Showing papers in "AAPG Bulletin in 2013"

Organic matter–hosted pore system, Marcellus Formation (Devonian), Pennsylvania

Abstract: The Marcellus Formation of Pennsylvania represents an outstanding example of an organic matter (OM)–hosted pore system; most pores detectable by field-emission scanning electron microscopy (FE-SEM) are associated with OM instead of mineral matrix. In the two wells studied here, total organic carbon (TOC) content is a stronger control on OM-hosted porosity than is thermal maturity. The two study wells span a maturity from late wet gas (vitrinite reflectance [Ro], 1.0%) to dry gas (Ro, 2.1%). Samples with a TOC less than 5.5 wt. % display a positive correlation between TOC and porosity, but samples with a TOC greater than 5.5 wt. % display little or no increase in porosity with a further increasing TOC. In a subset of samples (14) across a range of TOC (2.3–13.6 wt. %), the pore volume detectable by FE-SEM is a small fraction of total porosity, ranging from 2 to 32% of the helium porosity. Importantly, the FE-SEM–visible porosity in OM decreases significantly with increasing TOC, diminishing from 30% of OM volume to less than 1% of OM volume across the range of TOC. The morphology and size of OM-hosted pores also vary systematically with TOC. The interpretation of this anticorrelation between OM content and SEM-visible pores remains uncertain. Samples with the lowest OM porosity (higher TOC) may represent gas expulsion (pore collapse) that was more complete as a consequence of greater OM connectivity and framework compaction, whereas samples with higher OM porosity (lower TOC) correspond to rigid mineral frameworks that inhibited compactional expulsion of methane-filled bubbles. Alternatively, higher TOC samples may contain OM (low initial hydrogen index, relatively unreactive) that is less prone to development of FE-SEM–detectable pores. In this interpretation, OM type, controlled by sequence-stratigraphic position, is a factor in determining pore-size distribution.

Porosity of Devonian and Mississippian New Albany Shale across a maturation gradient: Insights from organic petrology, gas adsorption, and mercury intrusion

Abstract: The evolution of porosity in shales with increasing maturity was examined in a suite of five New Albany Shale samples spanning a maturity range from immature (vitrinite reflectance, Ro 0.35%) to postmature (Ro 1.41%). Devonian to lower Mississippian New Albany Shale samples from the Illinois Basin used in this study contain marine type II kerogen having total organic carbon contents from 1.2 to 13.0 wt. %. Organic petrology, CO2 and N2 low-pressure adsorption, and mercury intrusion capillary pressure techniques were used to quantify pore volumes, pore sizes, and pore-size distributions. Increasing maturity of the New Albany Shale is paralleled by many changes in the characteristics of porosity. The total porosity of 9.1 vol. % in immature New Albany Shale decreases to 1.5 vol. % in the late mature sample, whereas total pore volumes decrease from 0.0365 to 0.0059 cm3/g in the same sequence. Reversing the trend at even higher maturity, the postmature New Albany Shale exhibits higher porosity and larger total pore volumes compared to the late mature sample. With increasing maturity, changes in total porosity and total pore volumes are accompanied by changes in pore-size distributions and relative proportions of micropores, mesopores, and macropores. Porosity-related variances are directly related to differences in the amount and character of the organic matter and mineralogical composition, but maturity exerts the dominant control upon these characteristics. We conclude that organic matter transformation due to hydrocarbon generation and migration is a pivotal cause of the observed porosity differences.

Mechanisms of shale gas storage: Implications for shale gas exploration in China

Abstract: This article reviews the mechanisms of shale gas storage and discusses the major risks or uncertainties for shale gas exploration in China. At a given temperature and pressure, the gas sorption capacities of organic-rich shales are primarily controlled by the organic matter richness but may be significantly influenced by the type and maturity of the organic matter, mineral composition (especially clay content), moisture content, pore volume and structure, resulting in different ratios of gas sorption capacity (GSC) to total organic carbon content for different shales. In laboratory experiments, the GSC of organic-rich shales increases with increasing pressure and decreases with increasing temperature. Under geologic conditions (assuming hydrostatic pressure gradient and constant thermal gradient), the GSC increases initially with depth due to the predominating effect of pressure, passes through a maximum, and then decreases because of the influence of increasing temperature at greater depth. This pattern of variation is quite similar to that observed for coals and is of great significance for understanding the changes in GSC of organic-rich shales over geologic time as a function of burial history. At an elevated temperature and pressure and with the presence of moisture, the gas sorption capacities of organic-rich shales are quite low. As a result, adsorption alone cannot protect sufficient gas for high-maturity organic-rich shales to be commercial gas reservoirs. Two models are proposed to predict the variation of GSC and total gas content over geologic time as a function of burial history. High contents of free gas in organic-rich shales can be preserved in relatively closed systems. Loss of free gas during postgeneration uplift and erosion may result in undersaturation (the total gas contents lower than the sorption capacity) and is the major risk for gas exploration in marine organic-rich shales in China.

Organic-rich Marcellus Shale lithofacies modeling and distribution pattern analysis in the Appalachian Basin

Abstract: The Marcellus Shale is considered to be the largest unconventional shale-gas resource in the United States. Two critical factors for unconventional shale reservoirs are the response of a unit to hydraulic fracture stimulation and gas content. The fracture attributes reflect the geomechanical properties of the rocks, which are partly related to rock mineralogy. The natural gas content of a shale reservoir rock is strongly linked to organic matter content, measured by total organic carbon (TOC). A mudstone lithofacies is a vertically and laterally continuous zone with similar mineral composition, rock geomechanical properties, and TOC content. Core, log, and seismic data were used to build a three-dimensional (3-D) mudrock lithofacies model from core to wells and, finally, to regional scale. An artificial neural network was used for lithofacies prediction. Eight petrophysical parameters derived from conventional logs were determined as critical inputs. Advanced logs, such as pulsed neutron spectroscopy, with log-determined mineral composition and TOC data were used to improve and confirm the quantitative relationship between conventional logs and lithofacies. Sequential indicator simulation performed well for 3-D modeling of Marcellus Shale lithofacies. The interplay of dilution by terrigenous detritus, organic matter productivity, and organic matter preservation and decomposition affected the distribution of Marcellus Shale lithofacies distribution, which may be attributed to water depth and the distance to shoreline. The trend of normalized average gas production rate from horizontal wells supported our approach to modeling Marcellus Shale lithofacies. The proposed 3-D modeling approach may be helpful for optimizing the design of horizontal well trajectories and hydraulic fracture stimulation strategies.

