Showing papers in "AAPG Bulletin in 2001"

Segmentation of an obliquely rifted margin, Campos and Santos basins, southeastern Brazil

Abstract: We make the case for Early Cretaceous transfer zones that segment the obliquely rifted Atlantic margin of southeastern Brazil. Our interpretation is based on published literature, Bouguer-corrected gravity, regional reflection seismic profiles, and well data. In the Santos and Campos basins, Neocomian rift architecture was strongly influenced by preexisting fabric and structures of the Late Proterozoic (Brasiliano orogeny). The Atlantic margin inherited an east-northeast-west-southwest orientation so that rifting was oblique to the margin. On a regional map of Bouguer-corrected gravity, a nearshore belt of positive anomalies correlates with an interpreted broad Moho uplift in the footwall of Neocomian extensional faults. Farther offshore, a second belt of positive anomalies correlates with a presalt ridge of eroded volcanic or basement anticlines covered by thin Aptian evaporites, interpreted as a failed spreading center. Intervening negative anomalies coincide with the main rift basin. All three belts show apparent offsets along linear zones trending west-northwest-east-southeast, which we interpret as transfer zones. The vergence of half rifts tends to change across transfer zones, compartmentalizing the rifted margin into subbasins. Our results have implications for the risks associated with distribution, maturation, and migration of hydrocarbons within the prolific Early Cretaceous lacustrine petroleum system of the Campos and Santos basins.

Analog models of restraining stepovers in strike-slip fault systems

Abstract: Scaled sandbox models have successfully simulated the geometries and progressive evolution of antiformal pop-up structures developed in a weak sedimentary cover above restraining stepovers in offset sinistral strike-slip fault systems in rigid basement. Models were run both with and without synkinematic sedimentation, which was added incrementally to cover the growing antiformal structures. Vertical and horizontal sections of the completed models permit the full three-dimensional (3-D) structure of the pop-ups to be analyzed in detail. Three representative end-member experiments are described: 30° underlapping restraining stepovers; 90° neutral restraining stepovers; and 150° overlapping restraining stepovers. The experimental pop-ups are typically sigmoidal to lozenge-shaped, antiformal structures having geometries that are dependent on both the stepover angle and stepover width in the underlying basement faults. Underlapping restraining stepovers typically form elongate lozenge-shaped pop-ups; 90° neutral restraining stepovers produce shorter, squat rhomboidal pop-ups; and overlapping restraining stepovers produce sigmoidal antiformal pop-ups. Trans pop-up cross fault systems are characteristic at large displacements on the basement fault system. Above the offset principal displacement zones, the pop-ups are commonly small, narrow, positive flower structures, whereas in the stepover region, they widen out and become markedly asymmetric. This pop-up asymmetry switches across the center of the stepover, where the pop-ups are largely symmetical. Maximum rotations measured within the central highly uplifted region of the pop-ups increase from 7° counterclockwise for the underlapping (30°) stepovers, to 14° counterclockwise for the neutral (90°) stepovers, to 16° counterclockwise for the overlapping (150°) stepovers. In models where no synkinematic sediments were added during deformation, the pop-up structures are bound by convex, flattening-upward, oblique-slip reverse fault systems that link downward to the offsets in the basement fault system. In contrast, in the (Begin page 234) experiments where synkinematic sediments were added incrementally during deformation, the pop-ups are formed by oblique-slip reverse faults that steepen upward into the synkinematic strata with the formation of fault-propagation growth folds. The analog models are compared with natural examples of pop-up structures and show strong similarities in structural geometries and stratal architectures. These models may provide structural templates for seismic interpretation of complex contractional structures in offset strike-slip fault systems.

Lake-type controls on petroleum source rock potential in nonmarine basins

Abstract: Based on numerous empirical observations of lacustrine basin strata, we propose a three-fold classification of lacustrine facies associations that accounts for the most important features of lacustrine petroleum source rocks and provides a predictive framework for exploration in nonmarine basins where lacustrine facies are incompletely delineated. (1) The fluvial-lacustrine facies association is characterized by freshwater lacustrine mudstones interbedded with fluvial-deltaic deposits, commonly including coal. Shoreline progradation dominates basin fill, resulting in the stacking of indistinctly expressed cycles up to 10 m thick. In map view, the deposits may be regionally widespread but laterally discontinuous and contain strong facies contrasts. Transported terrestrial organic matter contributes to mixed type I-III kerogens that generate waxy oil (type I kerogen is hydrogen rich and oil prone; type III kerogen is hydrogen poor and mainly gas prone). The Luman Tongue of the Green River Formation (Wyoming) and the Honyanchi Formation (Junggar basin, China) provide examples of this facies association, which is also present in the Songliao basin of northeastern China, the Central Sumatra basin, and the Cretaceous Doba/Doseo basins in west-central Africa. (2) The fluctuating profundal facies association represents a combination of progradational and aggradational basin fill and includes some of the world's richest source rocks. Deposits are regionally extensive in map view, having relatively homogenous source facies containing oil-prone, type I kerogen. Examples include the Laney Member of the Green River Formation (Wyoming), the Lucaogou Formation (Junggar basin, China), the Bucomazi Formation (offshore west Africa), and the Lagoa Feia Formation (Campos basin, Brazil). (3) The evaporative facies association represents dominantly aggradational fill related to desiccation cycles in saline to hypersaline lakes and may include evaporite and eolianite deposits. Sublittoral organic-rich mudstone facies are relatively thin but may be (Begin page 1034) quite rich and widespread. The highest organic enrichment coincides with the deepest lake stages. Low input of land plant organic matter results in minimal lateral contrasts in organic content. In some cases a distinctive type I-S (sulfur-rich) kerogen may generate oil at thermal maturities as low as 0.45% vitrinite reflectance equivalent. Examples include the Wilkins Peak Member of the Green River Formation (Wyoming), the Jingjingzigou Formation (Junggar basin, China), the Jianghan and Qaidam basins (China), and the Blanca Lila Formation (Argentina).

