Showing papers in "AAPG Bulletin in 2010"

The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy

Abstract: Diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs. Variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability. Linking the types and distribution of diagenetic processes to the depositional facies and sequence-stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity. The heterogeneity patterns of sandstone reservoirs, which determine the volumes, flow rates, and recovery of hydrocarbons, are controlled by geometry and internal structures of sand bodies, grain size, sorting, degree of bioturbation, provenance, and by the types, volumes, and distribution of diagenetic alterations. Variations in the pathways of diagenetic evolution are linked to (1) depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; (2) detrital sand composition; (3) rate of deposition (controlling residence time of sediments at specific near-surface, geochemical conditions); and (4) burial thermal history of the basin. The amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and, therefore, can be predicted in the context of sequence stratigraphy, particularly in paralic and shallow marine environments. Changes in the relative sea level exert significant control on the types and extent of near-surface shallow burial diagenetic alterations, which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs. Carbonate cementation is more extensive in transgressive systems tract (TST) sandstones, particularly below parasequence boundaries, transgressive surface , and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter, bioturbation, and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates, which also enhance the formation of glaucony. Eogenetic grain-coating berthierine, odinite, and smectite, formed mostly in TST and early highstand systems tract deltaic and estuarine sandstones, are transformed into ferrous chlorite during mesodiagenesis, helping preserve reservoir quality through the inhibition of quartz cementation. The infiltration of grain-coating smectitic clays is more extensive in braided than in meandering fluvial sandstones, forming flow barriers in braided amalgamated reservoirs, and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution. Diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars, micas, and mud intraclasts under wet and warm climates, whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers. Turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls, as well as along layers of concentration of carbonate bioclasts and intraclasts. Commonly, hybrid carbonate turbidite arenites are pervasively cemented. Proximal, massive turbidites normally show only scattered spherical or ovoid carbonate concretions. Improved geologic models based on the connections among diagenesis, depositional facies, and sequence-stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies, as well as for CO2 geologic sequestration, but also support more effective hydrocarbon exploration through reservoir quality prediction.

Sandstone diagenesis and reservoir quality prediction: Models, myths, and reality

Abstract: Models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration. Not all of these are equally supported by quantitative data, observations, and rigorous hypothesis testing. Simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones. Calibrated numerical models that simulate compaction and quartz cementation, when linked to basin models, have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture, composition, and quartz surface area is available. Analysis of global, regional, and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality. (1) The effectiveness of grain coatings on quartz grains (e.g., chlorite, microquartz) as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results. (2) Vertical effective stress, although a fundamental factor in compaction, cannot be used alone as an accurate predictor of porosity for lithified sandstones. (3) Secondary porosity related to dissolution of framework grains and/or cements is most commonly volumetrically minor (2%). Exceptions are rare and not easily predicted with current models. (4) The hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model. (5) Heat-flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure, thereby slowing the rate of quartz cementation in some subsalt sands.

Geochemical constraints on the origin and volume of gas in the New Albany Shale (Devonian–Mississippian), eastern Illinois Basin

Abstract: This study involved analyses of kerogen petrography, gas desorption, geochemistry, microporosity, and mesoporosity of the New Albany Shale (Devonian–Mississippian) in the eastern part of the Illinois Basin Specifically, detailed core analysis from two locations, one in Owen County, Indiana, and one in Pike County, Indiana, has been conducted The gas content in the locations studied was primarily dependent on total organic carbon content and the micropore volume of the shales Gas origin was assessed using stable isotope geochemistry Measured and modeled vitrinite reflectance values were compared Depth of burial and formation water salinity dictated different dominant origins of the gas in place in the two locations studied in detail The shallower Owen County location (415–433 m [1362–1421 ft] deep) contained significant additions of microbial methane, whereas the Pike County location (832–860 m [2730–2822 ft] deep) was characterized exclusively by thermogenic gas Despite differences in the gas origin, the total gas in both locations was similar, reaching up to 21 cm3/g (66 scf/ton) Lower thermogenic gas content in the shallower location (lower maturity and higher loss of gas related to uplift and leakage via relaxed fractures) was compensated for by the additional generation of microbial methane, which was stimulated by an influx of glacial melt water, inducing brine dilution and microbial inoculation The characteristics of the shale of the Maquoketa Group (Ordovician) in the Pike County location are briefly discussed to provide a comparison to the New Albany Shale

Sandstone reservoir quality prediction: The state of the art

Abstract: Joanna Ajdukiewicz joined Exxon Production Research Company in 1980. She was Reservoir Quality Assessment and Prediction team lead there from 1991 to 1995 and at Imperial Oil Research Centre in Calgary from 1995 to 1997. Subsequently, she has worked a variety of Exploration Company assignments in the North Sea, Gulf of Mexico, and Middle East. Her current interests are in predicting the distribution of early diagenetic controls on deep reservoir quality. Rob Lander develops diagenetic models for Geocosm LLC. He obtained his Ph.D. in geology from the University of Illinois in 1991, was a research geologist at Exxon Production Research from 1991 to 1993, and worked for Rogaland Research and Geologica AS from 1993 to 2000. He is also a research fellow at the Bureau of Economic Geology. To guess is cheap; to guess wrongly is expensive (Chinese proverb). Reservoir-quality predictive models will be a useful element of risk analysis until remote-sensing tools are invented that accurately measure effective porosity and permeability ahead of the bit. This issue of the AAPG Bulletin highlights recent advances in a new generation of reservoir quality models that have successfully predicted porosity and permeability in diverse siliclastic reservoirs under many different burial conditions. Most previous attempts at predrill reservoir quality prediction have relied on empirical correlations or on first-principle geochemical simulations that incorporate laboratory-derived input parameters (Wood and Byrnes, 1994). The new reservoir quality models differ from previous approaches in that, although incorporating theory-inspired algorithms, they include terms with values that are explicitly designed to be calibrated by, and tested against, data sets of high-quality petrographic analyses that are linked to thermal and effective-stress histories. Petrographic observations therefore provide essential constraints in these models on the types, timing, and rates of key geologic processes affecting sandstone pore systems. This approach avoids the pitfalls inherent …

Oil generation as the dominant overpressure mechanism in the Cenozoic Dongying depression, Bohai Bay Basin, China