Jurassic evolution of the Gulf of Mexico salt basin

Abstract: We present a new hypothesis for the Jurassic plate-tectonic evolution of the Gulf of Mexico basin and discuss how this evolution influenced Jurassic salt tectonics. Four interpretations, some based on new data, constrain the hypothesis. First, the limit of normal oceanic crust coincides with a landward-dipping basement ramp near the seaward end of the salt basin, which has been mapped on seismic data. Second, the deep salt in the deep-water Gulf of Mexico can be separated into provinces on the basis of position with respect to this ramp. Third, paleodepths in the postsalt sequence indicate that salt filled the Gulf of Mexico salt basin to near sea level. Fourth, seismic data show that postsalt sediments in the central Louann and the Yucatan salt basins exhibit large magnitudes of Late Jurassic salt-detached extension not balanced by equivalent salt-detached shortening. In our hypothesis, Callovian salt was deposited in preexisting crustal depressions on hyperextended continental and transitional crust. After salt deposition ended, rifting continued for another 7 to 12 m.y. before sea-floor spreading began. During this phase of postsalt crustal stretching, the salt and its overburden were extended by 100 to 250 km (62–155 mi), depending on location. Sea-floor spreading divided the northern Gulf of Mexico into two segments, separated by the northwest-trending Brazos transform. The eastern segment opened from east to west, leaving the Walker Ridge salient in the center of the basin as the final area to break apart. In some areas, salt flowed seaward onto new oceanic crust, first concordantly over the basement as a parautochthonous province, then climbing up over stratigraphically younger strata as an allochthonous province.

A new theoretical approach to model sorption-induced coal shrinkage or swelling

Abstract: The shrinkage or swelling of coal as a result of gas desorption or adsorption is a well-accepted phenomenon. Its impact on permeability changes has also been recognized for two decades. Its importance has increased significantly because of the potential of coals that are not likely to be mined and depleted or nearly depleted coalbed methane reservoirs to serve as CO2 repositories. This article proposes a new theoretical technique to model the volumetric changes in the coal matrix during gas desorption or adsorption using the elastic properties, sorption parameters, and physical properties of coal. The proposed model is based on the theory of changes in surface energy as a result of sorption. The results show that the proposed model is in excellent agreement with the laboratory volumetric strain data presented in the literature during the last 50 yr. Furthermore, the proposed model can be extended to describe mixed-gas sorption behavior, which can be applied to enhanced coalbed methane and CO2 sequestration operations.

Upper Miocene to Quaternary unidirectionally migrating deep-water channels in the Pearl River Mouth Basin, northern South China Sea

Abstract: A series of short and steep unidirectionally migrating deep-water channels, which are typically without levees and migrate progressively northeastward, are identified in the Baiyun depression, Pearl River Mouth Basin. Using three-dimensional seismic and well data, the current study documents their morphology, internal architecture, and depositional history, and discusses the distribution and depositional controls on the bottom current–reworked sands within these channels. Unidirectionally migrating deep-water channels consist of different channel-complex sets (CCSs) that are, overall, short and steep, and their northeastern walls are, overall, steeper than their southwestern counterparts. Within each CCS, bottom current–reworked sands in the lower part grade upward into muddy slumps and debris-flow deposits and, finally, into shale drapes. Three stages of CCSs development are recognized: (1) the early lowstand incision stage, during which intense gravity and/or turbidity flows versus relatively weak along-slope bottom currents of the North Pacific intermediate water (NPIW-BCs) resulted in basal erosional bounding surfaces and limited bottom current–reworked sands; (2) the late lowstand lateral-migration and active-fill stage, with gradual CCS widening and progressively northeastward migration, characterized by reworking of gravity- and/or turbidity-flow deposits by vigorous NPIW-BCs and the CCSs being mainly filled by bottom current–reworked sands and limited slumps and debris-flow deposits; and (3) the transgression abandonment stage, characterized by the termination of the gravity and/or turbidity flows and the CCSs being widely draped by marine shales. These three stages repeated through time, leading to the generation of unidirectionally migrating deep-water channels. The distribution of the bottom current–reworked sands varies both spatially and temporally. Spatially, these sands mainly accumulate along the axis of the unidirectionally migrating deep-water channels and are preferentially deposited to the side toward which the channels migrated. Temporally, these sands mainly accumulated during the late lowstand lateral-migration and active-fill stage. The bottom current–reworked sands developed under the combined action of gravity and/or turbidity flows and along-slope bottom currents of NPIW-BCs. Other factors, including relative sea level fluctuations, sediment supply, and slope configurations, also affected the formation and distribution of these sands. The proposed distribution pattern of the bottom current–reworked sands has practical implications for predicting reservoir occurrence and distribution in bottom current–related channels.