Sandy fans--from Amazon to Hueneme and beyond

Abstract: Most submarine fans are supplied with both sand and mud, but these become segregated during transport, typically with the sand becoming concentrated in channels and channel-termination lobes. New data from high-resolution seismic reflection surveys and Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) wells from a variety of fans allow a synthesis of the architecture of those submarine fans that have important sand deposits. By analyzing architectural elements, we can better understand issues important for petroleum geology, such as the reservoir properties of the sand bodies and their lateral continuity and vertical connectivity. Our analysis of fan architecture is based principally on the Amazon and Hueneme fans, generally perceived to be classic examples of muddy and sandy systems, respectively. We recognize depositional elements, for example, channel deposits, levees, and lobes, from seismic reflection data and document sediment character in different elements from DSDP/ODP drill cores. We show the utility for petroleum geology of evaluating sandy and muddy elements rather than characterizing entire fans as sand rich or mud rich. We suggest that fan classification should include evaluation of source-sediment volumes and grain size, as well as the probable processes of turbidity-current initiation, because these factors control the character of fan elements and their response to changes in sea level, sediment supply, and autocyclic changes in channel pattern. Basin morphology, controlled by tectonics, influences overall geometry, as well as the balance between aggradation and progradation.

Hydraulic fracturing during the formation and deformation of a basin: A factor in the dewatering of low-permeability sediments

Abstract: The geological expression of hydraulic fracturing is varied and is controlled primarily by the magnitude of the differential stress and the intrinsic properties of the rock. The orientation and type of fractures that develop within a basin are determined by the state of stress, which in turn is controlled by the geological boundary conditions. During the early stages of burial and diagenesis the formation of hydraulic fractures is thought to be an important factor in the movement of fluids through and out of low-permeability, semilithified sediments. Unfortunately, these fractures are not generally preserved and are presumed to heal once the fluid pressure is relieved. The low-permeability Mercia Mudstones of the Bristol Channel Basin, southwest England, however, contain bodies of sand that, during the opening of the basin, were injected along some of the hydraulic fractures in the mudstones, preserving them as sedimentary dikes and sills. Field observations indicate that fluid pressures within the Mercia Mudstones were also very high during basin inversion and that hydraulic fracturing provided a transient permeability that relieved this excess pressure. The fractures are not visible in most of the mudstones but have been preserved within evaporite-rich horizons as a network of satin spar veins. Thus, the chance preservation of the sedimentary dikes and satin spar veins shows that at different times during the evolution of the basin, fluids migrated through low-permeability units along transient networks of hydraulic fractures. In addition, the orientation and spatial organization of these fractures reflect the boundary conditions operating at various stages in the basin history.

Evolution of deep-water Tertiary sinuous channels offshore Angola (west Africa) and implications for reservoir architecture

Abstract: Simple to complex, highly sinuous deep-water channels are common in the subsurface Tertiary off Angola, west Africa, and have been important exploration targets there in recent years. We discuss in this article three examples of these sinuous channels that are generally characterized by high-amplitude seismic reflection events, resulting from reservoir-prone lithologies. The planform characteristics of these channels--sinuosities and sinuous loop wavelengths--are similar to those of the fluvial channels; however, the planform combined with sectional seismic characteristics of the deep-water channels suggest that high sinuosities generally evolved through repeated channel aggradation and lateral migration. In detail, the influence of lateral migration vs. vertical aggradation in the evolution of the deep-water channel sinuosities varies from one channel to another and even along the length of a single channel or across a single sinuous loop. The lateral migrations may be continuous or discrete, separated in distance, and resolvable in seismic. Along some parts of the channels, very high initial sinuosities, not significantly affected by lateral migration, suggest that the initial sea-floor topography and gradients were a major factor in their development. In such cases, the channels essentially aggraded vertically. Thus, the modes of deep-water channel migration and sinuosity evolution are complex and different from those of fluvial channels that exhibit largely lateral (without much aggradational component) shifts within a meander loop. These differences are the result of unique hydrodynamic characteristics of the currents in these systems. The gross reservoir shapes in both the fluvial and deep-water channels are obviously dictated by their high sinuosities. The degree of lateral migration vs. vertical aggradation, however, determines the details of the lateral extent vs. vertical stacking of reservoir lithologies and their connectivities in deep-water sinuous channels. A fluvial, (Begin page 1374) point-bar-type continuity may be present in a part of a deep-water sinuous channel loop but is not expected to extend all across it.

Lowstand alluvial bypass systems: Incised vs. unincised

Abstract: Alluvial systems ranging in age from Miocene to late Pleistocene are observed beneath the southern Java Sea Shelf, offshore northwest Java. A combination of seismic reflection attributes, time slices, and horizon slices extracted from three-dimensional seismic volumes have enabled identification of these alluvial systems. The plan-view expression of these systems ranges from low sinuosity to high sinuosity, and incised to unincised. Widths of individual channels range from 100 to 250 m. Meander belt widths range from 2 to 6 km. In some instances, well-developed minor tributary feeder systems can be observed to be associated with major trunk valleys. Late Pleistocene alluvial systems imaged on the shelf were active during periods of lowered sea level when vast shelf areas were emergent. Of these systems only a select few are characterized by incision. Incision is inferred where trunk channels of fluvial systems are associated with minor, orthogonal, deeply etched tributary channels/valleys. The incised trunk valleys range from 0.5 to 5 km wide and contain channels within them; the incised tributary valleys are an order of magnitude narrower and are characterized by well-developed dendritic drainage patterns. Valley incision, which likely formed within a period of 3-5 k.y., can be traced more than 200 km inboard of the shelf edge. The presence of numerous unincised alluvial systems on marine shelves of the southern Java Sea suggests that valley incision likely characterizes only the lowest of lowstands. To the extent that the Pleistocene can be used as an analog to older sections, we conclude that unincised lowstand alluvial bypass systems can constitute a more common response to sea level lowering than do incised systems.