Abstract: The Dongying depression in the Bohai Bay Basin is a young, prolific oil-producing basin in China. The gray to black mudstones, calcareous mudstones, and oil shales of the Eocene Shahejie Formation (Es3 and Es4) are the major source rocks that are primarily dominated by type I kerogens with a high total organic carbon of up to 18.6%. The Es3 interval is characterized by a sedimentation rate of up to 500 m/m.y. Widespread overpressures are present in the Eocene Es3 and Es4 intervals in the depression, with pressure coefficients up to 1.99 from drillstem tests. Among the sonic, resistivity, and density logs, only the sonic-log displays an obvious response to the overpressure from which the top of the overpressure can be clearly identified. Acoustic traveltime versus effective vertical stress analysis of more than 300 wells in the Dongying depression suggests that the acoustic traveltime of the normally pressured and overpressured mudstones is reduced with increasing vertical effective stress. Pore pressures are accurately predicted in the Dongying depression using an Eaton (1972) exponent of 2.0 by comparing the predicted pressure coefficients with measured pressure coefficients. Disequilibrium compaction has been previously proposed as the sole cause of the high-magnitude overpressures in the Eocene strata of the Dongying depression because of rapid deposition of fine-grained sediments. However, our data indicate that the overpressures are caused by oil generation from the source rocks within the Es3 and Es4 intervals. The overpressured sediments display a normal compaction as evidenced from the overpressured mudstones exhibiting no anomalously low density, the apparent lack of correlation between mudstone densities and effective vertical stress, and the overpressured reservoir sandstones showing no anomalous high-matrix porosities or anomalous geothermal gradient. The depths to the top of the overpressure intervals range from 2000 to 3000 m (6562–9843 ft) following closely source rock depths. All the overpressured reservoirs and source rocks have a minimum temperature of approximately 87C, and overpressured source rocks generally have vitrinite reflectance (Ro) values of 0.6% or higher. Overpressures are not found in the strata within which the Ro values are less than 0.5%. The overpressured reservoirs in the Es3 and Es4 intervals are predominantly oil saturated or oil bearing. Organic-rich source rocks with overpressures are capable of generating hydrocarbons and thus can maintain an abnormally high pressure. The calcite precipitation in the calcareous mudstones may have caused a significant reduction in porosity and permeability to form an effective pressure seal. The presence of widespread microfractures in the source rocks may relate to episodic expulsion of hydrocarbons or overpressure dissipation. Overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks through active faulting and fracturing.

Geochemical characterization of gases from the Mississippian Barnett Shale, Fort Worth Basin, Texas

Abstract: Molecular and isotopic compositions of 51 gas and 7 condensate samples produced from the Barnett Shale were determined by gas chromatography–isotope ratio mass spectrometry to investigate their origin and maturity at the time of generation. Additionally, two hydrous pyrolysis experiments were performed to calibrate maturity values predicted for gas generation. Molecular and carbon and hydrogen isotopic compositions of the gases indicate that gas produced from the Barnett is thermogenic. To estimate generation maturity, additional analyses were performed on the condensates and available core samples. Diamondoid indices combined with vitrinite reflectance (Ro) measurements, isotopic and molecular composition, and hydrous pyrolysis experiments suggest that gas produced from the Barnett Shale was generated within the condensate–wet gas window (1.3–2.0% Ro) at a later stage than hydrocarbons accumulated in shallower reservoirs. The origin of the gas is thermogenic, probably derived from both kerogen cracking and secondary cracking of previously generated nonmigrated hydrocarbons. Additionally, the occurrence of a methane isotope reversal in group 1 gases corroborates that in-situ oil or gas cracking is occurring in the easternmost part of the study area.

Prediction of deep reservoir quality using early diagenetic process models in the Jurassic Norphlet Formation, Gulf of Mexico

Abstract: We have developed process-based models for early grain coats and their impact on deep reservoir quality in the Jurassic eolian Norphlet Formation, Alabama, with implications for exploration and development in other conventional and tight-gas continental reservoirs. The Norphlet, a major gas reservoir to depths of 21,800 ft (6645 m) and temperatures of 419F (215C), displays contrasting intervals of high and low reservoir quality within compositionally similar cross-bedded eolian sands. Study results show that grain coats formed soon after deposition are responsible for differences in deep Norphlet porosity of up to 20% and permeability up to 200 md. Three types of grain coats were identified in Norphlet dune sands, each formed in a different part of a shallow groundwater system, and each with distinctive impact on deep reservoir quality. Diagenetic chlorite coats, formed where dunes subsided into shallow hypersaline groundwater, preserve good deep porosity (to 20%) and permeability (to 200 md). Continuous tangential illitic coats, formed in the vadose zone of stabilized dunes exposed to periodic fresh-water influx, preserve good deep porosity (to 15%) associated with poor permeability (1 md) due to linked formation of later high-temperature diagenetic illite. Discontinuous grain coats, formed in active dunes where grains were abraded by eolian transport, are associated at depth with tight zones of pervasive quartz cement, low porosity (8%), and low permeability (1 md). These concepts plus data from 60 wells were used to derive bay-wide predictive tight and porous-zone isopachs that can be used for well placement, geologic models, and field development.

Delta-front hyperpycnal bed geometry and implications for reservoir modeling: Cretaceous Panther Tongue delta, Book Cliffs, Utah

Abstract: The Cretaceous Panther Tongue has an upward-coarsening and -thickening pattern and is well exposed in extensive large outcrops in the Book Cliffs area, west-central Utah. The deposits have been interpreted as having formed in a fluvial-dominated river delta environment that generated highly sediment-concentrated sustained (turbidity) flows during flooding, producing hyperpycnal-flow deposits on the delta front despite some resemblance to deep-water turbidites. The facies associations indicate terminal distributary channel, channel mouth, and proximal delta-front and distal delta-front depositional environments. The measured paleocurrents indicate a south-southwest transport of the sediments. The thickness of the hyperpycnal sandstone beds ranges from centimeters to meters. Sandstones are characteristically parallel laminated, sometimes structureless or rarely display inclined strata of cut-and-fill type. The sandstone hyperpycnal beds dominate the delta-front clinoforms and dip southward, consistent with the other paleocurrent indicators. Individual sandstone beds in the clinoforms have dips that range from 0.1 on the distal delta front (lower part of the outcrops) to 3 in the proximal parts (upper part of the outcrops). The hyperpycnal beds can be traced from a proximal mouth-bar environment to the distal delta front over a distance of hundreds of meters. As individual beds extend from mouth bar to distal delta-front environments, they become systematically finer grained and thinner. Over short distances (hundreds of meters), the beds thin with rates ranging between 0.0001 (i.e., dm/km) to 0.02 (i.e., tens of meters per kilometer). The sandstone beds thin to a greater degree in a dip direction than along strike, indicating a relatively strike-elongate (flow-normal) geometry of the hyperpycnal flows and of the delta lobes. The wider than longer geometry of the delta-front beds requires that reservoir development be more focused upon the downdip facies changes (heterogeneities) than the lateral (along strike) heterogeneities.