Evaluation of sampling methods for fracture network characterization using outcrops

Abstract: Outcrops provide valuable information for the characterization of fracture networks. Sampling methods such as scanline sampling, window sampling, and circular scanline and window methods are available to measure fracture network characteristics in outcrops and from well cores. These methods vary in their application, the parameters they provide and, therefore, have advantages and limitations. We provide a critical review on the application of these sampling methods and apply them to evaluate two typical natural examples: (1) a large-scale satellite image from the Oman Mountains, Oman (120,000 m2 [1,291,669 ft2]), and (2) a small-scale outcrop at Craghouse Park, United Kingdom (19 m2 [205 ft2]). The differences in the results emphasize the importance to (1) systematically investigate the required minimum number of measurements for each sampling method and (2) quantify the influence of censored fractures on the estimation of fracture network parameters. Hence, a program was developed to analyze 1300 sampling areas from 9 artificial fracture networks with power-law length distributions. For the given settings, the lowest minimum number of measurements to adequately capture the statistical properties of fracture networks was found to be approximately 110 for the window sampling method, followed by the scanline sampling method with approximately 225. These numbers may serve as a guideline for the analyses of fracture populations with similar distributions. Furthermore, the window sampling method proved to be the method that is least sensitive to censoring bias. Reevaluating our natural examples with the window sampling method showed that the existing percentage of censored fractures significantly influences the accuracy of inferred fracture network parameters.

Evidence for overpressure generation by kerogen-to-gas maturation in the northern Malay Basin

Abstract: Gas generation is a commonly hypothesized mechanism for the development of high-magnitude overpressure. However, overpressures developed by gas generation have been rarely measured in situ, with the main evidence for such overpressures coming from source rock microfractures, the physical necessity of overpressures for primary migration, laboratory experiments, and numerical modeling. Indeed, previous in-situ observations suggest that gas generation only creates highly localized overpressures within rich source rocks. Pore-fluid pressure data and sonic velocity–vertical effective stress plots from 30 wells reveal that overpressures in the northern Malay Basin are primarily generated by fluid expansion and are located basinwide within the Miocene 2A, 2B, and 2C source rock formations. The overpressures are predominantly associated with gas sampled in more than 83% of overpressure measurements and have a sonic-density response consistent with gas generation. The association of fluid expansion overpressures with gas, combined with the sonic-density response to overpressure and a regional geology that precludes other overpressuring mechanisms, provides convincing in-situ evidence for basinwide gas generation overpressuring. Overpressure magnitude analysis suggests that gas generation accounts for approximately one-half to two-thirds of the measured excess pore pressure in the region, with the remainder being generated by coincident disequilibrium compaction. Thus, the data herein suggest that gas generation, if acting in isolation, is producing a maximum pressure gradient of 15.3 MPa/km (0.676 psi/ft) and not lithostatic magnitudes as commonly hypothesized. The gas generation overpressures in this article are not associated with a significant porosity anomaly and represent a major drilling hazard, with traditional pore-pressure prediction techniques underestimating pressure gradients by 2.3 ± 1.5 MPa/km (0.1 ± 0.07 psi/ft).

Fractured tight sandstone oil and gas reservoirs: A new play type in the Dongpu depression, Bohai Bay Basin, China

Abstract: Although conventional reservoirs dominate the Bohai Basin, China, a new type of sandstone reservoir also exists in the Dongpu depression that has a low matrix porosity (tight) in which natural fractures govern both permeability and porosity. These fractured sandstones are located on a structurally modified buried hill underlying Paleogene mudstones, and are truncated along an angular unconformity. The fractured sandstone oils of the Triassic Liujiagou, Heshanggou, and Ermaying Formations are derived from the Paleogene Shahejie Formation, which reached peak oil generation and expulsion during the Oligocene to early Miocene (32.8–15.6 Ma). Gas was generated primarily during the Paleogene from Carboniferous and Permian coals. Petrographic evidence suggests that oil and gas emplacement followed the compaction and cementation of the Triassic sandstone reservoirs. Fluid inclusion evidence and burial history analysis suggest that fractures developed before oil emplacement but may have coincided with peak gas generation, which suggests that oil and gas mainly migrated and accumulated in fractures.

The role of fluid pressure and diagenetic cements for porosity preservation in Triassic fluvial reservoirs of the Central Graben, North Sea

Abstract: Anomalously high porosities and permeabilities are commonly found in the fluvial channel sandstone facies of the Triassic Skagerrak Formation in the central North Sea at burial depths greater than 3200 m (10,499 ft), from which hydrocarbons are currently being produced. The aim of our study was to improve understanding of sandstone diagenesis in the Skagerrak Formation to help predict whether the facies with high porosity may be found at even greater depths. The Skagerrak sandstones comprise fine to medium-grained arkosic to lithic-arkosic arenites. We have used scanning electron microscopy, petrographic analysis, pressure history modeling, and core analysis to assess the timing of growth and origin of mineral cements, with generation, and the impact of high fluid pressure on reservoir quality. Our interpretation is that the anomalously high porosities in the Skagerrak sandstones were maintained by a history of overpressure generation and maintenance from the Late Triassic onward, in combination with early microquartz cementation and subsequent precipitation of robust chlorite grain coats. Increasing salinity of pore fluids during burial diagenesis led to pore-filling halite cements in sustained phreatic conditions. The halite pore-filling cements removed most of the remaining porosity and limited the precipitation of other diagenetic phases. Fluid flow associated with the migration of hydrocarbons during the Neogene is inferred to have dissolved the halite locally. Dissolution of halite cements in the channel sands has given rise to megapores and porosities of as much as 35% at current production depths.

Insight into petrophysical properties of deformed sandstone reservoirs

Abstract: We use samples from undeformed and deformed sandstones (single deformation band, deformation band cluster, slip-surface cataclasite, and fault core slip zone) to characterize their petrophysical properties (porosity, permeability, and capillary pressure). Relationships between permeability and porosity are described by power-law regressions where the power-law exponent (D) decreases with the increasing degree of deformation (strain) experienced by the sample from host rock (D, ∼9) to fault core (D, ∼5). The approaches introduced in this work will allow geologists to use permeability and/or porosity measurements to estimate the capillary pressures and sealing capacity of different fault-related rocks without requiring direct laboratory measurements of capillary pressure. Results show that fault core slip zones have the highest theoretical sealing capacity (>140-m [459-ft] oil column in extreme cases), although our calculations suggest that deformation bands can locally act as efficiently as fault core slip zones in sealing nonwetting fluids (in this study, oil and CO2). Higher interfacial tension between brine and CO2 (because of the sensitivity of CO2 to temperature and pressure) results in higher capillary pressure and sealing capacity in a brine and CO2 system than a brine and oil system for the same samples.

