirs inferred from well tests, and examples of low fault transmissibilities from history-matching exercises from carbonate reservoirs. This paper also documents the range of compositions of fault rocks in carbonates and the range of permeabilities that have been reported from low-permeability fault cores in carbonate fault zones, as well as the implications of the observed range of fault permeabilities in carbonates for sealing behaviours. The purpose of this paper is not to argue that every fault in a carbonate reservoir will seal or will even be capable of sealing. There are, however, enough examples of faults in carbonates that are sealing in a dynamic sense, and in a static sense, that the topic of carbonate fault seal should warrant much more study. Creation of predictive models will ultimately require a considerable amount of subsurface data, but these models should be created.
Fractured carbonate reservoirs are commonly extremely heterogeneous. This heterogeneity is caused by fracturing at multi scales superimposed on inherent textures from deposition and diagenesis. In addition, there is an interrelationship between diagenesis and fracture formation, e.g. the matrix properties may change due to reactions with fluids provided by the fracture network or fractures are filled with calcite cement. The aim of this study is first to describe individual fractures and the fracture network of some typical fractured carbonate reservoirs, and secondly to discuss their importance for fluid flow in the reservoir. Finally, it will be outlined how these findings should be incorporated into static and dynamic reservoir models. Traditionally fluid flow in fractured reservoirs is simulated using dual porosity or dual porosity/dual permeability formulation based on the work of Warren and Root (1963). Each cell in the simulation models is typically 100*100*10m and characterized by one value of fracture permeability (tensor) and one fracture porosity. The matrix-fracture fluid exchange is described using a transfer function which is commonly regarded to be proportional via shape factor to the block-size, which is kept constant within one simulation cell. However, this simplistic image of a fractured reservoir only partly captures the internal fracture features and fracture network geometry as observed in core or outcrop analogues of reservoir rocks. Firstly, the majority of fractures in core are only partly open. This implies that flow within fractures will tend to be channelized instead of fissure type flow. Furthermore, the parts of the fracture which are open do show a variable amount of calcite cement between the matrix and the open void. The calcite cement on the fracture planes inhibits transfer of fluid between matrix and fracture. This implies that the fracture matrix transfer function is not only controlled by the geometry of the matrix blocks, but also by the area of the fracture plane, adjacent to the matrix block, which is not coated with cement. Finally, the fracture fill form bridges between the two sides of the fracture will keep the fractures open during pressure depletion in the reservoir. Layering and bed parallel stylolites do also have a significant effect on fracture network geometry as fractures commonly terminates against these features. The combination of stylolites and open fractures do result in complicated flow patterns as demonstrated by use of CT-scan monitoring of gas flooding experiments of core samples (Wennberg et al 2009) Outcrop studies in the Zagros of SW Iran show that several fracture sets coexist in anticlines which are outcrop analogues to hydrocarbon fields in the area. The fractures generate very complex fracture network geometries and four distinct fracture sets have been recognized. Set A and B are parallel and perpendicular to the fold axis respectively. Set C and D are oblique and symmetric to the fold axis. The presence of more than 2 fracture sets implies that the matrix block in general has a non-rectangular shape. It is well known that fracture spacing is dependent on the mechanical stratigraphy, i.e. the spatial variation in mechanical properties due to changes in lithology or other matrix properties. The mechanical stratigraphy is controlled by the depositional environment and subsequent diagenesis. In this study the distribution of fracture spacing have been investigated using the coefficient of variance (Cv) which is the standard deviation divided by the average fracture spacing (Gillespie et al. 2001). Cv equals 0 for a regular spacing, Cv equals 1 for random spacing distribution and Cv > 1 implies clustering of fractures. Fracture spacing investigated in Cretaceous and Tertiary outcrops of reservoir analogues is not regular as shown by a coefficient of variance ranging between 0.4 and 2. The majority of Cv values are close to 1 i.e. the fracture spacing do dominantly have a random distribution. Hence, both the shape and the size of matrix blocks do indeed have large variation within a rock volume equivalent of a simulation model cell. Both the distribution of cement within a fracture and the fracture spacing characteristics described above will have an important impact on the fluid flow and recoverable reserves in fractured reservoirs. Hence, these characteristics should be taken into account when building reservoir models and when analysing the results of the simulation model. Furthermore, the observations above demonstrate the importance of an integrated multi-disciplinary study of fractures characterization and modelling. Improving static modelling, followed by dynamic simulation, of naturally fractured reservoirs, requires involvement of all disciplines including sedimentology, diagenesis, structural geology, petrophysics, geophysics, well test analysis, core analysis and reservoir engineering as well as drilling and production engineering.
References:
Gillespie, P.A., Walsh, J.J., Watterson, J., Bonson, C.G. and Manzocchi, T. 2001. Scaling relationships of joints and vein arrays from The Burren Co, Clare, Ireland. Journal of Structural Geology 23, 183-201.
Warren, J.E and Root, P.J. 1963. The behaviour of naturally fractured reservoirs. SPE-426-PA.