Cenozoic Qaidam basin, China: A stronger tectonic inversed, extensional rifted basin

Abstract: Qaidam basin is a composite sedimentary basin developed on typical continental crust. It comprises a Jurassic foreland basin and a Cenozoic extensional basin. The geologic history of the Cenozoic Qaidam basin can be divided into two megastages on the basis of stratigraphy and structure. The first megastage, from latest Cretaceous to Oligocene, consisted of two periods of rifting due to upwelling of the hot upper mantle. This is demonstrated by two tectono-sequences formed by the rifting, the synrifting magmatic thermal activity, and the control of protobasin development by syndepositional faults. The second megastage comprises three tectono-sequences that developed in the Miocene and Pliocene and was a period of structural inversion that consisted of compressive downwarping and reverse faulting. This tectonic inversion may be ascribed to the long-distance propagation of compressive stress caused by the collisions of the Indian and Eurasian plates. The thick sequence of primary oil source rocks was deposited in the major rifted depression and the restricted drainage graben of the rifted protobasin. During the tectonic inversion megastage they were subject to deep burial and prolonged heating. A major and a minor oil-generating basin have developed. The tectonic inversion processes produced several structural features that may contain potential hydrocarbon reservoirs and traps.

Sequence architecture, depositional systems, and controls on development of lacustrine basin fills in part of the Erlian basin, northeast China

Abstract: Sequence architecture and depositional systems of a Jurassic-Cretaceous lacustrine rift succession in the Wuliyashitai subbasin, and Erlian basin in northeastern China were investigated using seismic profiles, complemented by well logs and cores. Five second-order or composite sequences are identified on the basis of basin-scale unconformities. Constituent third-order sequences are defined by unconformities along the basin margins and correlative conformities within the central basin. Lowstand-transgressive and highstand systems tracts are separated by major lake flooding surfaces. Depositional systems identified in the basin include alluvial fan, incised valley, fan delta, braid delta, and sublacustrine fan. Three types of lacustrine sequences, consisting of distinctive depositional systems, formed during different tectonic stages: (1) alluvial-shallow-lacustrine sequences (type A) developed during the initial rifting stage; (2) shallow-lacustrine to deep-lacustrine sequences (type B) formed during the early and late periods of rifting; and (3) deep-lacustrine sequences (type C) developed in response to rapid tectonic subsidence during the middle, maximum-rifting stage. Uplift following each rifting episode related to changes in the paleostress field appears to have been the major control on the formation of high-order sequence boundaries (second-order and some third-order sequences). Block rotation, coupled with lake level fluctuations, controlled the formation of most third-order sequence boundaries. Forward simulation modeling suggests that development of different lacustrine sequence types can be related to a balance between tectonic subsidence and sediment input. (Begin page 2018) Deltaic sandstone bodies and incised valley fills along the hinged margins within the shallow-lacustrine and deep-lacustrine sequences and sublacustrine-fan sandstones in the central basin are the economically most important reservoir sandstones in the basin. Fan-delta deposits along the escarpment margin constitute another potential reservoir type.

Upper Oligocene lacustrine source rocks and petroleum systems of the northern Qaidam basin, northwest China

Abstract: Our organic geochemical study of oils from the northern Qaidam basin defines a family of genetically related oils that contain biomarkers indicative of source rocks deposited in Tertiary hypersaline, anoxic lacustrine settings. Although Cenozoic outcrop samples from northern Qaidam are too organic lean to be of source quality, dark laminated upper Oligocene mudstones containing gypsum crystals and pyrite from the Shi 28 well yield total organic carbon (TOC) and Rock-Eval data indicative of fair to good source rocks. Organic matter is derived from algae and bacteria and there apparently was little contribution from terrestrial material. Biomarker data provide a good correlation between the produced oils and the upper Oligocene Shi 28 core samples. Hydrocarbons derived from these source rocks are contained in upper Oligocene, Miocene, and Pliocene reservoirs. Although eight of the oil samples are from the northwest corner of the basin, one sample in this genetic family of hypersaline oils comes from northeast Qaidam, an area previously believed to only produce oils derived from Jurassic freshwater lacustrine source rocks. This sample thus indicates the presence of an unidentified and undocumented source rock in the northeast part of the basin. Hypersaline oils and the associated source rocks have low biomarker maturity parameters. Thermal modeling indicates that hydrocarbon generation probably occurred in northwestern Qaidam within the last 3 million years.

Stress, pore pressure, and dynamically constrained hydrocarbon columns in the South Eugene Island 330 field, northern Gulf of Mexico

Abstract: Hydrocarbon phase pressures at the peak of two severely overpressured reservoirs in the South Eugene Island 330 field, Gulf of Mexico, converge on the minimum principal stress of the top seal. We interpret that the system is dynamically constrained by the stress field present through either fault slip or hydraulic fracturing. In two fault blocks of a shallower, moderately overpressured reservoir sand, hydrocarbon phase pressures are within a range of critical pore pressure values for slip to occur on the bounding growth faults. We interpret that pore pressures in this system are also dynamically controlled. We introduce a dynamic capacity model to describe a critical reservoir pore pressure value that corresponds to either the sealing capacity of the fault against which the sand abuts or the pressure required to hydraulically fracture the overlying shale or fault. This critical pore pressure is a function of the state of stress in the overlying shale and the pore pressure in the sand. We require that the reservoir pore pressure at the top of the structure be greater than in the overlying shale. The four remaining reservoirs studied in the field exhibit reservoir pressures well below critical values for dynamic failure and are, therefore, considered static. All reservoirs that are dynamically constrained are characterized by short oil columns, whereas the reservoirs having static conditions have very long gas and oil columns.