Migration of Cenozoic deformation in the Eastern Cordillera of Colombia interpreted from fission track results and structural relationships: Implications for petroleum systems

Abstract: Previously unreleased fission-track results and regional structural relationships are used to interpret the migration of deformation during Cenozoic orogenesis in the Eastern Cordillera (Cordillera Oriental) of the Colombian Andes. Low-temperature thermochronological results are based on apatite and zircon fission-track analyses of 41 samples collected along vertical and horizontal transects across the Eastern Cordillera at 4–7N latitude. Inverse modeling of fission-track results helps delimit the most probable cooling histories caused by exhumation linked to upper-crustal deformation. These inverse models are constrained by known structural geometries, chronostratigraphy, biostratigraphy, and vitrinite reflectance data. Fission-track data and modeling results indicate a close correspondence in the timing and style of deformation along the western and eastern flanks of the Eastern Cordillera. East-directed fold-thrust deformation along the eastern boundary with the Llanos foreland basin was underway by the late Oligocene and early Miocene. Similarly, west-directed fold-thrust structures along the western boundary with the intermontane middle Magdalena Valley Basin became active at approximately the same time. Less well known is the time of initial shortening within the axial segment of the Eastern Cordillera; although fission-track results suggest active exhumation by the early Miocene, shortening may have commenced much earlier during the late Eocene. Timing relationships for the Eastern Cordillera have important implications for the generation, migration, and accumulation of petroleum in the middle Magdalena Valley intermontane basin and the Llanos foreland basin. Our study provides a regional context to assess the timing of structural trap development and improve exploration and development of new and existing reservoirs in Colombia and analogous fold-thrust systems elsewhere.

Reservoir quality modeling of tight-gas sands in Wamsutter field: Integration of diagenesis, petroleum systems, and production data

Abstract: The Late Cretaceous Almond Formation is a lithic-rich, tight-gas sandstone reservoir in Wamsutter field, Wyoming. The Almond has measured porosities of less than 12% and permeabilities that are generally less than 0.1 md. This is the result of significant mechanical and chemical compaction, precipitation of carbonate cements and authigenic clays, and deep-burial cementation by quartz. Despite the advanced diagenesis and poor reservoir quality of this tight-gas sandstone, Touchstone diagenetic modeling was used successfully to simulate rock properties during burial. Basin modeling and fluid inclusion data were integrated with Touchstone™ simulations to constrain the timing of petroleum charge into the reservoir. Model results indicate that the Almond was initially charged by liquid petroleum, when the reservoir retained high porosity and permeability. Subsequent gas charging flushed most of the liquids from the system as the reservoir continued to experience porosity and permeability reduction from continued quartz cementation. The Almond Touchstone model was applied to 15 well locations throughout the field, ranging from areas of low thermal stress to high thermal stress. A comparison of Touchstone-predicted permeability and standard thermal stress yielded a correlation that was used to build a pseudopermeability reservoir quality risk map across the Wamsutter area. A comparison of this map with historical production data demonstrates that the map successfully predicts areas of enhanced well performance. The method used to build this risk map may be applied elsewhere as a quick means of high grading areas of risk during field development.

Stratigraphy and U-Pb detrital zircon geochronology of Wrangel Island, Russia: Implications for Arctic paleogeography

Abstract: Wrangel Island represents a small but unique exposure of Neoproterozoic basement and its upper Paleozoic and Mesozoic cover within the mostly unexplored East Siberian Shelf. Its geology is critical for testing the continuity of stratigraphic units and structures across the Chukchi Sea from Alaska to Arctic Russia, for evaluating the hydrocarbon potential of this offshore region, and for constraining paleogeography and plate reconstructions of the Arctic. Upper Paleozoic platform carbonates and shales on Wrangel likely match those of the Chukchi Shelf and adjacent North Slope of Alaska (e.g., Sherwood et al., 2002), but Triassic basinal turbidites contrast with Alaska's thin shelfal units. Detrital zircon suites from upper Paleozoic strata on Wrangel reveal that local basement-derived detritus (500–800 Ma) decreases up section, replaced by 900–2000-Ma zircon populations compatible with a Baltic shield provenance. Cambrian–Ordovician–Silurian zircons (420–490 Ma) are present in lesser abundance in most samples and are inferred to have been derived from the Arctic part of the Caledonide belt. Triassic detrital zircon suites contrast with those from underlying strata: Precambrian zircons have less of an age range (1700–2000 Ma), and Devonian and younger (400 Ma) zircons are much more abundant. This change reflects breakup of the carbonate platform during Permian–Triassic rifting, with zircon age populations in Triassic strata compatible with sediment sources in the Urals, Taimyr, and Siberia. Detrital zircon data suggest that Wrangel Island, Chukotka, and northern Alaska (the Arctic Alaska-Chukotka microplate) restore against the Lomonosov Ridge upon closure of the Amerasia Basin and to the edge of the Barents Shelf after closing the Eurasia Basin. The detrital zircon data thus suggest that the Barents Shelf lays close to the paleo-Pacific margin in the early Mesozoic and that subduction-driven tectonics may have been a greater factor in the evolution of the Amerasia Basin of the Arctic than previously suspected.

Bioturbation influence on reservoir quality: A case study from the Cretaceous Ben Nevis Formation, Jeanne d'Arc Basin, offshore Newfoundland, Canada

Abstract: The delineation Ben Nevis L-55 well, located in the Hebron-Ben Nevis field, offshore Newfoundland, targeted the Cretaceous Ben Nevis Formation in the petroleum-rich Jeanne d'Arc Basin. This case study focuses on the bioturbated net-pay horizons and assesses the importance of animal-sediment interactions in controlling the porosity and permeability of sandstone reservoir intervals. Our data reveal that bioturbation can either reduce permeability and porosity by as much as approximately 33% or enhance it by up to 600%, dependent on burrow type and behavior of the trace-making organism. The net-pay interval in the cored interval of Ben Nevis L-55 is characterized by Ophiomorpha-dominated ichnofabrics. The action of bioturbators can be classified in terms of sediment mixing, sediment cleaning, sediment packing, and pipe-work-building strategies. Bioturbation has the potential to (1) increase isotropy or uniformity of grain size by destroying sedimentary laminae through burrow homogenization, or (2) decrease isotropy by selectively sorting grains into burrow lining and fill by grain size, and through creation of open-burrow systems filled with later sediments of differing character to the host sediment. The petrophysical characteristics of the reservoir facies are highly dependent on trace fossil morphology, presence or absence of burrow linings, nature of burrow fills, burrow size, and bioturbation intensity. Mudstone-rich facies and ichnofabrics containing mudstone-filled and/or lined burrows (e.g., Ophiomorpha and clusters of Chondrites) have the net effect of permeability reduction. In contrast, permeability enhancement is documented from muddy sandstone facies with clean sand-filled burrows (e.g., Thalassinoides) and clean sandstones with burrow-mottled or diffuse to massive textures.