Wennberg, O.P., Rennan, L., Basquet, R. 2009. CT-Scan imaging of natural open fractures in a porous rock geometry and fluid flow. Gephysical Prospecting 49, 239-249.
Even though evaporites constitute less than 2 % of the world’s sedimentary rocks, one-half of the world’s largest oilfields are sealed by evaporites, the other half are sealed by shales (Fig. 10.1; Grunau 1987). Kirkland and Evans (1981) argued that evaporites overlie or seal carbonates containing an estimated 50 % of the world’s known total petroleum reserve. Of the world’s 25 largest gas fields, nine are sealed by evaporites and sixteen by shales and hydrates. Sixteen are capped by Mesozoic seals, 7 by Palaeozoic seals, and only two by Tertiary seals. Fourteen are in the 1,000–2,000 m seal-depth interval, nine in the 2,000–3,000 m interval; and two in the 0–1,000 m interval. As one would expect, more gas fields than oilfields are sealed by Palaeozoic caprocks, and more oilfields than gas fields are sealed by Tertiary caprocks. Surprisingly, the seal depth intervals for the 25 largest oil and gas fields do not differ significantly. However, Grunau argues many “supergiant” gas accumulations below depths of 3,000 m and have either not yet been discovered, or have not yet been put in production (e.g. much of the Khuff-hosted gas in North Dome in the Middle East).
Permeability in fractured carbonate reservoirs is very heterogeneous due to fracturing at different scales superimposed on inherent textures from deposition and diagenesis. Observations of fractures in core and outcrop indicate that flow in open fractures in carbonate rock tends to be channelled rather than through fissures. Most of the flow takes place along a few dominating channels in the fracture plane, whereas most of the fracture plane is not effective for fluid flow. The formation of flow channels is caused by a combination of mechanical and, in particular, diagenetic processes. Single extension fractures occur as partly open or vuggy fractures, and their hydraulic properties are controlled by dissolution and cementation. Single shear fractures are typically open at local steps in the fault plane controlled by shearing along irregular fracture surfaces. Fault damage zones tend to be concentrated at fault tips, intersections, pull-aparts and overlap zones that represent areas of dilation. These damage zones represent elongated features in three dimensions with a high fracture density that will result in channelled flow at reservoir scales. The effect of channelled flow should be taken into account during evaluation of fractured carbonate reservoirs and when building dynamic flow models.
Tectonic stress is one of the comparatively active energies in the earth's crust. It is of guiding significance in oil and gas exploration and development to study tectonic stress patterns and transformation processes. Through studies on upright folds, conjugate joints, slickenside on fracture planes and other structural vestiges, we rebuilt the Late Mesozoic-Cenozoic tectonic stress field of Xuanhan-Daxian region and examined regular patterns of its tectonic activities. The result shows that this region experienced at least two tectogeneses in Late Mesozoic-Cenozoic, one occurred during Late Yanshanian, when a series of NE-direction faults and folds developed in Xuanhan region of east Sichuan under the SE to NW direction compression; and the other happened in Himalayan period, when the east Sichuan tectonic belt subsided gradually while NW-direction faults and folds were formed by the NE to SW direction extrusion in Daba mountain. The superposition of various tectonic stress fields with different directions and intensities in different periods comprehensively controlled the formation and evolution of oil and gas reservoirs of this region. The superimposition of staggered tectonic activities in the study area has improved reservoir properties, substantially controlled the development of hydrocarbon traps and adjusted lithologic reservoirs into structural-lithologic composite ones. Moreover, tectonic stress and fractures formed consequently in rocks have provided oil and gas migration with dynamic power and pathways.
Based on the analysis of 3D seismic date and regional geology data, lots of NW and NWW trending compression faults and NE trending strike-slip faults, but with less exlensional faults, are identified in the Central uplift of the Tarim craton, which has the main characters of diversity of styles, multiple epochs and inheritance of evolution, and segmentation along strike. Most of the compression faults are detachment thrusts slipped in Middle Cambrian salt layer but not basement involved as previous cognition, which formed in the end of Early Ordovician with 4 kinds of fault styles, and had transpression effects and segmentation along the fault belt. Many strike-slip faults are discovered in the northern slope of the Central uplift by the 3D seismic data in recent years. There were tulip flower structure, upstanding structure and positive flower structure in profiles, and en echelon structure, feathered structure, brush structure, and so on, converged to southwest in plane. NE-trending sinistral strike-slip faults were developed by the intensive three stages of oblique collision from the south Tarim plate boundary in Silurian, Middle Devonian and Late Permian. The fault system, mostly distributed in Palaeozoic, can be divided into 3 types and 3 levels and 4 groups of orientation, which controlled the structure framework and shape of the Central uplift with layering in vertical and the zoning from north to south and blocking from west to east. The fault system of the Central uplift undergone 4 stages with 9 periods' evolution; extensive extension-weak compression stage in Neoprolerozoic, local weak extension period in Cambrian to Early Ordovician-powerful thrusting period in Middle-Late Ordovician, strike-slip fault developed stage in Silurian-Middle Devonian, and partly inherited faulting stage in Carboniferous-Early Triassic. It is obvious that the fault system played important roles in the hydrocarbon accumulation of Lower Palaeozoic marine carbonate in the Central uplift. It is not only the oil and gas composite accumulation in vertical, but also the favorable reservoirs and enriching hydrocarbon controlled by faults. Different type and stage faults had varied effects which caused the segmentation of hydrocarbon distribution along the fault bell. The migration-accumulation system consisted by hydrocarbon source fault and adjacent reservoir controlled the fluid orderly distribution.