Sedimentation, stratigraphy, and petroleum potential of Krishna-Godavari basin, East Coast of India

Abstract: The Krishna-Godavari basin is located in the central part of the eastern passive continental margin of India. The structural grain of the basin is northeast-southwest. Exposures of Upper Cretaceous sedimentary rocks demarcate the basin margin toward the north west, where the northwest-southeast-trending Pranahita-Godavari graben abuts the basin. The basin contains thick sequences of sediments with several cycles of deposition ranging in age from Late Carboniferous to Holocene. A major delta with a thick, argillaceous facies that has prograded seaward since the Late Cretaceous is a hydrocarbon exploration target. Magnetic and gravity data predicted the basin architecture, which was subsequently confirmed by a multichannel seismic sur vey. The basin is divided into subbasins by fault-controlled ridges. Sediments accumulated in subbasins more than 5 km thick. Above the basement ridges, thin sediments are found. Until the Jurassic period, sediments were deposited in the rift valley and in topo graphic lows. This sequence is completely overlain by a Lower Cretaceous, transgressive sedimentary wedge. Later, continued delta progradation characterized basin sedimentation. With an areal extent of approximately 45,000 km2, this proven petroliferous basin has potential reservoirs ranging in age from the Permian to the Pliocene. Exploratory drilling of more than 350 wells in more than 160 structures has resulted in the discovery of 42 oil and gas bearing structures. Good source rocks are known from sequences ranging in age from Permian-Carboniferous to early Miocene. Because the reservoir sand bodies have limited lateral variation, understanding the stratigraphy and depositional sub environments in different sequences is essential to decipher the favorable locales for reservoir sands. Tilted fault blocks, growth faults, and related rollover anticlines provide the structural traps. (Begin page 1624)

Lowstand deltas in the Frontier Formation, Powder River basin, Wyoming: Implications for sequence stratigraphic models

Abstract: Deposits of lowstand deltas formed on the floor of the Cretaceous Interior seaway of North America are found in the Cenomanian, lower Belle Fourche Member of the Frontier Formation, central Wyoming. Sandstones located in similar distal basin locations, hundreds of kilometers basinward of highstand shoreline deposits, form important hydrocarbon reservoirs isolated within marine shales, but interpretation of their origin has been highly controversial. The distribution, geometry, and internal facies of these sandstones are documented by an extensive outcrop study and regional subsurface correlations to develop genetic facies models for these deposits. This integrated record of lithofacies, ichnofacies, palynofacies, paleocurrent data, bedding relationships, and isolith maps incorporates observations from nearly 100 measured outcrop sections and about 550 subsurface well logs. Four episodes of sediment progradation and subsequent transgression each left behind gradually upward-coarsening deltaic sandstones that have eroded tops. These deltaic sandstones have a lobate to elongate geometry, basinward-dipping internal clinoform bedding, radiating paleocurrents, a low to moderate degree of shallow-marine burrowing, and show variable wave influence and tidal influence on deposition. Delta plain, paralic, and nonmarine facies have been eroded from the top of deltaic successions. Erosion surfaces capping progradational deltaic successions are the only stratal discontinuities that can be mapped regionally, and they appear to record transgressive ravinement enhanced over areas of structural uplift, compared with lowstand surfaces of erosion, which record the bypass of sediments basinward. Low accommodation during lowstands left little room for sandstones to stack vertically, and successive episodes of delta progradation were offset along strike. More tide-influenced delta deposits formed within shoreline embayments (Begin page 262) defined by the topography of older wave-influenced delta lobes and subtle syndepositional deformation of the basin floor. Standard sequence stratigraphic terminology is difficult to use in broad lowstand systems like the Frontier Formation because sandstones do not show simple vertical stacking patterns, major stratal discontinuities can form by processes other than lowstand fluvial erosion, and minor syndepositional deformation of the basin floor exerts a first-order influence on depositional and sediment preservation patterns. Although many basin-distal sandstones have been interpreted to be deposits of offshore bars, shelf-isolated valley fills, and stranded shorelines, the Frontier Formation examples documented here suggest that many of these deposits may be top-eroded deltas formed where rivers delivered sediment to lowstand coastlines. The external geometry and internal heterogeneities of hydrocarbon reservoirs found in these types of deposits reflect processes active on the low accommodation deltaic shoreline, even in cases where subsequent ravinement has significantly truncated the deposits during transgression.

Nonmarine sequence stratigraphy: Updip expression of sequence boundaries and systems tracts in a high-resolution framework, Cenomanian Dunvegan Formation, Alberta foreland basin, Canada