Modern internal waves and internal tides along oceanic pycnoclines: Challenges and implications for ancient deep-marine baroclinic sands

Abstract: Thus far, the subject of deep-marine sands emplaced by baroclinic currents associated with internal waves and internal tides as potential reservoirs has remained an alien topic in petroleum exploration. Internal waves are gravity waves that oscillate along oceanic pycnoclines. Internal tides are internal waves with a tidal frequency. Internal solitary waves (i.e., solitons), the most common type, are commonly generated near the shelf edge (100–200 m [328–656 ft] in bathymetry) and in the deep ocean over areas of sea-floor irregularities, such as mid-ocean ridges, seamounts, and guyots. Empirical data from 51 locations in the Atlantic, Pacific, Indian, Arctic, and Antarctic oceans reveal that internal solitary waves travel in packets. Internal waves commonly exhibit (1) higher wave amplitudes (5–50 m [16–164 ft]) than surface waves (2 m [6.56 ft]), (2) longer wavelengths (0.5–15 km [0.31–9 mi]) than surface waves (100 m [328 ft]), (3) longer wave periods (5–50 min) than surface waves (9–10 s), and (4) higher wave speeds (0.5–2 m s–1 [1.64–6.56 ft s–1]) than surface waves (25 cm s–1 [10 in. s–1]). Maximum speeds of 48 cm s–1 (19 in. s–1) for baroclinic currents were measured on guyots. However, core-based sedimentologic studies of modern sediments emplaced by baroclinic currents on continental slopes, in submarine canyons, and on submarine guyots are lacking. No cogent sedimentologic or seismic criteria exist for distinguishing ancient counterparts. Outcrop-based facies models of these deposits are untenable. Therefore, potential exists for misinterpreting deep-marine baroclinic sands as turbidites, contourites, basin-floor fans, and others. Economic risks associated with such misinterpretations could be real.

Methods for identification of isolated carbonate buildups from seismic reflection data

Abstract: Isolated carbonate buildups (ICBs) are commonly attractive exploration targets. However, identifying ICBs based only on seismic data can be difficult for a variety of reasons. These include poor-quality two-dimensional data and a basic similarity between ICBs and other features such as volcanoes, erosional remnants, and tilted fault blocks. To address these difficulties and develop reliable methods to identify ICBs, 234 seismic images were analyzed. The images included proven ICBs and other features, such as folds, volcanoes, and basement highs, which may appear similar to ICBs when imaged in seismic data. From this analysis, 18 identification criteria were derived to distinguish ICBs from non-ICB features. These criteria can be grouped into four categories: regional constraints, analysis of basic seismic geometries, analysis of geophysical details, and finer-scale seismic geometries. Systematically assessing the criteria is useful because it requires critical evaluation of the evidence present in the available data, working from the large-scale regional geology to the fine details of seismic response. It is also useful to summarize the criteria as a numerical score to facilitate comparison between different examples and different classes of ICBs and non-ICBs. Our analysis of scores of different classes of features suggests that the criteria do have some discriminatory power, but significant challenges remain.

A hierarchical approach to architectural classification in marginal-marine systems: Bridging the gap between sedimentology and sequence stratigraphy

Abstract: A new hierarchical architectural classification for clastic marginal-marine depositional systems is presented and illustrated with examples. In ancient rocks, the architectural scheme effectively integrates the scales of sedimentology (core, outcrop) and sequence stratigraphy (wireline-log correlation, reflection seismic). The classification also applies to modern sediments, which allows for direct comparison of architectural units between modern and ancient settings. In marginal-marine systems, the parasequence typically defines reservoir flow units. This classification addresses subparasequence scales of stratigraphy that commonly control fluid flow in these reservoirs. The scheme consists of seven types of architectural units that are placed on five architectural hierarchy levels: hierarchy level I: element (E) and element set (ES); hierarchy level II: element complex (EC) and element complex set (ECS); hierarchy level III: element complex assemblage (ECA); hierarchy level IV: element complex assemblage set (ECAS); and hierarchy level V: transgressive-regressive sequence (T-R sequence). Architectural units in levels I to III are further classified relative to dominant depositional processes (wave, tide, and fluvial) acting at the time of deposition. All architectural units are three-dimensional and can also be expressed in terms of plan-view and cross-sectional geometries. Architectural units can be linked using tree data structures by a set of familial relationships (parent-child, siblings, and cousins), which provides a novel mechanism for managing uncertainty in marginal-marine systems. Using a hierarchical scheme permits classification of different data types at the most appropriate architectural scale. The use of the classification is illustrated in ancient settings by an outcrop and subsurface example from the Campanian Bearpaw–Horseshoe Canyon Formations transition, Alberta, Canada, and in modern settings, by the Mitchell River Delta, northern Australia. The case studies illustrate how the new classification can be used across both modern and ancient systems, in complicated, mixed-process depositional environments.