Low-temperature deformation is characterized by heterogeneous strain in which the bulk of the material clearly retains its primary texture. Deformation is by grain-scale crystal plasticity, rotation, fracture, and pressure solution, and by transgranular mechanisms that crosscut numerous grains. The important low-temperature crystal-plastic features are twin lamellae, deformation bands, and undulatory extinction. Strain magnitudes and stress and strain tensor orientations can be determined with a variety of methods that are based on mechanical twins, platy grain orientation, grain center distribution, and fault geometry and slip directions. Different deformation mechanism associations, expressed by the partitioning of the total strain into different mechanisms, are related to the ductility and environment of deformation. Deformation fronts separating different mechanism associations are defined on the basis of changes in the crystal-plastic component of strain. -from Author
Conjugate strike-slip fault zones that cover metre-scale areas at Beadnell, Northumbria, and Kilve, Somerset, were initiated as conjugate vein arrays. Early conjugate faults are linked by the propagation of one fault that eventually by-passes the other fault. A model for the development of strike-slip faults is presented, using fault and vein geometries and the position of damage zones with respect to the master faults as an indication of the propagation direction. This model includes the evolution of networks from (1) the initial random development of vein arrays, to (2) the isolated development of several unconnected conjugate fault segments that pass into vein arrays, through (3) the intersection of a conjugate set of master faults and linkage with minor antithetic faults, and the formation of new vein arrays with extensional geometries after a linked network of faults is established, to (4) breaching of intersection points by dominant faults, and finally (5) the propagation towards oversteps that are breached to form a through-going fault. The geometry of the active structures simplifies with time, as strain is localised along the master fault, but the complexities are preserved in the fault walls.
This paper is included in the Special Publication entitled 'Faulting, fault sealing and fluid flow in hydrocarbon reservoirs', edited by G.Jones, Q.J. Fisher and R.J. Knipe. Pressure solution plays a major role in fault initiation and development in carbonate rocks. Strike-slip faults in the Triassic and Jurassic limestones of Somerset, UK, initiated as en echelon extension fractures, which became linked by pressure solution seams. Shear occurred along the pressure solution seams as the bridges between the veins rotated. The linked vein-pressure solution seam systems developed into pull- aparts and eventually into through-going faults. Normal fault planes in the Cretaceous Chalk at Flamborough Head, Yorkshire, UK, commonly have the pitted appearance of slickolites. Phyllosilicates are often concentrated along faults in Chalk, the thickness of the phyllosilicate gouge being proportional to fault displacement. The enrichment of phyllosilicates along the faults is due to pressure solution rather than simply to phyllosilicate smear. Pressure solution can be concentrated at the contractional quadrants of faults, particularly where there is a contractional overstep onto an adjacent fault. Metre-scale oversteps between strike-slip faults in Somerset often have pressure solution seams, while contractional oversteps and bends at Flamborough Head are accommodated by compaction of beds, with pressure solution apparently being important. The concentration of phyllosilicates by pressure solution can hinder fluid flow along and across faults in carbonate rocks. A high density of pressure solution seams between overstepping faults can effectively link these faults, increasing both the effective length of the barrier to fluid flow and the fault seal potential.