Microfracturing in the Upper Triassic Sichuan Basin tight-gas sandstones: Tectonic, overpressure, and diagenetic origins

Abstract: The western Sichuan Basin located at the front of the Longmen Mountain in the western Sichuan Province of China is a foreland basin formed in the Late Triassic. The Upper Triassic Xujiahe Formation in the basin is an ultralow-permeability and low-porosity tight-gas sandstone reservoir. Microfractures, such as intragranular microfractures, grain-edge microfractures, and transgranular microfractures, are abundant in the tight sandstones. Microfractures improve storage and permeability and impact distribution of natural gas. Microfractures reflect tectonic, overpressure, and diagenetic origins. Using quartz and calcite fluid inclusions and burial history, tectonic microfractures were determined to be formed at the end of the Triassic, the end of the Cretaceous, and the end of the Neogene–early Pleistocene. Microfractures related to overpressure, being tension microfractures, are commonly filled with bitumen and were formed at the middle to Late Cretaceous when the maximum pressure coefficients were 1.6 to 2.1. In the middle to late Neogene, the pressure coefficient was reduced to less than 1.3 because of fault activity and tectonic uplift in the southwestern Sichuan Basin, and these tension microfractures closed and were filled with calcite. The transition of stress state from compression to tension by overpressure is the reason that tension microfractures were formed in the compression setting. Diagenetic fractures were formed at the end of the Late Triassic to Jurassic. Under intense compaction, grain-crushing crackle fractures in quartz and cleavage fractures in feldspar formed intragranular microfractures. Some transgranular microfractures caused by diagenesis are along the bedding plane and parallel to the directional mineral grains.

A model for fibrous illite nucleation and growth in sandstones

Abstract: We have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in-situ K-feldspar grain dissolution or imported into the model reference frame. Illite fiber nucleation and growth are modeled using Arrhenius expressions that consider saturation state in addition to temperature and time. Nucleation occurs on pore walls, and muscovite and detrital illite may be defined as energetically favorable substrates. The model is integrated with other Touchstone™ models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations. We evaluated the illite model performance on two data sets: (1) Jurassic quartzose samples from offshore mid-Norway with maximum temperatures ranging from 108 to 173C (226 to 343F) and (2) Miocene lithic samples from offshore Southeast Asia that have maximum temperatures ranging from 157 to 182C (315 to 360F). The model matches measured abundances of illite, kaolinite, and K-feldspar in both data sets using identical kinetic parameters. Predicted K-Ar ages are consistent with available data given uncertainties associated with detrital contaminants. Although no illite particle-size data are available from the analyzed samples, modeled crystallite thicknesses from the Norway data set are comparable to published measurements of 0.004 to 0.012 m from North Sea samples with similar temperature histories.

Reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection

Abstract: Reactive transport models (RTMs) permit quantitative investigation of diagenesis and its effects on reservoir quality. The RTM TOUGHREACT is used to investigate diagenesis in an isolated platform driven by geothermal (Kohout) convection of seawater, which has been invoked to explain dolomitization during early burial. Previous short (0.1 m.y.) RTM simulations suggested that convection can drive dolomitization, mostly at greater than 50C, and anhydritization, but complete dolomitization requires greater than 30–60 m.y. Our more extended RTM simulations (30 m.y.) indicate significant nonlinearities in the system, consistent with high-temperature experiments, with parts of the platform completely dolomitized within 10–15 m.y. As dolomitization proceeds, the process becomes predominantly flux controlled, with development of a wedge-shaped dolomite body, which thins from the margin to the interior, at considerably shallower depth and cooler temperatures (20–30C) than suggested by short simulations. Dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms. Sediment permeability and reactive surface area, commonly inversely related, are key controls. Dolomitization is limited to the margin of low-permeability muddy platforms despite a high reactive surface area. Dolomitization of more permeable grainy platforms is limited by a lower reactive surface area, occurring only in the platform core due to widespread cooling. Sedimentary layering produces a complex diagenetic stratigraphy, dolomitization favoring more reactive beds at shallow depth where permeability is not limiting, but switching to more permeable beds at depth. Bank-marginal fracturing limits dolomitization of the platform interior, whether the fractures are baffles or conduits for flow.

Structural and sedimentary evolution of the southern Songliao Basin, northeast China, and implications for hydrocarbon prospectivity

Abstract: The southern Songliao Basin manifests itself as a wide-rift system that developed in northeast China from the Late Jurassic to Early Cretaceous. Individual basins in the system experienced marked rift subsidence, but the postrift subsidence was insignificant in most of the basins, contrasting strikingly with the tectonic subsidence history of the northern Songliao Basin. Two types of rift basins are defined according to whether the basins underwent prominent postrift subsidence. Type 1 basins are characterized by thick postrift accumulations. Type 2 basins, although experiencing minor postrift subsidence, represent most of the southern Songliao Basin and can be subdivided in accordance with bounding-fault geometry and areal extent: basins bounded by high-angle faults (type 2a), basins bounded by low-angle faults (type 2b), and basins with limited spatial area (type 2c). Many rift basins are expressed as narrow belts in map view and are composed internally of several segments linked through different types of transfer zones. Depositional processes and facies architecture of the basins are controlled primarily by dips and migration of active bounding faults. Synrift depocenters occur close to high-angle bounding faults, and deep-lake deposition commonly persists through much of the synrift subsidence. Lacustrine deposition can be enhanced by the backward (or toward the footwall) stepping of active bounding faults. Depocenters controlled by low-angle bounding faults, in contrast, tend to shift basinward through time, and deep-lake facies commonly develop in the middle stage of rifting. At the end of the Cretaceous, basin inversion was evident in type 1 basins, such as the Shiwu Basin on the north, but other basins appear to have mostly escaped the contractional deformation. Gravitational collapse of the previously thickened crust is considered the cause for the generation of the rift basins, and lateral flow of the ductile lower crust may explain the significantly induced postrift subsidence of type 2 basins. Effective plays occur in basins bounded by high-angle faults, and therefore, type 1 and type 2a basins are suggested to be the main targets of future oil exploration. Synrift source rock maturation might be partially attributed to the heating of Early Cretaceous magmatism because shallower burial alone could not elevate temperatures high enough for petroleum generation.