Ordered low-temperature dolomite mediated by carboxyl-group density of microbial cell walls

Abstract: Abundant in the ancient rock record, early dolomite remains scarce in modern systems at low temperatures (50C), even those systems supersaturated with respect to dolomite. This scarcity is attributed to kinetic inhibition including complexation of Mg2+ by water and sulfate, carbonate activity, and Mg:Ca ratio. Recent investigations point to a function for microbial metabolisms and surfaces, in which disordered phases are formed. Here, we report the precipitation of primary ordered dolomite at 30C, facilitated solely by the cell walls of two nonmetabolizing archaea from saline solutions with an Mg:Ca ratio of 1:1, 5:1, and 10:1, and slightly saturated with respect to dolomite. Control experiments using bacteria and functionalized microspheres did not precipitate dolomite. Archaeal cell wall functional groups were approximately one order of magnitude higher than the bacteria and spheres used in this study. From these results, we propose a mechanistic model in which carboxyl groups associated with cell wall biomass and exopolymeric substances dehydrate Mg ions, further promoting carbonation and leading to dolomite nucleation. These data explain reports of low-temperature dolomite formation associated with numerous microbial metabolic guilds, including bacteria and archaea, and those reported in association with exopolymeric substances or cell wall surfaces, and identify a key and widespread mechanism in the formation of disordered dolomite and ordered primary phases of dolomite at low temperature. Importantly, the functionalized dead and nonmetabolizing biomass is the key in low-temperature dolomite precipitation, not active microbial metabolism. These observations may lead to new predictive models for the distribution of dolomite.

Diagenesis and quartz cement distribution of low-permeability Upper Triassic–Middle Jurassic reservoir sandstones, Longyearbyen CO2 lab well site in Svalbard, Norway

Abstract: Reservoir properties of Upper Triassic–Middle Jurassic sandstones, Spitsbergen, are studied as part of a CO2 storage pilot project in Longyearbyen. The reservoir formations show large contrasts in sandstone compositions, with unexpected low permeability despite moderate porosity values. Petrographic analyses were performed to investigate the influence and distribution of diagenesis. It is concluded that, because of various compaction, cementation, and dissolution processes, the sandstone porosity is mainly isolated molds and micropores and associated with fibrous illite and chamosite, explaining the low permeability. Diagenesis and the distribution of quartz cement is influenced by lithofacies and detrital compositions. Mineralogically immature sandstones (De Geerdalen Formation) show a homogeneous distribution of quartz cement overgrowths on quartz grains, distributed interstitial to labile grains and other cements (e.g., late calcite). The main silica source was from the dissolution of adjacent feldspar and labile grains as part of the chemical compaction. In contrast, quartz-dominated sandstones (Knorringfjellet Formation) show a heterogeneous patchy distribution of quartz cement influenced by the sedimentary bioturbation pattern, with silica sourced also from dissolution at clay-rich microstylolites. Phosphatic beds at the base and top of the formation are strongly influenced by marine eogenesis and reworking processes and associated with concentration of iron-rich authigenic minerals. The highest porosity appears in sand-supported conglomerate where moldic clay-mineral ooids contributed to reduce quartz cementation. The stratigraphic change from mineralogical immature (Triassic) to mature (uppermost Triassic–Jurassic) sandstone compositions is detected in wide areas of the Barents Shelf and has considerable implications for the distribution of sandstone reservoir properties.

Influence of deep Louann structure on the evolution of the northern Gulf of Mexico

Abstract: Three aspects of basement structure and rift-related salt distribution have especially influenced the evolution of the deep-water northern Gulf of Mexico: (1) creation of a basement high (Toledo Bend flexure), separating a chain of interior basins from the central Louann salt basin, (2) segmentation of the central Louann salt basin by the Brazos transfer fault into eastern and central domains, and (3) salt provinces formed during basin opening. The Toledo Bend flexure was reactivated as a hinge during the Cenozoic uplift of the North American craton. This uplift triggered gravity gliding, forming fold belts in the seaward parts of the continental margin. The geometry of the Toledo Bend flexure influenced the position of these fold belts. The Brazos transfer fault separates the west sector of the study area from the central and east sectors. Most of the salt in the deep-water northern Gulf of Mexico lay in the central sector, which sourced most of the Sigsbee salt canopy. The western sector was narrower and was subdivided by the East Breaks basement high. Splitting the Callovian salt basin in two as the gulf opened created a southward-thinning wedge of salt at the seaward end of the northern Gulf of Mexico. We divide this wedge into a series of provinces on the basis of the geometry of the base of the deep salt. Original salt thickness influenced diapir location, the geometry of the Sigsbee canopy, the geometry and style of later compressional fold belts, and petroleum systems.

Western Greece and Ionian Sea petroleum systems

Abstract: In western Greece, the Ionian and pre-Apulian zones represent, respectively, the basin and the transitional zone (slope) to the Apulian platform. The Apulian platform constitutes the weakly deformed foreland of the external Hellenides. The pre-Apulian zone appears in the Ionian Islands and the eastern Ionian Sea, whereas the Apulian platform is exclusively found in the Ionian Sea. The Ionian zone consists of Triassic evaporites, Jurassic– upper Eocene (mostly pelagic carbonates, minor cherts, and shales), overlainbytheOligoceneflysch.Organic-richsourcerocks occur within Triassic evaporites and Jurassic–Cretaceous pelagic argillaceous-siliceous rocks. The pre-Apulian zone consists of Triassic to Miocene deposits, mainly mixed neritic-pelagic carbonates. Hydrocarbon source rocks include pelagic and hemipelagic deposits rich in marine organic material, although terrigenous organic matter is also found in siliciclastic layers. Apulian platformsourcerocksaremainlytheorganic-richshaleswithinthe Triassic Burano evaporites. Western Greece contains major petroleum systems, which extend into the Ionian Sea. Ionian, pre-Apulian, and Apulian petroleum systems contribute to the probable hydrocarbon accumulations within the big offshore (Ionian Sea) anticlines. Western Greece contains important oil and gas shale reservoirs with a potential of unconventional exploration. Promising areas for hydrocarbons need systematic and detailed threedimensional seismic data. Exploration for conventional petroleum reservoirs, through the interpretation of seismic profiles and the abundant surface geologic data, will provide the subsurface geometric characteristics of the unconventional reservoirs. Their exploitation should follow that of conventional hydrocarbons to benefit from the anticipated technological advances, eliminating environmental repercussions.