Static stress fields impose a permeability anisotropy from stress-controlled features such as faults, extension fractures, and stylolites which, depending on the tectonic regime, may enhance, or counteract existing anisotropic permeability in layered rock sequences. Textural evidence from hydrothermal veins suggests, however, that fluid flow in fault-related fracture systems generally occurs episodically and that dynamic stress cycling effects are widespread. In the vicinity of active faults that undergo intermittent rupturing, permeability and fluid flux may be tied to the earthquake cycle through a range of mechanisms, leading to complex interactions between stress cycling, the creation and destruction of permeability, and fluid flow. -from Author
The strata of the N Umbrian Apennine fold belt are cut by an array of mesoscopic faults that generally display strike- or oblique-slip offset. The unusual orientation pattern of mesoscopic faults of the study area may indicate that the mechanics of initiating faults in rocks undergoing pressure-solution deformation is different from that in rocks undergoing purely brittle deformation. Alternatively, the fault pattern may indicate that the faults represent slip on pre-existing fractures. If this latter situation is true, the geometry of the fault array may merely reflect the geometry of the pre-existing joint array.-from Authors
We propose that discrete solution surfaces originate at stress concentrations and propagate through rock as anticracks. As material is dissolved and removed, the anticrack walls move toward each other; stress and displacement fields are identical to those for the conventional opening crack, but with a change of sign. Observations of entire traces of solution surfaces are consistent with the anticrack concept: (1) the surfaces are bounded in extent; (2) the dissolved thickness varies from a maximum near the center to zero at the tips; and (3) the maximum dissolved thickness is proportional to the length of the surface. Local dissolution and in-plane propagation are suggested by the large isotropic compressive stress at the anticrack tip. Propagating solution surfaces will interact to form a regular array corresponding to some bulk strain rate. Anticracks may also interact with opening and shear cracks; observations of interacting solution surfaces, veins, and faults illustrate these configurations. Intersecting arrays of cracks, anticracks, and shear cracks operate to yield a mode of bulk deformation similar to diffusion-accommodated grain-boundary sliding in polycrystalline solids. *Present address: Center for Tectonophysics, Texas A&M University, College Station, Texas 77843
Spaced cleavage formed by rock dissolution can represent major amounts of shortening parallel to bedding; much so-called fracture cleavage is of this origin. We classify the solution cleavage developed in Mesozoic pelagic limestones of the Umbrian Apennines into four intensity types (weak, moderate, strong, very strong) on the basis of qualitative attributes and mean spacing of cleavage surfaces. Shortening can be determined from imbricated chert beds and reaches 50% in rocks with very strong cleavage. In the Umbrian Apennines, solution cleavage is commonly associated with detachment thrusts. We describe an example in which the dissolution mechanism "damaged" the rock beneath a thrust by creating closely spaced discontinuities; fragments bounded by these discontinuities were torn up and incorporated in a nearly chaotic shear zone as the thrust sheet moved forward.
A hand-specimen-size buckle fold from a fault zone in the Appalachian Plateau of New York shows a fanning spaced cleavage attributed to pressure solution because of the truncation of fossils and the presence of residual material in the cleavage laminae. Apparent slip on the cleavage is interpreted to be the result of the removal of material along laminae oblique to bedding. The fold is cut along the cleavage and partially unfolded by rigid-body rotation of the segments, thereby eliminating the offsets of bedding without slip parallel to the cleavage. Field evidence suggests that the material removed from the cleavage laminae is present locally as cement.
Well-exposed strike-slip faults in limestones in the north-western part of Gozo show damage zones that can be grouped into three categories based on their location along faults; tip damage, linking damage and distributed damage. The predominant fracture types within damage zones include extension fractures and secondary faults. Tip damage zones usually show wedge-shaped patterns formed by antithetic faults and extension fractures, commonly accompanied by block rotation. Several fractures are combined at linking damage zones, typically with the concentration of a high intensity of fractures. Structures in distributed damage zones are typically similar to the classical Riedel shear pattern. Evolutionary and 3D models are proposed in terms of the geometries of damage zones for small displacement strike-slip fault zones. Different evolutionary routes depend on fault tip modes and locations.
Fault zones focus fluid expulsion in the muddy northern Barbados Ridge accretionary prism with fault-parallel permeabilities about 1000 times greater than intergranular permeabilities in the adjacent sediment. In the Oregon prism the low bedding-perpendicular permeability (due to mudstones) inhibits intergranular dewatering; however, intergranular flow is concentrated where submarine erosion breaches high permeability sandy layers. Even so, faults can capture fluid flow from these exposed sandy layers suggesting the faults have a still higher permeability. Such observations coupled with laboratory measurements permeabilities suggest that faults off Oregon may have fault-parallel permeabilities at least 10-10000 times greater than the adjacent sediments. Results from Barbados and Oregon suggest fluid flow is concentrated along the most active faults. At the toe of prisms the fault zones are being progressively loaded by the thickening wedge and are undergoing compaction. Preliminary experiments show that permeability decreases relative to the surrounding wall rocks along faults within this compactive deformation regime; we believe that these faults must undergo dilation, perhaps linked to transient increases in pore pressure if they are to be preferential fluid conduits. Farther upslope erosion exposes rocks that are more consolidated, commonly more cemented, and generally of lower intergranular permeability than rocks of equivalent burial further seaward. Because of their lithification and overconsolidation these rocks dilate during faulting, locally enhancing fracture permeability. In such dilative regimes, faults become evermore focused zones of fluid expulsion relative to occluded intergranular pathways.
Bedding-parallel (BPV), strike (SV), and cross-fold (CFV) veins represent a sequence of polyphase fracturing during the development of a fault-bend fold in lower Paleozoic carbonate beds in the Appalachian Valley and Ridge province, Pennsylvania. Brecciation and layer-parallel shearing played important roles in the development of the earliest vein set (BPV) which propagated prior to folding. Fluid inclusions in calcite BPV are highly saline brines (23.4 wt% NaCI) trapped at conditions close to lithostatic (P ≤ 180 MPa, T ≤ 267 °C). While passing through the lower kink plane of a fault-bend fold, strike joints propagated in dolomitic beds located on the extensional side of neutral surfaces. Stylolitization of strike joint surfaces accompanied slip of the hanging wall up a ramp. Renewed extension upon passing through a second kink plane led to propagation of antitaxial SV along the stylolitized joints by the crack-seal process. Slightly less saline fluids (22.4 wt% NaCI) were trapped in SV at fluid pressures ≤144 MPa and temperatures ≤217 °C. With the concurrent formation of lateral ramps, the carbonates moved to the upper flat and were subjected to strike parallel extension as manifested by the propagation of antitaxial CFV. Due to further mixing of fresh waters, the salinity of fluids forming CFV decreased (20.5 wt% NaCl) with trapping conditions at P ≤ 116 MPa and T ≤ 179 °C. The fluid evolution path from BPV to CFV through SV shows a modest decrease in salinity with a sharp decrease in possible trapping pressures.