Abstract: Ten allomembers, labeled J-A, have been mapped in the deltaic Dunvegan Formation using 2340 wells and 63 outcrop sections. Each allomember spans about 200 k.y., is bounded by regional transgressive surfaces, and consists of 3-10 component shingles, apparently of autogenic origin. Allomembers H-E are discussed in this article. Their internal stacking patterns and bounding surfaces permit interpretation as depositional sequences. Sequences H-F have a sigmoidal-prismatic geometry, downlap to the southeast, and thin northwest onto the coastal plain. No differential thickening exists toward the orogen in the southwest. In contrast, sequence E thickens markedly toward the northwest. The tabular geometry of sequences H-F suggests accommodation was controlled by sea level change, whereas the wedge geometry of sequence E suggests significant tectonic tilting. Subaerial unconformities that bound valleys and extend onto adjacent interfluve paleosols can be traced 120-330 km updip into outcrop. The base of each sequence is characterized by shoreline backstep of 80-150 km. Transgressive and highstand shingles in each sequence comprise marine deltaic deposits that pass landward into coastal plain strata. Falling-stage shingles offlap seaward with little or no nonmarine equivalents updip, whereas lowstand shingles show renewed coastal onlap. In outcrop, three nonmarine systems tracts are recognized: (1) A channel-dominated, low-accommodation systems tract fills valleys up to 35 m deep and is equivalent to the early transgressive systems tract. (Begin page 1968) (2) A lacustrine-dominated, high-accommodation systems tract comprises mudstone enclosing ribbon channel sandstones, blankets interfluves, and forms the bulk of each sequence. It is equivalent to late transgressive and early highstand systems tracts. (3) A paleosol-dominated, low-accommodation systems tract occupies the upper part of each sequence and is capped by the sequence boundary. It is equivalent to the late highstand systems tract. Sequence-bounding interfluve paleosols represent tens of thousands of years of pedogenesis related to slow aggradation, followed by fluvial entrenchment and clastic bypass, and represent the late highstand, falling-stage, lowstand, and early transgressive systems tracts. Coastal plain deposits record only part (perhaps

Process controls on the development of stratigraphic trap potential on the margins of confined turbidite systems and aids to reservoir evaluation

Abstract: Stratigraphic trapping at pinch-out margins is a key feature of many turbidite-hosted hydrocarbon reservoirs. In systems confined by lateral or oblique frontal slopes, outcrop studies show that there is a continuum between two geometries of pinch-out configuration. In type A, turbidites thin onto the confining surface--although the final sandstone pinch-out is commonly abrupt--and individual beds tend not to erode into earlier deposits. In type B, turbidite sandstones commonly thicken toward the confining slope, and beds may incise into earlier deposits. These two types may occur in combination, to give a wide spectrum of pinch-out characteristics. Our analysis suggests the principal control in determining pinch-out character is flow magnitude, with smaller flows producing type A and larger flows producing type B. In areas of poor seismic control it can be difficult to assess either pinch-out character or the proximity of wells to confining slopes. Because estimates of paleoflow magnitude can be made from core or high-quality log image data, however, it is possible to make reasonable estimates of pinch-out character even from wells such as exploration wells, which may be placed conservatively, away from the field margins. Furthermore, systematic paleoflow variations and thickness trends are commonly seen in individual turbidite sandstones as they approach confining slopes. For example, dispersal directions indicate flow deflection parallel with the strike of confining topography; beds thin toward type A onlaps and thicken toward type B onlaps. These relationships can be exploited via analysis of vertical successions to constrain well position with respect to the slope. Similarly, the presence, location, and frequency of locally derived debrites can provide information on the presence and proximity of confining slopes. (Begin page 972)

Geochemical Characterization of Natural Gas: A Physical Multivariable Approach and its Applications in Maturity and Migration Estimates

Abstract: Gas geochemistry has recently been shown to enhance information regarding the geological history of hydrocarbons. In this paper, graphical representations of physico-chemical processes affecting the chemical and isotopic signatures of natural gases are exemplified. These diagrams are based on experimental studies and the use of basic statistics to extract significant and synthetic parameters from the geochemical data. From 11 chemical and isotopic ratios, a statistical analysis (PCA) yields two very important parameters. The first parameter, using mainly the C2+ fraction of the gas, relates to maturity and the second parameter, involving the proportions and d13C values of methane, indicates mainly segregative migration. Positive values of the second parameter indicate that gases accumulated far from their source, whereas negative values correspond to residual gas pools after leakage of a part of the fluids. A tentative reconstruction of the gas history has been performed in two Brazilian basins: the Espirito Santo basin and part of the Reconcavo basin. The Espirito Santo basin is located on the passive continental margin of the Atlantic Ocean, and the Reconcavo basin corresponds to an intracontinental aborted rift. In both cases, the source rocks are mainly lacustrine, with thermal maturities ranging between the oil window and the beginning of the gas window. Results show that in the Reconcavo basin, a major fault (the Mata Catu fault) acts as a drain for hydrocarbon migration at the basin scale, associating a major isotopic fractionation to the gas migration with a clear correlation between isotope fractionation and the distance of migration. In the Espirito Santo basin, this segregation appears in the platform sediments to a lesser extent and is absent in the gas pools located in the paleocanyons filled with turbidites. This long-distance migration in the platform sediments suggests that a hydrocarbon kitchen is located offshore.

The Calcarenite di Gravina Formation in Matera (Southern Italy): New Insights for Coarse-Grained, Large-Scale, Cross-Bedded Bodies Encased in Offshore Deposits