Tectonic versus diagenetic origin of fractures in a naturally fractured carbonate reservoir analog (Nerthe anticline, southeastern France)

Abstract: Field analogs allow a better characterization of fracture networks to constrain naturally fractured reservoir models. In analogs, the origin, nature, geometry, and other attributes of fracture networks can be determined and can be related to the reservoir through the geodynamic history. In this article, we aim to determine the sedimentary and diagenetic controls on fracture patterns and the genetic correlation of fracture and diagenesis with tectonic and burial history. We targeted two outcrops of Barremian carbonates located on both limbs of the Nerthe anticline (southeastern France). We analyzed fracture patterns and rock facies as well as the tectonic, diagenetic, and burial history of both sites. Fracture patterns are determined from geometric, kinematic, and diagenetic criteria based on field and lab measurements. Fracture sequences are defined based on crosscutting and abutting relationships and compared with geodynamic history and subsidence curves. This analysis shows that fractures are organized in two close-to-perpendicular joint sets (i.e., mode I). Fracture average spacing is 50 cm (20 in.). Fracture size neither depends on fracture orientation nor is controlled by bed thickness. Neither mechanical stratigraphy nor fracture stratigraphy is observed at outcrop scale. Comparing fracture sequences and subsidence curves shows that fractures existed prior to folding and formed during early burial. Consequently, the Nerthe fold induced by the Pyrenean compression did not result in any new fracture initiation on the limbs of this fold. We assume that the studied Urgonian carbonates underwent early diagenesis, which conferred early brittle properties to the host rock.

Sequence-stratigraphic architectures and sand-body distribution in Cenozoic rifted lacustrine basins, east China

Abstract: Cenozoic rifted lacustrine basins in east China display three main basin types: (1) basins with steeply dipping boundary fault, whose hanging walls tilt along pivot points; (2) basins with listric boundary faults, whose hanging walls bend along flexural bending points; and (3) basins formed by earlier extensional rifting with later strike-slip movement superimposed. The sequence development is intimately linked to the tectonic movements in the area, where second-order sequences are regionally correlatable from basin to basin and relate to the large-scale tectonic movements in the region. Third-order sequences are related to local tectonic activity and are correlatable within basins, between subbasins, and sometimes, between neighboring basins. Detailed sequence-stratigraphic analysis and mapping of depositional systems demonstrate that sand-body distribution patterns are related to sequence-stratigraphic frameworks. For the three kinds of basins, the positions of pivot point zones, flexural bend zones, and strike-slip faults plus the syndepositional faults all control the distribution of depositional systems, systems tract, and sand bodies. These controlling factors can be attributed to different structural and stratigraphic features that change the accommodation. Structural elements include boundary faults, syndepositional faults, and abrupt changes in dip. Stratigraphic controls include preexisting surfaces with local channelization, paleobathymetric lows, and onlap onto clinoform slopes. The lowstand sand bodies deposited at the downdip end of these controlling factors constitute the current and future exploration targets for conventional reservoirs.

Variable gas content, saturation, and accumulation characteristics of Weibei coalbed methane pilot-production field in the southeastern Ordos Basin, China

Abstract: Using diverse geologic and geophysical data from recent exploration and development, and experimental results of analysis of gas content, gas capacity, and gas composition, this article discusses how geologic, structural, and hydrological factors determine the heterogeneous distribution of gas in the Weibei coalbed methane (CBM) field. The coal rank of the Pennsylvanian no. 5 coal seam is mainly low-volatile bituminous and semianthracite. The total gas content is 2.69 to 16.15 m3/t (95.00–570.33 scf/t), and gas saturation is 26.0% to 93.2%. Burial coalification followed by tectonically driven hydrothermal activity controls not only thermal maturity, but also the quality and quantity of thermogenic gas generated from the coal. Gas composition indicates that the CBM is dry and of dominantly thermogenic origin. The thermogenic gases have been altered by fractionation that may be related to subsurface water movement in the southern part of the study area. Three gas accumulation models are identified: (1) gas diffusion and long-distance migration of thermogenic gases to no-flow boundaries for sorption and minor conventional trapping, (2) hydrodynamic trapping of gas in structural lows, and (3) gas loss by hydrodynamic flushing. The first two models are applicable for the formation of two CBM enrichment areas in blocks B3 and B4, whereas the last model explains extremely low gas content and gas saturation in block B5. The variable gas content, saturation, and accumulation characteristics are mainly controlled by these gas accumulation models.

Outcrop-based reservoir characterization of a kilometer-scale sand-injectite complex

Abstract: This study documents that Danian-aged sand remobilization of deep-water slope-channel complexes and intrusion of fluidized sand into hydraulically fractured slope mudstones of the Great Valley sequence, California, generated 400-m (1312 ft)–thick reservoir units: unit 1, parent unit channel complexes for shallower sandstone intrusions; unit 2, a moderate net-to-gross interval (0.19 sand) of sills with staggered, stepped, and multilayer geometries with well-developed lateral sandstone-body connectivity; unit 3, a low net-to-gross interval (0.08 sand) of exclusively high-angle dikes with good vertical connectivity; and unit 4, an interval of extrusive sandstone. Unit 2 was formed during a phase of fluidization that emplaced on an average 0.19 km3 (0.046 mi3) of sand per cubic kilometer of host sediment. Probe permeametry data reveal a positive relationship between sill thickness and permeability. Reservoir quality is reduced by the presence of fragments of host strata, such as the incorporation of large rafts of mudstone, which are formed by in-situ hydraulic fracturing during sand injection. Mudstone clasts and clay- and silt-size particles generated by intrusion-induced abrasion of the host strata reduce sandstone permeability in multilayer sills (70 md) when compared to that in staggered and stepped sills (586 and 1225 md, respectively). Post-injection cementation greatly reduces permeability in high-angle dikes (81 md). This architecturally based reservoir zonation and trends in reservoir characteristics in dikes and sills form a basis for quantitative reservoir modeling and can be used to support conceptual interpretations that infer injectite architecture in situations where sands in low net-to-gross intervals are anticipated to have well-developed lateral and vertical connectivity.