Depositional setting and diagenetic processes and their impact on the reservoir quality in the late Visean–Bashkirian Kashagan carbonate platform (Pre-Caspian Basin, Kazakhstan)

Abstract: The Kashagan isolated carbonate platform developed from the Visean to the Bashkirian in the Pre-Caspian Basin. Sedimentation is organized in shallowing-upward cycles marked at the top by subaerial exposure surfaces. This study is based on analyses of 12 cored wells, integrated sedimentology, petrography, fluid inclusion, stable isotopes, and trace element data. We reconstructed the pore-system evolution in the platform interior and margin. Near-surface diagenesis includes marine cement fringes followed by meteoric dissolution and cementation. Meteoric diagenesis was stronger in the Bashkirian sequence and progressively decreased down section in the Visean. The main karst surfaces are evidenced by root traces, sharp 13C depletion, and meteoric water-fluid inclusions in calcites. Porosity in the platform interior follows a cyclic pattern due to the interaction of the freshwater dissolution and cementation and compaction that destroyed the porosity at the tops and bases of the cycles. Burial diagenesis, represented by some calcite cementation, did not significantly alter the cyclic porosity distribution in the platform interior. In the platform margin, porosity distribution is more heterogeneous due to more poorly defined sedimentary cyclicity and stronger burial diagenetic overprint linked to exotic fluid circulation. Basin-sourced fluids caused dissolution, cementation, and local dolomitization. These cements show bright luminescence, a high manganese content, depleted 18O, a high homogenization temperature, and low-salinity fluid inclusions. The complex diagenetic evolution diagenesis produced better matrix porosity in the platform interior and heterogeneous porosity with larger pores and fractures but with poor matrix porosity in the platform margin.

The response of turbidite slope channels to growth-induced seabed topography

Abstract: The morphology of turbidite slope channels and the distribution of reservoir and nonreservoir facies within them are commonly complicated by the interaction of the channels with the development of structurally induced topography. The pattern of channel response may be dictated by the timing of structural growth related to channel development, the size and shape of the structures, the orientation of the structure to depositional dip, and the erosional power of the channels. Several three-dimensional seismic data sets have been examined from passive margins deformed by gravity-driven tectonics to investigate the range of responses that large, third-order erosional channel complex systems can show to slope topography. The examples are examined with respect to timing of the growth-related seabed topography versus timing of channel formation and the erosive power of the flows within the channel. Structures that develop aerially limited sea-floor topography, which predates channel development, cause turbidite channels to take a moderate diversion as they traverse the slope. Where similar structures are more laterally extensive, channels may take extreme diversions, commonly kilometers along slope before continuing down the regional slope. When the erosion of the flows is strong enough and can overcome the rate of growth of the structure, channels can continue to incise across the growing structure. If the rate of growth of the structure is higher, the channel systems shift systematically sideways to avoid the rising topography. The style of the sedimentation-topography interaction has a strong but commonly subtle effect on the geometry, internal stratigraphic architecture, and nature and distribution of the facies deposited within and around the channels.

Interaction between prerift salt and detachment faulting in hyperextended rift systems: The example of the Parentis and Mauleon basins (Bay of Biscay and western Pyrenees)

Abstract: Prerift salt layers associated with extensional detachment faults exhuming mantle and deeper crustal rocks at the sea floor are observed in the Parentis and Arzacq-Mauleon basins located at the eastern termination of the Bay of Biscay. How detachment faults interact with salt in hyperextended rift systems is yet little understood. Based on field observations and drill-hole and seismic data, we propose a new model to explain the interaction between salt tectonics and extensional detachment systems. We demonstrate that the presence of a thick prerift salt layer in an area undergoing extreme crustal thinning can control the geometry and evolution of rift systems and obscure the rift-related structures in the underlying basement. During an initial stage of rifting, prerift salt layers act as a decoupling horizon between sub- and suprasalt units and hinder crustal detachment faults to cut through the salt layers and form breakaways at the sea floor. As a consequence, they sole out along the ductile salt layer and no subsalt material can be exhumed to the sea floor. Thus, sub- and suprasalt layers deform by different deformation modes, which makes that detachment fault difficult to identify on seismic images. In a later stage, when salt has migrated and thinned out, sub- and suprasalt layers can locally couple and detachment faults can be exposed at the surface, resulting in windows of exhumed basement surrounded by extensional allochthons formed by suprasalt sedimentary units.

Three-dimensional facies modeling of carbonate sand bodies: Outcrop analog study in an epicontinental basin (Triassic, southwest Germany)

Abstract: This article is the first part of an integrated study to characterize the anatomy and geometries of carbonate shoal reservoir bodies in epicontinental settings. It is based on outcrop analog data from Triassic layer-cake carbonates in the south German Basin, which were deposited along an epicontinental, very gently inclined carbonate ramp. The database of this study consists of 56 measured sections supplemented by six shallow cores and wireline logs, covering an area of 25 by 36 km (15 by 22 mi). The potential reservoir bodies consist generally of midramp shoal and shoal-fringe facies types, which are composed of skeletal and oolitic carbonate packstones and grainstones with significant amounts of porosity. The upper Muschelkalk is composed of a lower transgressive and an upper regressive interval; within these, shoal bodies show not only similarities but also major differences in character, geometry, and distribution. These reflect the conditions of an epeiric system, which reacts strongly to small changes in accommodation. The accommodation seems to be mainly triggered by the interaction of hierarchically organized large-, medium-, and small-scale relative sea level changes and a subtle paleorelief. At the level of medium-scale cycles, distinct two-dimensional cyclicity styles can be differentiated, which record the lateral facies distribution along the depositional gradient. Different styles of medium-scale cycles include different types of shoal bodies: (1) transgressive crinoidal shoal style: shoal bodies (mean width: 21 km [13 mi], mean length: 37 km [23 mi], mean thickness: 4.2 m [13.7 ft]) with low facies differentiation, deposited on top of subtle paleohighs; (2) skeletal sheets style: thin reservoir sheets (mean width: 5.1 km [3.1 mi], mean length: 11.1 km [6.8 mi], mean thickness: 0.3 m [0.9 ft]) of reservoir facies on top of the paleohighs; (3) regressive oolitic shoal style: shoal bodies (mean width: 14.6 km [9.0 mi], mean length: 20.8 km [12.9 mi], mean thickness: 0.69 m [2.2 ft]) with high facies differentiation on the flanks of paleohighs; and (4) low-accommodation style: patchy and mosaic distribution of shoal bodies (mean width: 11.2 km [6.9 mi], mean length: 26 km [16 mi], mean thickness: 0.69 m [2.2 ft]). All data were loaded into three-dimensional (3-D) modeling software to distribute 14 facies types and model the 3-D stratigraphic architecture. The resulting facies distribution implies that volume and dimensions of the shoal bodies are mainly controlled by the combination of stratigraphic cycles and a subtle paleorelief, which is indicated by overall thickness changes of succession. High-energy shoal facies types occur only in and around areas with a reduced overall thickness, whereas areas with a thicker development are dominated by low-energy, muddy facies types. All observations combined point to the presence of a subtle paleorelief, which could be induced by slight differential subsidence of inherited paleotectonic basement blocks. On the kilometer to regional scale, previous studies suggest simple layer-cake stratal patterns. However, in the full 3-D view, the apparent layer-cake stratigraphy turned out to be a pseudolayer cake, with very gentle (0.01–0.001 dip) clinoform geometries. These very subtle offlapping, pinch-out geometries may have been overlooked in epeiric reservoir systems elsewhere.