Vein arrays are often composed of pull-aparts which are linked by shear fractures, good examples of which occur in the Lower Jurassic limestones of Somerset, southwestern England. Such pull-apart arrays have displacement-distance characteristics which are similar to fault zones, with maximum displacement (indicated by the largest pull-apart widths) near the centre of the array, and with displacement decreasing towards the tips. Pull-apart arrays usually die out into enéchelon or pinnate veins. Evidence for pressure solution along the shear fractures which connect pull-aparts include their dark and braided nature, their obliquity to the displacement direction, the high dihedral angles (often > 90°) between conjugate shear fractures, and the dissolution of earlier structures. A range of geometries occurs, with varying relative amounts of veins and pressure solution being related to varying amounts of transtension or transpression. There is a general trend for an increase in the angle between vein segments and the shear fractures as contraction increases. There is therefore a trend for increased pressure solution on the shear fractures in more contractional arrays. The concentration of insoluble material along shear fractures has important implications for the mechanics and sealing of faults.
Two methods for measuring joint spacing are described and compared. The area method is a constant for a given outcrop area and is not affected by joint distribution within that area; in contrast, the line method depends on the location of the linear traverse. Two kinds of joint sets are distinguished on bedding surfaces: (1) a poorly-developed set represents the early stages of development when typical joint lengths are less than typical spacing; (2) a well-developed set represents later stages when lengths are much greater than spacing. The area method is applicable to both poorly- and well-developed sets, whereas the line method produces inconsistent results for poorly-developed joint sets. Surface textures on many joints indicate point fracture origins and propagation parallel to bedding, whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding. The laboratory experiments described in this paper do not suffer from these restrictions. An important concept, confirmed during these experiments, is fracture saturation. When the applied strain reaches a certain value, fracture spacing stops evolving and remains nearly constant: these fracture sets are well-developed. Spacing at saturation is a function of layer thickness but is independent of strain, whereas spacing before saturation varies strongly with applied strain. Thus, plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly-developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties. On the other hand, spacing may be a sensitive indicator of strain for layers with poorly-developed joint sets. Assessing fracture saturation is a lirst-order consideration when gathering spacing data.
The Bolivian Sub-Andean Zone (SAZ) corresponds to a Neogene thrust system that affects an about 10-km thick Palaeozoic to Neogene siliciclastic succession. The analysis of macro and microstructures and cement distribution in thrust fault zones shows that they are sealed by quartz at depths >∼3 km, due to local silica transfer by pressure-solution/precipitation activated at temperatures >70–90 °C. At shallower depths, faults have remained open and could be preferential drains for lateral flow of carbonate-bearing fluids, as shown by the occurrence of carbonate cements in fractures and their host-sandstone. Due to decreasing burial, resulting from foothill erosion during fault activity, critically buried fault segments can be affected by non-quartz-sealed structures that post-date initial quartz-sealed structures. The integration of textural, fluid inclusion and isotopic data shows that carbonates precipitated at shallow depth (<3 km), low temperature (<80 °C) and relatively late during the thrusting history. Isotopic data also show that precipitation occurred from the mixing of gravity-driven meteoric water with deeper formation water bearing carbonate carbon derived from the maturation of hydrocarbon source rocks (Silurian and Devonian shales). The combined microstructural and isotopic analyses indicate that: (i) fluid flow in fault zones often occurred with successive pulses derived from different or evolving sources and probably related to episodic fault activity, and (ii) at a large-scale, the faults have a low transverse permeability and they separate thrust sheets with different fluid histories.