Outcrop analog for an oolitic carbonate ramp reservoir: A scale-dependent geologic modeling approach based on stratigraphic hierarchy

Abstract: Considerable effort has been devoted to the development of simulation algorithms for facies modeling, whereas a discussion of how to combine those techniques has not existed. The integration of multiple geologic data into a three-dimensional model, which requires the combination of simulation techniques, is yet a current challenge for reservoir modeling. This article presents a thought process that guides the acquisition and modeling of geologic data at various scales. Our work is based on outcrop data collected from a Jurassic carbonate ramp located in the High Atlas mountain range of Morocco. The study window is 1 km (0.6 mi) wide and 100 m (328.1 ft) thick. We describe and model the spatial and hierarchical arrangement of carbonate bodies spanning from largest to smallest: (1) stacking pattern of high-frequency depositional sequences, (2) facies association, and (3) lithofacies. Five sequence boundaries were modeled using differential global position system mapping and light detection and ranging data. The surface-based model shows a low-angle profile with modest paleotopographic relief at the inner-to-middle ramp transition. Facies associations were populated using truncated Gaussian simulation to preserve ordered trends between the inner, middle, and outer ramps. At the lithofacies scale, field observations and statistical analysis show a mosaiclike distribution that was simulated using a fully stochastic approach with sequential indicator simulation. This study observes that the use of one single simulation technique is unlikely to correctly model the natural patterns and variability of carbonate rocks. The selection and implementation of different techniques customized for each level of the stratigraphic hierarchy will provide the essential computing flexibility to model carbonate settings. This study demonstrates that a scale-dependent modeling approach should be a common procedure when building subsurface and outcrop models.

Lithofacies, stable isotopic composition, and stratigraphic evolution of microbial and associated carbonates, Green River Formation (Eocene), Piceance Basin, Colorado

Abstract: Lacustrine carbonates of the Eocene Green River Formation crop out on the western margin of the Piceance Basin and the eastern margin of the Uinta Basin, in western Colorado. This area allows tracing of vertical and horizontal facies variation over hundreds of meters. Limestone beds consist of littoral to sublittoral lithofacies: bioclastic and oolitic grainstones, oolitic wackestone, intraclastic rudstone, stromatolites, and thrombolites. Facies form upward-deepening cycles that start with sharp-based grainstones and packstones followed by stromatolites or thrombolites and capped by fine-grained stromatolites and/or oil shale deposits. The vertical succession of carbonate deposits correlates with evolutionary lake stages. The succession starts with grainstone deposits rich in ostracods and gastropods that correspond to an initial freshwater lake. Thrombolites capped by laminated stromatolites or coarse-agglutinated stromatolites correlate with a higher-salinity transitional lake. Deepening-upward cycles, as much as 5 m (16 ft) thick, of thrombolites, agglutinated stromatolites, and fine-grained stromatolites occur in the highly fluctuating lake. The upper section is dominated by laminated stromatolites that correspond to a rising lake. Stable isotope 18O and 13C values covary and range from 8 to +0.8 and 3 to +5, respectively. The 18O values indicate carbonate-precipitating water evolved from fresh to saline and became less saline in the upper Green River. Negative excursions of 13C values correspond to lake level rises, and positive excursions of 13C values occur during lake level falls. Syndepositional to burial diagenesis modified carbonate porosity. Early dissolution is followed by burial compaction and fracturing. Compaction and late calcite cements occluded primary and secondary porosity.

Seismic modeling in the analysis of deep-water sandstone termination styles

Abstract: Innovative seismic forward modeling is used to illustrate the sensitivity within seismic data, and its application in the interpretation of onlap and pinch-out of terminating deep-water sandstones, two critical components in deep-water exploration and production. Sandstone quality, net-to-gross estimates, volume calculations, vertical connectivity, and stratigraphic trapping are all dependent on the sandstone extent and their seismic characteristics in these settings. However, seismic resolution is commonly insufficient to resolve the critical reservoir parameters. Seismic modeling of termination styles based on integrated outcrop and subsurface properties allows for depth- and resolution-focused predictive models to be built for improved subsurface analysis. This technique is currently underused as a method to better understand the sensitivity of seismic data to the target lithologies and their geometries. The Gres d'Annot Formation is a well-studied sand-prone deep-water system of Paleogene age, deposited in a bathymetrically complex setting. Six end-member termination styles are discussed, including three sand-prone styles—simple onlap (O s), draping onlap (O d), and bed thickening (O t)—and three heterolithic styles—advancing pinch-out (P a), convergent pinch-out (P c), and convergent thickening and pinch-out (P ct). Local thickening close to the system margins is common in both sand-prone and heterolithic terminating strata and plays an important function in the appropriate distribution of sandstone. The outcrops are interpreted as potential (process) analogs for the complex sandstone distribution and termination patterns observed in plays like the Paleogene of the Gulf of Mexico and the Jurassic of the northern North Sea.