Abstract: Excellent outcrops of the upper Pliocene-lower Pleistocene Calcarenite di Gravina around Matera (southern Italy) provide continuous exposure of coarse-grained, clastic basin-margin, shoreline to offshore facies. Among these facies, the most conspicuous and volumetrically important are the transition-slope deposits that form large-scale, high-angle, cross-bedded lithosomes. These are laterally extensive, parallel with the paleoshoreline, and show seaward progradation. We interpret them to represent avalanches of sediment swept out onto a depositional slope, below wave base, from the shoreface zone by storm waves and wind-driven currents. Three types of building blocks are recognized based on bedding patterns and facies architecture: embryonic parasequences, mature parasequences, and simple sequences. Parasequences formed during stillstands of sea level and simple sequences during high-frequency cycles of relative change of sea level. These building blocks are stacked in a backstepping configuration and onlap onto Cretaceous limestone substrate. Backstepping is believed to be due to a tectonically forced transgression that is punctuated by high-frequency cycles of sea level. Modern analogs for these building blocks are the Holocene prograding prisms detected in high-resolution seismic lines of the Mediterranean shelves. The reflection patterns of these seismic units resemble the bedding architecture of the Matera simple sequences and parasequences and show similarities of shape, size, position, and orientation of coastal setting, and direction of progradation. Comparison of the Holocene prisms and the Pliocene-Pleistocene accretional units in Matera indicates that they may represent the same (Begin page 662) genetic process: progradation of clastic prisms below the wave base level. The Matera accretional units also show similarities with other examples of laterally extensive, large-scale, cross-bedded sand bodies encased in offshore deposits, such as some sandbanks. Differentiating between these laterally extensive and seaward-prograding cross-bedded sand bodies and some coarse-grained Gilbert-type deltas is difficult, however, if interpretations are based only on two-dimensional (2-D) outcrops in dip section. Without high-resolution data it is also difficult to distinguish between transition-slope and prograding shoreface lithosomes. This difficulty may be acute where only seismic or well-log data are available; however, certain other architectural characteristics, such as stacking and preservation of facies belts and position of bounding surfaces, as well as differences in associated sedimentary structures and fossil content, may be used for interpretation. The Matera example provides a mechanism for emplacement in offshore settings of elongate and strandline-parallel sand and gravel deposits that prograde seaward and that preserve a coarsening-upward internal succession. This article offers an interpretation for other ancient examples of large-scale, cross-bedded lithosomes encased in offshore deposits.

Geomorphology and sequence stratigraphy due to slow and rapid base-level changes in an experimental subsiding basin (XES 96-1)

Abstract: Subsidence is a major factor in the accumulation and architecture of natural basin fills. A recently built experimental facility (Experimental Earthscape Facility [XES]) at St. Anthony Falls Laboratory of the University of Minnesota incorporates, for the first time, a flexible subsiding floor in its design. Thus the experimental basin can model erosion and deposition associated with independent variations in sediment supply, absolute base-level change, and rates and geometries of subsidence. The results of the first experiment in a prototype basin (1 × 1.6 × 0.8 m) are described here, wherein the stratigraphic development associated with first slow and then rapid base-level cycles in a basin that has a sag geometry has been analyzed. A videotape of the experiment and subsequent serial slicing of the dried strata in the basin allow interpretation of the sequence development under conditions of precisely known changes of absolute base level, subsidence, and sedimentation. Relative base-level changes, which strongly varied in the basin owing to the sag geometry of subsidence, seem to exert primary control on sedimentary patterns, although autocyclic changes were also important. Style of sequence boundaries differed between slow and fast base-level falls. During the slow base-level fall, an incised valley developed once the shoreline prograded out of the zone of maximum subsidence, suggesting that incision at the shoreline may be very sensitive to changes in relative base level. Once started, however, the valley quickly widened, by knickpoint retreat, into a broad, low-relief erosion surface that stretched across the entire basin. As erosion took place at the knickpoint, deposition occurred immediately downflow, so both the knickpoint and the upstream limit of deposition migrated landward together, producing a strong time-transgressive erosion and onlap sequence. The stratigraphic (Begin page 818) record of this sequence boundary is a single yet very subtle widespread unconformity that becomes conformable downstream, which is difficult to trace in stratigraphic cross section. In contrast, the incised valley that formed during the rapid base-level fall was relatively narrow, deep, and lengthened over time as deposits at the mouth of the valley were gradually exposed and incised through. Wholesale backfilling of the incised valley did not begin until the rapid base-level rise started. As a result, the rapid base-level change produced a more easily recognized incised valley in the stratigraphic record than did the slow base-level change. Potential reservoir development within the strata is evaluated by means of a gray-scale proxy for porosity. Four distinctive zones of enhanced reservoir quality occurred in the basin: the most proximal part of the basin; the upper part of growth-fault-bounded sedimentary wedges; deep-water forced regressive systems tract composed of grainflow deposits; and transgressive systems tract formed during the rapid base-level rise. This distribution of relatively porous units suggests that, for a variety of reasons, rapid sea level cycles may produce the best reservoir units.

Subsurface Mesozoic basins in the central Atlas of Tunisia: Tectonics, sequence deposit distribution, and hydrocarbon potential

Abstract: The Mesozoic and Cenozoic outcrops of the central and meridional Atlas of Tunisia have been studied by structural, biostratigraphic, and sedimentologic approaches. We extend these studies to the subsurface, employing seismic and well data addressing structures, sequence stratigraphy, and hydrocarbon potential. The basin structures are related to strike-slip fault systems associated with Triassic evaporites. Mesozoic sedimentary sections in the areas of the Gafsa, Sidi Ali Ben Oun, Sidi Aich, and Majoura blocks were studied by an integrated seismic sequence stratigraphic and tectonic approach calibrated to wells and surface geologic controls. We identified tectonic corridors where Upper Triassic evaporites intruded younger Mesozoic rocks beginning in the Jurassic. The organization of the tectonic blocks is characterized by the distribution of subsiding basin zones of graben and rim synclines and resistant areas of platforms. Triassic intrusions induced by the strike-slip movements formed salt pillows and domes that accentuate the border elevations between the rim syncline depocenters and the platforms. Mesozoic sequence deposits are distributed along the flanks of the basin-platform borders according to lowstand prograding downlap systems tracts overlapped by aggrading and retrograding onlap and toplap deposits. The time-space distribution of the systems tract deposits indicates the possibility of formation of Triassic, Jurassic, and Cretaceous carbonate and sandstone prograding turbidites, reefs, structural and stratigraphic pinch-outs, and unconformities in the flanks of salt domes and basins. The geometry and distribution of reservoir and seal systems tracts permit the consideration of new Triassic, Jurassic, and Cretaceous combined stratigraphic and structural hydrocarbon traps around the basins and the platforms. This (Begin page 886) is a new tectonic model of the central Atlas of Tunisia that shows new potential objectives and traps.