The south-east Adriatic region contains four exploration provinces that are still in an immature state of exploration, the Apulian foreland, the external margin of Hellenides-Dinarides thrust belt offshore, the thrust belt onshore (where covered by thick, but deformed, Neogene deposits in Albania), and the inland coastal regions of south Albania and western Greece (where folds lack a Neogene cover). This is widespread evidence of the occurrence of source rocks and of their adequate maturation, especially in relation to overthrusting and to deep burial in the Neogene foredeep. Play types include palaeo-horst blocks in the foreland, biogenic gas in Mio-Pliocene sands, structural traps in Mesozoic-Eocene folds, and structural (and perhaps stratigraphical) traps in Neogene sands. -from Authors
Thrust-fault surfaces are interconnected with a thrust belt through a three-dimensional system of decollement flats, frontal ramps, and lateral connectors. Lateral connectors and transverse zones are synkinematic with respect to thrust-belt structures and are integral components of the overall kinematic plan of the thrust belt. Although some lateral connectors are non-systematically distributed within the allochthon, the systematic alignment of lateral connectors into transverse zones suggests some fundamental control on the location. Possible fundamental controls are 1) sub-thrust basement faults, 2) basement-rooted faults in the cover strata, 3) drape folds in the cover strata over basement faults, 4) stratigraphic variations in the decollement-host strata, and 5) a combination of stratigraphic variations and sub-thrust basement structures. -from Author
Monkshood anticline is a well-exposed surface anticline located in the Foothills in northeastern British Columbia. Extension fractures are well developed in the Prophet Formation (a carbonate-shale-chert sequence) throughout this fold. Virtually all of these fractures are filled with some combination of calcite and quartz cements. The fractures formed and the mineral cements were deposited during the fold development. Most fractures on Monkshood anticline have formed at high angles to bedding, and they commonly fall into two to three distinct orientation sets. The dominant fracture trend is aligned with the fold axis through the backlimb of the fold, but there is considerable variance in the dominant orientation in the forelimb. An order of magnitude variation is found in fracture densities across this fold, and over two orders of magnitude variation occur in mean fracture aperture. These variations in density and aperture do not correlate with particular structural positions. Fracture trace lengths exhibit power-law distribution patterns, suggesting fractal character. Fracture aperture displays a roughly linear correlation to volumetric fracture strain, but shows no consistent association with either density or trace length. Stochastic modeling of the fracture networks on Monkshood anticline suggests density is the primary factor affecting fracture network connectivity, hut fracture size also can play an important supporting role. The occurrence of a dominant fracture orientation set can impart a significant directional bias to connectivity. Measurement stations on Monkshood anticline that have both high fracture densities (for connectivity) and substantial fracture apertures (for conductance) occur primarily in midlimb positions.
Stylolite zones consist of an irregular seam and often a variety of associated fracture sets. The geometry of this zone can be used to determine paleostress directions. Of the fracture sets commonly observed in close proximity to stylolites, one is generally induced by the coring procedure (unloading fractures), and another is natural and possibly associated with stylolites of tectonic origin (so-called 'tension gashes'). Within a unit possessing abundant stylolites the stylolites are most common in zones of relativley low porosity. In addition, because of the relativley compact, poorly sorted, fine-grained nature of the seam itself, stylolite zones commonly have much reduced permeability compared to the underformed rock above and below. Laboratory-measured permeability associated with stylolite zones can, however, be misleadingly large owing to the presence of unloading fractures and/or tension gashes. These fractures may dramatically increase laboratory permeability but commonly are either not present in the reservoir or are relevant only to the size of the laboratory measurement sample. It is critical, therefore, to understand the origin and scale of associated fracture development in interpreting such data.-Author
La Paz oil field is one of the most famous, well-documented fractured reservoirs in the world. Since 1945 it has produced more than 830 million bbl of oil from both low porosity Cretaceous limestones and underlying granitic basement. The field is situated on a large inversion structure, partially uplifted in the late Eocene, having the majority of the inversion occurring in the Miocene-Pliocene. Fracture distributions, initial rate and cumulative production distributions, and trends in formation water chemistry all suggest that reservoir quality and reserves are controlled by natural fractures associated with Miocene-Pliocene and younger strike-slip faults and possibly by secondary microporosity related to the geometry of the earlier Eocene block uplift. Production levels within the area of Eocene uplift are anomalously high, with one well producing 59 million bbl of oil. Typical wells in other parts of the field produce 1 million bbl of oil. Analysis of the distributions cited previously suggests that production has occurred from highly elliptical to linear drainage areas surrounding faults. These restricted zones of fault-related fractures allow for communication of hydrocarbons stored in the low porosity rock carbonate matrix to the wellbore. In addition, diagenetic microporosity may be an important component in matrix storage in the carbonates and may be due to local subaerial exposure during the Eocene uplift. This article shows that analysis of the natural fracture system with respect to the production characteristics in even old fields can give rise to new reservoir models, leading to new infill locations within the field limits or additional exploration opportunities in the area.
Detailed mapping of the structures exposed in northeastern Tennessee reveals a complex deformation sequence consisting of the following stages: 1) early thrust faulting, 2) folding, 3) intermediate thrust faulting, 4) intermediate folding, 5) normal faulting, 6) strike-slip faulting, and 7) jointing. Each stage is composed of one or more structures. Once a structure formed, it may have remained active over several stages of deformation. Based on cross-cutting relationships among the structures and the transport directions determined from thrust faults, it appears that the deformation stages up to and including normal faulting (Stage 5) are consistent with a series of incremental deformation events driven by similar causative forces associated with the Alleghanian Orogeny. In particular, the results in this study reveal three nested fracture networks which were potential pathways for fluid migration during tectonic deformation and may still play a role in current fluid migration. -from Authors
Field studies of low-permeability siliceous shale units of the Monterey Formation in the southern San Joaquin Valley and coastal California show evidence for fault control on hydrocarbon transport important for both migration and production. Shearing along preexisting discontinuities, such as bedding planes and joints, locally increases permeability in the sheared zone and surrounding fractured rock. As the rock is subjected to shear, it begins to systematically fragment and subsequently to brecciate, thereby creating interconnected voids for hydrocarbon transport. A outcrop-based conceptual model for the development of hydrocarbon pathways in the Monterey Formation is applied to the subsurface using formation microscanner (FMS) data and core. Bed-parallel breccia zones are identified in the Antelope Shale at Buena Vista Hills oil field.