Methanogenic biodegradation of petroleum in the West Siberian Basin (Russia): Significance for formation of giant Cenomanian gas pools

Abstract: Approximately 1700 tcf (48 trillion m3) of dry gas (99% methane) reserves and resources occur in western Siberia, mostly in shallow (1500 m [4921 ft]) Cenomanian pools in the northern part of the basin. This dry gas constitutes about 11% of the world's conventional gas endowment and about 17% of the annual gas production. The origin of the dry gas has been debated extensively over the last 45 yr but remains controversial. Widely discussed hypotheses on the origin include early-mature thermogenic gas from coal, primary microbial gas from dispersed organic matter or coal, and thermogenic gas from deep source rocks. However, all these hypotheses are in some ways inconsistent with the molecular or isotopic composition of the gases or the results of basin and petroleum systems modeling. Here, I present geochemical and geological evidence that a significant (although yet not quantified) part of the shallow dry gas in the northern West Siberian Basin originated from methanogenic biodegradation of petroleum. Circumstantial evidence includes the occurrence of heavily biodegraded oil legs and residual oil in many Cenomanian gas pools, as well as geochemical evidence of heavy to slight biodegradation in Jurassic–Albian reservoirs commonly underlying the Cenomanian pools. Direct evidence includes, most importantly, 13C-enriched CO2 in pools with biodegraded oil (although data are limited), which indicates 40–70 wt.% conversion of oil-derived CO2 to secondary microbial methane. Distinctive hydrocarbon molecular and isotopic compositions of most gases in Cenomanian pools (average dryness C1/(sum C1-C5) is 0.9976; average 13C of methane is 51.8) suggest that they represent mixtures of biodegraded thermogenic gases from deep, mainly Jurassic, source rocks and secondary microbial methane with an occasional small addition of primary microbial methane. Contribution of early-mature coal-derived gas is possible in areas with the most significant thermal stress of Hauterivian–Aptian sediments but remains speculative. Review of petroleum habitats of five representative oil-gas-condensate fields in western Siberia (including the world's second largest gas field, Urengoyskoe) suggests that methanogenic biodegradation may best explain the observed distribution and properties of fluids in the shallow reservoirs of those fields. Recognition of secondary microbial gas in western Siberia helps explain the observed dominance of gas in the shallow, cool northern part of the basin, where conditions were more favorable for prolonged petroleum biodegradation than in the central and southern parts of the basin. Secondary microbial gas has been recognized worldwide and may (1) represent a volumetrically significant exploration target in shallow reservoirs (perhaps more significant than primary microbial gas) and (2) indicate effective thermogenic petroleum systems in the deeper sections. Large volumes (up to 66,500 tcf [1884 trillion m3]) of secondary microbial methane could have been generated from biodegraded petroleum accumulations worldwide. Although a part of that gas accumulated as oil-dissolved, free, and hydrate-bound gas, most gas apparently escaped into the overburden, atmosphere, and ocean and could have affected global climate in the geologic past.

Delineating and quantifying depositional facies patterns in carbonate reservoirs: Insight from modern analogs

Abstract: Modern carbonate settings have long been used as analogs for interpreting various environments of deposition in subsurface data sets based on well-log and core data and have provided examples of how these environments can be interrelated within a larger context. This study focuses on carbonate sands in the Exuma Islands part of the Great Bahama Bank to show how modern settings can be better used for reservoir analogs by emphasizing visualization and quantification techniques that conceptually delineate aspects of potential grainstone reservoir distribution and heterogeneity. Along the windward margin of the Exumas, the association of tidal deltas, channels, and islands forms an exploration-scale linear belt of mostly ooid sand, approximately 5–10 km (3.1–6 mi) wide and 170 km (106 mi) long, paralleling the platform margin but set back from the platform edge. The distribution of carbonate sands in the Exumas was assessed with a digital elevation model and satellite imagery. By selecting different water-depth intervals, parts of the sand belt were highlighted for visual analysis and extracted for measurements of size and shape. For example, 37% of the sand belt comprises shallowest sands in which the largest areas, greater than 100,000 m2 (1.07 million ft2), are highly sinuous and maintain connectivity. The sand belt was also subdivided into various depositional settings; flood-tidal delta lobes comprise 85% of the sand belt. Tidal channels and islands add further heterogeneity to the sand belt. Channels average about 3 km (1.8 mi) in length and have regular but locally variable spacing. A representative island shows how the syndepositional topography of Holocene ridges develops complexly around antecedent Pleistocene topography.