Basin evolution in western Newfoundland: New insights from hydrocarbon exploration

Abstract: The Humber zone is the most external zone of the Appalachian orogen in western Newfoundland. It records multiphase deformation of the Cambrian-Ordovician passive margin and of the Ordovician to Devonian foreland basins by the Taconian, Salinian, and Acadian orogenic events. The recent phase of exploration drilling has provided new evidence for structural, stratigraphic, reservoir, and source rock maturation models of western Newfoundland. The first well, Port au Port 1, supported the hypothesis that the Round Head thrust had an earlier extensional history prior to the Acadian compressional inversion that created the present-day structural high of the Port au Port Peninsula. The well tested a small anticline formed in a footwall shortcut fault of the Round Head thrust. The second well, Long Point M-16, was drilled at the northern tip of Long Point to test a triangle zone identified by previous workers. This well demonstrates that the frontal monocline at the western edge of the triangle zone is elevated by a stack of imbricate thrusts composed of rocks of the Taconian allochthon and compressional basement-involved faults that have uplifted the Cambrian-Ordovician carbonate platform. The structural model developed in the Port au Port area with the aid of these wells has been extended throughout the Humber zone in western Newfoundland. Changes in structural style illustrated by regional cross sections suggest that prospective trap geometries are only developed in the southern and central parts of the region. The reservoir model proposed invokes exposure and karsting of the footwalls of extensional faults formed as the carbonate platform collapsed during a Middle Ordovician hiatus, the St. George unconformity. Structural relief became more pronounced as extensional collapse continued through the Middle Ordovician. (Begin page 394) These structurally high fault footwalls became the foci for dolomitizing and mineralizing fluids that used major faults as fluid conduits during the Devonian. Fluids deposited sulphide ores and created zebra and sparry dolomite and some sucrosic hydrothermal dolomites in the St. George Group and the Table Point Formation. The reservoir model, maturity and source rock data, and the structural models have been combined with seismic and onshore surface geology. This enables the prospectivity of the western Newfoundland Cambrian-Ordovician play trend to be evaluated for further exploration.

Detailed internal architecture of a fluvial channel sandstone determined from outcrop, cores, and 3-D ground-penetrating radar: Example from the middle Cretaceous Ferron Sandstone, east-central Utah

Abstract: Ideally, characterization of hydrocarbon reservoirs requires information about heterogeneity at a submeter scale in three dimensions. Detailed geologic information and permeability data from surface and cliff face outcrops and boreholes in the alluvial part of the Ferron Sandstone are integrated here with three-dimensional (3-D) ground-penetrating radar (GPR) data for analysis of a near-surface sandstone reservoir analog in fluvial channel deposits. The GPR survey covers a volume with a surface area of 40 x 16.5 m and a depth of 12 m. Five architectural elements are identified and described in outcrop and well cores, using a sixfold hierarchy of bounding surfaces. Internally, the lower four units consist of fine-grained, parallel-laminated sandstone, and the upper unit consists of medium-grained, trough cross-bedded sandstone. The same sedimentary architectural elements and associated bounding surfaces are distinguished in the GPR data by making use of principles developed in seismic stratigraphic analysis. To facilitate comparison of geologic features in the depth domain and radar reflectors in the time domain, the radar data are depth migrated. The GPR interpretation is carried out mainly on migrated 100 MHz data with a vertical resolution of about 0.5 m. Measures of the spatial continuity and variation of the first- and second-order bounding surfaces are obtained by computing 3-D experimental variograms for each architectural element (each radar (Begin page 1584) facies). The maximum correlation length of the dominant internal features ranges between 4 and 6 m, and the anisotropy factor ranges between 0.6 and 0.95.

Petroleum systems of Oman: Charge timing and risks

Abstract: After 35 years of exploration, creaming of the conventional plays in Oman is nearly complete, and consequently, the search has commenced for new, less obvious plays. Many of the new opportunities occur beyond the known hydrocarbon provinces and are considered to have significant charge risks. To define these risks, extensive basin modeling studies have been conducted in recent years. Modeling and empirical data show that Mesozoic and Cenozoic kitchen areas are restricted to western north Oman, the only areas currently buried at their maximum temperature. Large parts of north and central Oman depend on lateral migration from these kitchens for their charge. Progressive uplift of the east flank and basin inversion since the middle Paleozoic provides favorable conditions for long-distance migration in the post-Carboniferous interval. In central Oman, geochemical tracer molecules (benzocarbazoles) suggest that a north-south-trending, reactivated basement grain has funneled charge up to 300 km southeastward. Charge risks increase in the deeper sequence, in which eastward migrating hydrocarbons have to traverse the Ghaba salt basin, a pronounced syncline at depths greater than 3 km. The south Oman salt basin is currently cool because of shallow depths and hydrodynamic fluid-flow activity. The shallow post-Cambrian reservoirs rely on storage of early (Cambrian-Ordovician) charge by the Ara salt (Cambrian) sequence, followed by release of hydrocarbons as the salt edge retreats through time. Basin modeling has outlined the extent of the different petroleum systems and provided us with risk maps to guide our next exploration phase. It has revitalized some of the mature plays, for instance the Gharif Formation, where oil exploration is now focused along Late Cretaceous and Tertiary migration paths. Deeper sections are envisaged to have significant scope for gas.