Rates of paleo-fluid flow within a fault zone exposed in an exhumed coastal section of the Monterey Formation are estimated based on: (1) the size of rock fragments apparently transported in moving fluid along the fault; and (2) alternating fault cement layers containing high- and low-temperature fluid inclusions, interpreted to reflect pulses of hot fluid flowing up along the fault. The size of entrained ro ck fragments provides a minimum estimate of flow velocities on the order of 105 m/yr (10-2 m/s), interpreted to represent flow conditions during coseismic fluid expulsion events. Flow velocities of 104 to 108 m/yr (10-4 to 10° m/s) are obtained using a conductive model of heat loss for pulses of hot fluid moving up along the fault. The second flow velocity estimate is interpreted to reflect flow conditions over significant post-seismic time intervals. Sustained flow rates on this order of magnitude require effective focusing of fluid from the source rock volume into the fault conduit. Episodic fluid expulsion along the fault system is also reflected in cyclic cathodoluminescence patterns of fault cement, in the variable stable isotopic composition of cement, and in alternating stages of fault cementation and cement brecciation due to fault slip. The number of cyclic cement layers exceeds the number of brecciation events along this fault, suggesting that the cement cycles are largely the result of fluid pulses triggered by earthquakes along other faults in the basin rather than by the exposed fault itself.
The frontal zone of the Dinarides and its foreland are characterized by the presence of major late orogenic northwest-southeast faults trending parallel to and locally dissecting the leading edge of the thin-skinned thrust belt. On the basis of their geometry and regional distribution, these structures are interpreted as right-lateral strikeslip faults and are related to the formation of en echelon compressional structucres in the Split-Dubrovnik area of the central Adriatic region and to the opening of the post-Messinian pull-apart Albanian foredeep (South Adriatic Basin) in the Pliocene-Holocene. These strike-slip faults represent the last stage of the mountain-building process in the Pliocene and Holocene, when the forelandward propagation of the Dinaric thrust belt was inhibited by stacking of thrust sheets and by overall thickenning of the colliding continental crust. In that respect, the Dinaric side of the Apulia (Adria) differs from the Apenninic side, where the thin-skinned thrusting progressed normally toward the foreland until the Pliocene. On a regional scale, these northwest-southeast-trending strikeslip faults in the frontal Dinarides and their apparent continuation to the Hellenides of western Greece, as well as the dextral Vardar fault system, indicate the existence of escape tectonics in the Dinaric-Hellenic region. During the last stage of orogeny, the strike-slip faults provided a means of tectonic transport from the collision zone of Apulia with Europe toward the subduction zone of the Hellenic Trench. In southern Greece, the southeastward-moving Dinaric-Hellenic blocks apparently interfered with the westward-moving Anatolian plate and deflected it southwestward. The Albanian foredeep, in terms of tectonic setting, resembles the pull-apart Vienna basin, which formed within the attenuated southwest-northeast-trending transfer zone of the West Carpathians during the last stages of the Alpine orogeny.
Matrix investigations (X-Ray tomography, porosimetry by mercury injection, SEM analysis) around stylolites revealed major zones that represent different states in the propagation of the pressure solution structure.
Near the stylolite termination, a significant increase of porosity relative to the far-field host rock porosity and variations in the shape of matrix particles are associated with the lateral propagation of the dissolution zone in the plane of the seam. Close to the sides of the seam, this porosity enhancement zone is found again and may be responsible for vertical development of the stylolite style. Above and below the stylolite seam, the rock matrix is less porous than the reference state and this region appears to have been a site of precipitation of diffused solute. These observations imply that the enhanced porosity state around the stylolite tip is a transient one. This zone becomes a site of deposition as the stylolite tip propagates through it.
This paper summarises the results of a study of the regional geological development of Albania, carried out with the objective to develop a better understanding of the complicated geometry and kinematics within the framework of the tectonic development. The available geological information has been integrated into a model that is consistent with all the data. The data included: the geological map of Albania, well data, interpreted satellite imagery, existing and new (Shell) seismic (offshore and onshore), published paleomagnetic data and radiometric ages and own field work. The model offers a geomechanical explanation for the tectonic style and kinematics of the Albanian fold and thrust belt. This includes an explanation for out-of-sequence thrusting as a function of the mechanical stratigraphy (brittle limestones and viscous salt) and the structural style in the Peri-Adriatic Depression. The timing of the tectonic events is consistent with plate tectonic movements as modelled for the eastern Mediterranean.The result of the study demonstrates the value of regional geology for a better understanding of the local geology and the importance of an integrated approach.