The function of fractures and in-situ stresses in the Khuff reservoir performance, onshore fields, Saudi Arabia

Abstract: The highly variable performance of the Permian–Triassic Khuff reservoir in onshore Saudi Arabia has been attributed to the presence of natural fractures. Similar preproduction pressure profile and hydrocarbons in the different reservoir units in some fields have been attributed to vertical communication through large faults. To validate these assumptions, we studied the static and dynamic data from the Khuff reservoir in 19 major structural traps. We identified two distinctive fracture domains based on fracture orientation and density. Fracture evolution is mainly controlled by the extensional and consequent compressional plate tectonics instead of local structures. In-situ stresses are dominated by the Zagros plate tectonics and affect fracture aperture differently in the two fracture domains. The fracture impact on the Khuff reservoir performance is mostly subtle because of the nature and distribution of the fractures. High fracture-enhanced productivity occurs locally in some of the producing wells, and it results from high-density fracture clusters (including mesoscopic faults) with channel-type apertures. The following findings challenge the perceived major functions of fractures in the Khuff reservoir performance in onshore fields: (1) Individual fractures are dominantly tensile and small (mesoscopic and microscopic); (2) individual faults are small and not readily resolvable at seismic scale; (3) the depth and carbonate nature of the Khuff reservoir make the fractures highly susceptible to fast healing unless preserved within the hydrocarbon column; (4) initial vertical pressure gradient changes with production indicate a lack of present-day communication across the anhydrite sealing layers, between the different Khuff reservoir units; (5) horizontal well direction does not generally have an impact on productivity; and (6) sustained and heavy losses of circulation are rarely encountered in the Khuff reservoir wells. Mohammed S. Ameen received his Ph.D. and Diploma of Imperial College in structural geology and geomechanics from Imperial College, London, 1988. He has more than 20 years of academic and industrial experience and has patented a new method for the characterization of microfractured reservoirs. He has published 25 articles on fractures and folds, and has edited three special publications for the Geological Society (London). He conducted the first classic work on the fractures and folds across the Taurus-Zagros Range, Iraq, covering 30 major fold traps. The work is published in the AAPG Bulletin, the Geological Magazine, and the Journal of Petroleum Geology. He joined the Reservoir Characterization Department at Saudi Aramco in 1998. Since 2004, he has been leading the Structural Geology and Rock Mechanics Group in the Geological Technical Services Division, Saudi Aramco. He is an active member of the AAPG, Society of Petroleum Engineers, European Association of Geoscientists and Engineers, and the Geological Society (London). Ismail M. Buhidma is a petroleum engineering consultant in the Gas Reservoir Management Division of Saudi Aramco with 32 years of diverse industrial experience. For the last 12 years, he has been actively involved in Aramco's nonassociated gas development program. His areas of interest include reservoir management, well performance analysis, well test analysis, phase behavior, geomechanics, reservoir simulation, reservoir characterization, and well stimulation. Prior to joining Aramco, Ismail worked for Exxon in Libya, Schlumberger and Atlantic Richfield in the USA, and Qatar Petroleum in Qatar. Ismail holds B.S. and M.S. degrees in petroleum engineering from the University of Tulsa. He is a member of Society of Petroleum Engineers (SPE) and has published numerous SPE appears. Zillur Rahim is a petroleum engineering consultant with Saudi Aramco working in gas field development. He received his B.S. degree from Algerian Petroleum Institute and M.S. degree and Ph.D. from Texas A&M University, College Station, all in petroleum engineering. He has 25 years of industry experience, has published more than 50 technical papers, and has taught numerous industry courses. Previously, he worked with Holditch and Associates Petroleum Consultants and with Schlumberger Reservoir Technology group. His area of expertise includes reservoir engineering, hydraulic and acid fracturing, and reservoir management.

Fault linkage and graben stepovers in the Canyonlands (Utah) and the North Sea Viking Graben, with implications for hydrocarbon migration and accumulation

Abstract: Segmented graben systems develop stepovers that have important implications in the exploration of oil and gas in extensional tectonic basins. We have compared and modeled a representative stepover between grabens in Canyonlands, Utah, and the North Sea Viking Graben and, despite their different structural settings, found striking similarities that pertain to other graben systems. In both cases, the stepovers represent relatively high parts within the graben systems that are likely to be among the first to be filled with hydrocarbons generated in deeper parts of the grabens. Furthermore, the relay ramps and smaller fault offsets in stepovers ease hydrocarbon migration and allow stepovers to act as preferred migration routes from deep graben kitchens to structurally higher traps in the basin. Graben stepovers and their related structures should be paid special attention during exploration because they may represent hydrocarbon accumulations complementary to larger traps along the graben flanks. These observations explain the location of the Kvitebjorn, Valemon, and Huldra fields in a stepover structure of the Viking Graben and encourage increased focus on similar graben stepovers in the Viking Graben and other graben systems.

Origin and timing of late diagenetic illite in the Permian–Carboniferous Unayzah sandstone reservoirs of Saudi Arabia

Abstract: Diagenetic illite is one of two major porosity occluding cements in the Unayzah sandstone reservoirs (Permian–Carboniferous) of Saudi Arabia. The other is diagenetic quartz. This article focuses on the origin of illite and its timing. Illite has been formed by a reaction of detrital K-feldspar and early diagenetic kaolinite as temperatures increased due to burial. When either of the two reactants is exhausted, illite ceases to precipitate. There is no evidence that hydrocarbon emplacement, deep brine migration, or unique thermal events are factors in illite precipitation. Modeling of illite precipitation as a kinetically controlled reaction using burial histories of the samples studied generally yields a reasonable match between measured and modeled ages and amounts of illite. This lends further support to the gradual formation of illite over a time-temperature interval. Although quartz overgrowths and diagenetic illite may occur in the same thin section, they appear to be mutually exclusive locally. Quartz overgrowths do not occur on detrital quartz grains that are coated with diagenetic illite, and illite is rarely observed on quartz overgrowths. Therefore, it appears that not only does diagenetic illite inhibit nucleation of quartz overgrowths but quartz overgrowths may also inhibit precipitation of diagenetic illite. The two cements appear to compete for surface area on uncoated detrital quartz grains.

Extensional fault segmentation and linkages, Bonaparte Basin, outer North West Shelf, Australia