Coeval extension and shortening above and below salt canopies on an uplifted, continental margin: Application to the northern Gulf of Mexico

Abstract: Recent ultradeep exploration in the northern Gulf of Mexico has revealed a broad diffuse zone of salt-cored folding beneath the present continental shelf. This zone is a pillow fold belt, where salt pillows grew halokinetically and were then mildly shortened. Below the Louisiana shelf, a contractional early-to-late Miocene pillow fold belt is separated by a partly welded canopy from an overlying early Miocene–to–Pliocene extensional system. This anomalous juxtaposition raises two paradoxes: (1) Why was mid-Miocene shortening close to the Miocene shelf break, where extension is expected? and (2) Why did shortening below the canopy overlap in time with extension above the canopy? Coastal uplift can explain both paradoxes. Cenozoic uplift and exhumation of the north rim of the Gulf of Mexico created the observed coastal offlap and truncation around the rim. Uplift tilted the continental margin and overpowered the influence of the paleoshelf break, causing shortening much farther updip than before uplift. Physical models confirm that this hypothesis is mechanically sound. Our other models had two stacked detachments, each pinned in different locations. Because of this, deep shortening below the canopy was coeval with shallow extension above the canopy. The deep detachment was pinned far inland, equivalent to the uplifted continental interior. Extension above this deep detachment was partly balanced by shortening far downdip to form a pillow fold belt where a network of thrusts linked the squeezed pillows. In contrast, the shallow extensional system above the canopy was pinned farther seaward, equivalent to the upper continental slope.

Deposits of the sandy braided South Saskatchewan River: Implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization

Abstract: Estimation of the dimensions of fluvial geobodies from core data is a notoriously difficult problem in reservoir modeling. To try and improve such estimates and, hence, reduce uncertainty in geomodels, data on dunes, unit bars, cross-bar channels, and compound bars and their associated deposits are presented herein from the sand-bed braided South Saskatchewan River, Canada. These data are used to test models that relate the scale of the formative bed forms to the dimensions of the preserved deposits and, therefore, provide an insight as to how such deposits may be preserved over geologic time. The preservation of bed-form geometry is quantified by comparing the alluvial architecture above and below the maximum erosion depth of the modern channel deposits. This comparison shows that there is no significant difference in the mean set thickness of dune cross-strata above and below the basal erosion surface of the contemporary channel, thus suggesting that dimensional relationships between dune deposits and the formative bed-form dimensions are likely to be valid from both recent and older deposits. The data show that estimates of mean bankfull flow depth derived from dune, unit bar, and cross-bar channel deposits are all very similar. Thus, the use of all these metrics together can provide a useful check that all components and scales of the alluvial architecture have been identified correctly when building reservoir models. The data also highlight several practical issues with identifying and applying data relating to cross-strata. For example, the deposits of unit bars were found to be severely truncated in length and width, with only approximately 10% of the mean bar-form length remaining, and thus making identification in section difficult. For similar reasons, the deposits of compound bars were found to be especially difficult to recognize, and hence, estimates of channel depth based on this method may be problematic. Where only core data are available (i.e., no outcrop data exist), formative flow depths are suggested to be best reconstructed using cross-strata formed by dunes. However, theoretical relationships between the distribution of set thicknesses and formative dune height are found to result in slight overestimates of the latter and, hence, mean bankfull flow depths derived from these measurements. This article illustrates that the preservation of fluvial cross-strata and, thus, the paleohydraulic inferences that can be drawn from them, are a function of the ratio of the size and migration rate of bed forms and the time scale of aggradation and channel migration. These factors must thus be considered when deciding on appropriate length:thickness ratios for the purposes of object-based modeling in reservoir characterization.

Improved geostatistical models of inclined heterolithic strata for McMurray Formation, Alberta, Canada

Abstract: The McMurray Formation of northern Alberta in Canada contains multiscale complex geologic features that were partially formed in a fluvial-estuarine depositional environment. The inclined heterolithic strata deposited as part of fluvial point bars contain continuous centimeter-scale features that are important for flow characterization of steam-assisted gravity drainage processes. These channels are common, extensive, and imbricated over many square kilometers. Modeling the detailed facies in such depositional systems requires a methodology that reflects heterogeneity over many scales. This article presents an object-based facies modeling technique that (1) reproduces the geometry of multiscale geologic architectural elements seen in the McMurray Formation outcrops and (2) provides a grid-free framework that models these geologic objects without relating them to a grid system. The grid-free object-based modeling can be applied to any depositional environment and allows for the complete preservation of architectural information for consistent application to any gridding scheme, local grid refinements, downscaling, upscaling, drape surface, locally variable azimuths, property trend modeling, and flexible model interaction and manipulation. Features millimeters thick or kilometers in extent are represented very efficiently in the same model.

Sedimentologic and diagenetic controls on pore-network characteristics of Oligocene–Miocene ramp carbonates (Majella Mountain, central Italy)

Abstract: This article addresses the controls exerted by sedimentologic and diagenetic factors on the preservation and modification of pore-network characteristics (porosity, pore types, sizes, shapes, and distribution) of carbonates belonging to the Bolognano Formation. This formation, exposed at the Majella Mountain, Italy, is composed of Oligocene–Miocene carbonates deposited in middle- to outer-ramp settings. The carbonates consist of (1) grainstones predominantly composed of either larger benthic foraminifera, especially Lepidocyclina, or bryozoans; (2) grainstones to packstones with abundant echinoid plates and spines; and (3) marly wackestones to mudstones with planktonic foraminifera. The results of this field- and laboratory-based study are consistent with skeletal grain assemblages, grain sizes, sorting, and shapes, all representing the sedimentologic factors responsible for high values of connected primary macroporosity in grainstones deposited on the high-energy, middle to proximal outer ramp. Cementation, responsible for porosity reduction and overall macropore shape and distribution in grainstones to packstones deposited on the intermediate outer ramp, was mainly dependent on the following factors: (1) amount of echinoid plates and spines, (2) grain size, (3) grain sorting and shapes, and (4) clay amount. Differently, in the wackestones to mudstones, laid down on the low-energy, distal outer ramp, matrix is the key sedimentologic factor responsible for low values of scattered macroporosity and dominance of microporosity. The aforementioned results may be useful to improve the prediction of reservoir quality by means of mapping, simulating, and assessing individual carbonate facies with peculiar pore-network characteristics.