Detailed field studies of turbiditic sequences from South Georgia (South Atlantic), North Devon (England) and Cardigan Bay (Wales) show that flexural slip occurs on discrete movement horizons between rock packets in which the beds have welded contacts. Stair-stepping displacements of sedimentary dykes and of early quartz veins show that the movement horizons generally have a decimetre to metre spacing and are marked by bedding-parallel quartz veins. These veins are from 1 mm to several cm thick and can be used to identify movement horizons in the absence of displaced markers; they consist of several sheets of quartz fibres which each carry a slickenfibre lineation and which together preserve a record of the displacements on an individual surface. Complex slickenfibre patterns, and departures from ‘ideal’ behaviour, in which slip occurs orthogonal to the fold hinge, probably result from changes in slip vector on the limbs of growing non-cylindrical folds. Movement horizons show many of the features associated with large-scale thrusting, such as ramps, duplexes and imbricate structures, and the shear sense given by fibre steps on these surfaces and by duplexes within stratigraphically-restricted packages changes across fold hinges. Chevron folds are thought to develop mainly by flexural flow in the early stages, with flexural slip becoming dominant later as the beds become lithified; new slip surfaces are generated as the dip of the fold limbs increases.
This paper presents an outcrop-analog study of fractures in low-porosity sandstones in the Subandean thrust belt. We analyze the evolution of fault and joint systems in these sandstones, quantify their density along the structural trend, and identify the main factors controlling their variability. We show that faults and joints occur at different scales in a hierarchical fashion, as a consequence of progressive shearing. The first generation is an orthogonal set of joints, one parallel and the other perpendicular to the bedding azimuth. Shearing along these joints transformed them into small faults and created new sets of fractures, oblique to the bedding attitude. Linkage of these small faults facilitated the formation of larger faults with significant strike-slip offset. Shearing along bedding planes created subvertical splay joints that induced the formation of conjugate normal faults In. this thrust belt, subordinate strike-slip and normal faults are concomitant products of compressive deformation. This study documents a hierarchical correspondence between spacing of structural heterogeneities and stratigraphic architecture. We measured spacings of joints and outcrop-scale faults along the backlimb of the Abra del Condor anticline. We subdivided the structural discontinuities into four main groups: joints, small faults, intermediate faults, and fault zones. Spacing of joints, small faults, and intermediate faults has a lognormal distribution, whereas spacing of fault zones shows a normal distribution. The mean of these distributions is about the same as the thickness of the confining stratigraphic intervals. Therefore, spacing and dimensions of joints and faults have a first-order relationship to the thickness of the confining stratigraphic sequences.
Normal faults were initiated and grew through hierarchical formation of pressure-solution structures and their subsequent shearing in Cretaceous carbonates in the leading thrust front of Maiella Mountain, Italy. Through mapping in the field, we have documented the detailed architecture of faults with increasing slip values from a few millimeters to ˜50 m and have identified pretilting structural elements and four stages of fault development, each stage representing addition of a new structural element. The result is a conceptual model that begins with pretilting structures (bed-parallel and bed-perpendicular solution surfaces) that were reactivated in shear upon tilting of the beds at the frontal limb of the Maiella anticline. Slip on mechanical-layer boundaries and on bed-perpendicular solution surfaces resulted in oblique solution surfaces, linkage of solution surfaces, and fragmentation of rock. Oblique zones of fragmented rock in adjacent mechanical layers linked to form a continuous breccia and facilitated fault growth. These normal faults formed through mechanical processes strictly in a compressional regime.
This study investigates the reasons for the superimposition of several maximum principal stress directions (σ1) in the same area, and examines the contrast between unperturbed areas (stable direction of σ1) and perturbed areas (changing σ1 direction). We studied mesoscale structures on a 1000 m2 continuous limestone exposure near a regional scale strike-slip fault. Local σ1 directions were deduced from a high concentration of minor strike-slip faults, extension fractures and stylolites formed during the Pyrenean shortening in Languedoc, France. Most of the stylolites were formed in a stress field which was homogeneous on the exposure scale. This was followed by the reactivation of pre-existing extension fractures as strike-slip faults whose activity determined stress perturbations. A very heterogeneous stress field was produced leading to the formation of new localized stylolites and extension fractures, especially at fault terminations and at oversteps. Thus the final pattern shows the superimposition of all these structures. Reactivation of structures was caused by slight temporal changes in the orientation and intensity of the stress field produced by the nearby regional strike-slip faults. Our study suggests that the origin of stress deviations or superimpositions cannot be explained by random measurements of σ1. It is essential to be able to synthesize the fault pattern and the stress trajectories which it determines, and to do this, a very high density or a selection of data from mesoscale structures is needed.
This book is an entirely new one-volume text and reference that covers the procedures essential to geologic field studies. It presents information not found in other field geology texts, including identification and modern classification of rocks, means of recognizing and interpreting primary structures in rocks (which reveal their origin), coverage of engineering geologic maps, and recognition of recently active faults. The procedures are described thoroughly enough so that students can proceed largely on their own. It follows the typical sequence of a student's experience in learning field work-starting with the first observation of an outcrop and proceeding to methods of measurement and mapping, ending with preparation of a full geologic report.