Abstract: A detailed structural analysis of the Zone of Cooperation A-2 (ZOCA-2) three-dimensional (3-D) seismic survey on the outer northwestern edge of the Sahul Platform, northern Bonaparte Basin, outer North West Shelf, Australia, has identified three major populations of extensional faults. From oldest to youngest, these are (1) Jurassic north-south–trending extensional synrift faults, (2) Jurassic–Cretaceous east-west– to east-northeast–west-southwest–trending extensional faults, and (3) Neogene to present-day northeast-southwest–striking, right-stepping en-echelon faults. Seismic attribute analyses combined with fault-displacement analyses have illustrated the initial segmentation of all of the fault systems and characterized both the horizontal and vertical linkages formed by soft-linkage relay structures. The strongly segmented Neogene to present-day northeast-southwest–striking faults are interpreted to be a result of Neogene reactivation of underlying zones of weakness generated by preexisting east-northeast–trending Jurassic to Early Cretaceous faults. The Neogene to present-day northwest-southeast–directed extension was oblique to the underlying zones of weakness and thus formed characteristic strongly segmented en-echelon fault arrays. The Jurassic to Early Cretaceous extensional faults together with the overlying northeast-southwest–striking Neogene to present-day fault systems form a pseudoconjugate fault system separated by a Cretaceous sequence that acted as a decoupling horizon. Within this Cretaceous interval, which is characterized by polygonal fault systems, vertically segmented, isolated, and overlapping extensional fault arrays form a zone of soft linkage between the underlying Jurassic to Early Cretaceous rift faults and the overlying Neogene to present-day fault systems. This study shows that extension oblique to preexisting deeper fault systems produces en-echelon segmented extensional faults in the overlying sequences. Such en-echelon segmentation does not indicate major strike-slip deformation. The results of this research have implications for understanding the distribution, segmentation, linkages, and ages of extensional faults in many other rift basins as well as for the northern Bonaparte Basin. In addition, the complex 3-D linkage patterns shown in this study have significant implications for understanding trap geometries and fault-seal characteristics in other extensionally faulted basins.

Submarine channel response to intrabasinal tectonics: The influence of lateral tilt

Abstract: Lateral tilting is a common deformation style in extensional basins; its influence on subaerial channels is, to a degree, understood and may be significant, controlling the style of channel development and the resultant sand-body architecture. Growth faulting and lateral tilting in turbidite channel systems have been demonstrated from three-dimensional seismic data, but the resultant architecture of channels within these settings has not yet been documented. In the Carboniferous of northern England, a sand-rich slope channel, developed within a basin undergoing late-stage extension, underwent progressive and unidirectional migration toward a topographic low on a laterally tilting block. The resultant sandstone body is wedge shaped in cross section and composed dominantly of sigmoidal lateral accretion deposits. The channel returned to an axial course before undergoing lateral migration in the same direction, creating a multistory, multilateral channel sandstone body. The repeated unidirectional migration combined with evidence of syndepositional deformation suggests that active tectonism strongly influenced channel evolution and deposition. A model of submarine channel evolution in extensional basins is presented; in systems where large displacements occur, the channel system may avulse, creating isolated sand ribbons, which are connected updip; where the lateral dip is always more influential than the regional dip, the system may pond in the hanging-wall syncline. The model is compared to a subsurface channel within the Pliocene of the Nile Delta slope, which was influenced by syndepositional fault movement; application of the outcrop-derived model allows some simple architectural interpretations to be made.

Evaluating hydrocarbon trap integrity during fault reactivation using geomechanical three-dimensional modeling: An example from the Timor Sea, Australia

Abstract: Three-dimensional (3-D) coupled deformation and fluid-flow numerical modeling are used to simulate the response of a relatively complex set of trap-bounding faults to extensional reactivation and to investigate hydrocarbon preservation risk for structural traps in the offshore Bonaparte Basin (Laminaria High, the Timor Sea, Australian North West Shelf). The model results show that the distributions of shear strain and dilation as well as fluid flux are heterogeneous along fault planes inferring lateral variability of fault seal effectiveness. The distribution of high shear strain is seen as the main control on structural permeability and is primarily influenced by the structural architecture. Prereactivation fault size and distribution within the modeled fault population as well as fault corrugations driven by growth processes represent key elements driving the partitioning of strain and up-fault fluid flow. These factors are critical in determining oil preservation during the late reactivation phase on the Laminaria High. Testing of the model against leakage indicators defined on 3-D seismic data correlates with the numerical prediction of fault seal effectiveness and explains the complex distribution of paleo- and preserved oil columns in the study area.

Impact of deltaic clinothems on reservoir performance: Dynamic studies of reservoir analogs from the Ferron Sandstone Member and Panther Tongue, Utah

Abstract: Subsurface reservoir models are typically limited by a lack of spatially accurate geometric data on bedform architecture and geometry. These factors are key controls on fluid flow. Outcrop analogs have long been used as a source of such data, but the capture of sufficiently precise outcrop data is a challenge. The study presented in this article used highly accurate geometrical digital geological outcrop data collected using ground-based laser scanning (light detection and ranging [LIDAR]) to build and test three-dimensional geocellular models of deltaic reservoir analogs.Two well-exposed ancient river-dominated delta systems, the Panther Tongue and the Ferron Sandstone Member, which both crop out in central Utah, were digitally mapped to precisely recreate their clinothem and clinoform geometries in geocellular reservoir modeling software. Such clinoforms are commonly draped with low-permeability mudstones that produce reservoir heterogeneity by subdividing the deltaic sand body into a series of dipping sandstone beds (clinothems). A key aspect of the modeling was to accurately capture these geometries and their effect on simulated fluid flow.Portions of the two deltaic systems were dynamically analyzed in a reservoir modeling software by simulating production in 41 models. These models tested a range of mudstone barrier continuities and permeabilities. Results quantify how the continuation of the heterogeneities governed the production rate and recovery factor in the Panther Tongue models. Mudstone permeability values were more important in the Ferron Sandstone models with steeper dipping and closer spaced clinothems, although production was still influenced by the continuation of the heterogeneities.

Seismic geomorphology of offshore Morocco's east margin, Safi Haute Mer area

Abstract: The lower continental slope of Morocco's west coast consists of Triassic-age salt manifested in the form of diapirs, tongues, sheets, canopies, and toe thrusts. Active salt diapirism and regional tectonics greatly influence the morphology of the modern sea floor, forming a severely rugose expression with ongoing minibasin development and episodic submarine failure. Detailed mapping of a 1064-km2 (411-mi2) seismic survey acquired in the Safi Haute Mer area revealed that Jurassic to Holocene salt mobilization continually affected distribution of sediment, causing a range of depositional flow styles, from slumps to sheet slides and mass-transport complexes (MTCs). Large sediment waves (20 km [12 mi] long, 1.5-km [0.9-mi] wavelength) were also documented at the end of the Aptian. An east-west–trending structural anticline downdip of the salt activated during initiation of the Atlas uplift in the latest Cretaceous to earliest Tertiary and shaped much of the lower slope into the Tertiary with a persistent canyon system and slope channels. The largest of the debris flows is a Cretaceous-age MTC, a 500-m (1640-ft)-thick flow that spans an area of up to 20,000 km2 (7722 mi2). Composing the MTC are (1) chaotic, mounded seismic facies; (2) internal syndepositional thrusts; and (3) transported megablocks (3.3 km2 [1.3 mi2]) with preserved internal stratigraphy. The MTC originated from an upslope collapse of a narrow shelf during the earliest phases of the Alpine orogeny.