Geologic evolution of the Cuu Long and Nam Con Son basins, offshore southern Vietnam, South China Sea

Abstract: Rifting and regional subsidence characterize the Cuu Long and Nam Con Son basins, offshore southern Vietnam. Initial rifting began in the Eocene-early Oligocene, followed by the uplift and rotation of the crustal blocks in the late Oligocene. The erosion of the uplifted blocks marked the transition from rifting to regional subsidence in the Cuu Long Basin. A second phase of rifting began in the Nam Con Son Basin, lasting until the late Miocene. Parts of the Nam Con Son Basin experienced inversion in the middle to late Miocene. The synrift and postrift units in the Cuu Long Basin consist of nonmarine deposits and paralic to shallow-marine sediments, respectively. The synrift deposits in the Nam Con Son Basin can be divided into the early synrift unit, corresponding to the initial rifting phase, and the late synrift unit, deposited during the second rifting phase. The early synrift and late synrift units consist of nonmarine sediments and nonmarine to shallow-marine sediments, respectively; the postrift unit is composed of shelf and deeper marine deposits. The data compiled from the published reports suggest that the Cuu Long Basin is oil-prone, with the oil reservoired mainly in fractured basement highs, whereas the Nam Con Son Basin is generally gas-prone, with the gas trapped in Miocene sands and late Miocene carbonates. These distinct trends may be attributed to differences in timing of trap formation and the disruption of trap integrity caused by prolonged rifting and inversion in the Nam Con Son Basin. Gwang H. Lee is associate professor at Kunsan National University, Korea. His research interests include application of seismic reflection to basin research and sequence stratigraphy. He received his B.S. and M.S. degrees in oceanography from Seoul National University, Korea, in 1981 and 1983, respectively, and a Ph.D. in geological/geophysical oceanography from Texas A&M University in 1990. From 1991 to 1994 he worked for Shell Offshore Inc. in New Orleans.Keumsuk Lee received a B.S. degree in mathematics (1994) and an M.S. degree in geological oceanography (1999) from Kunsan National University, Kunsan, Korea. He is currently attending Florida Atlantic University, Boca Raton, Florida. His main research interest is application of geophysical methods to various geological and environmental problems. Joel S. Watkins is the Earl F. Cook Professor, Department of Geology and Geophysics, Texas AM co-chief scientist on DSDP Leg 66 (Middle America Trench), and project director, manager, and vice president with Gulf Oil, 1977-1985. He is the author or co-author of more than 130 scientific publications and senior editor of AAPG Memoirs 29, 34, and 53. His interests are structure and stratigraphy of small ocean basins and seismic reservoir characterization.

Use of Spectral Gamma-Ray Signature to Interpret Stratigraphic Surfaces in Carbonate Strata: An Example from the Finnmark Carbonate Platform (Carboniferous-Permian), Barents Sea

Abstract: Spectral gamma-ray (GR) profiles were examined in well 7128/6-1, the stratigraphic reference section of the entirely subsurface Finnmark carbonate platform. Detailed bulk-chemical profiling of selected GR peaks shows that potassium (K) and thorium (Th) are mutually correlated and are a direct index of siliciclastic (aluminosilicate) content, whereas uranium (U) is uncorrelated with K, Th, and all other chemical components measured. Uranium tends to be enriched in thin shale and argillaceous carbonate layers within otherwise carbonate-dominated intervals. Uranium is thus associated with aluminosilicate minerals and is not particularly concentrated in dolomite. Two types of GR peaks are observed. Potassium-thorium-dominated peaks are suggested to indicate relatively major transgressions during which aluminosilicate detritus was derived from sources interior to the Fennoscandian shield. Uranium-dominated peaks correspond with relatively minor transgressions within intervals of cyclic shallow-water carbonate deposits. Uranium-enriched aluminosilicate detritus is suggested to be the product of extended subaerial exposure of the platform, during which U was concentrated by groundwater movement. These results can be useful as a basis for applying spectral GR signature as a tool for stratigraphic interpretation in uncored or incipiently understood carbonate sections.

The use of satellite-based radar interferometry to monitor production activity at the Cold Lake heavy oil field, Alberta, Canada

Abstract: The Cold Lake heavy oil field has been studied by geoscientists for more than 30 years and has been producing bitumen for 20 years using the cyclic steam stimulation (CSS) process. Future development options can be improved by the resolution of steam movement and the avoidance of areas of faults and fractures. To locate these features, remote sensing has recently been investigated as a cheaper alternative to four-dimensional seismic surveys. Advances in satellite and radar technology have made it possible to measure very small movements of the earth's surface found in earthquake zones and volcanic regions. The technique uses synthetic-aperture radar interferometry (InSAR), allowing the measurement of deformation using a vertical resolution, in optimal conditions, on the order of millimeters between repeat acquisitions. This accuracy has only been achieved over dry areas without significant vegetation growth. At the Cold Lake oil field, it has been reported previously that the injection of steam to mobilize the bitumen causes the pump jacks to heave and subside by as much as 30 cm during the first steam cycle. The present project was instigated to determine if such small positive and negative vertical movements could be resolved over the field. Data from three satellite radar sensors were selected: ERS (European remote sensing satellite), JERS (Japanese remote sensing satellite) and Radarsat (the Canadian radar satellite). The present article has provided the first results of repeat-pass InSAR using JERS SAR data for the land subsidence application area. Contrary to the widespread belief that this technology can only be used for dry areas, we show that accurate results, on the order of a centimeter in resolution, can be obtained in a forested area such as Cold Lake using the JERS L-band spaceborne SAR system. (Begin page 782)

A Devonian hydrothermal chert reservoir: the 225 bcf Parkland field, British Columbia, Canada

Abstract: The 225 bcf (original gas in place [OGIP]) Parkland A pool was discovered in 1956 with the drilling of Imperial Pacific 6-29-81-15w6. This well initially production tested at 19 mmcf gas/day and has produced more than 95 bcf of sweet gas. The Parkland play type (as defined by the Geological Survey of Canada) consists principally of fracture-associated hydrothermally karsted and dolomitized reservoirs hosted in the Upper Devonian (Famennian) Wabamun Group medial ramp carbonates resting on the axis of the Peace River arch and includes major fields such as Tangent, Teepee, and Gold Creek. Ironically Parkland itself owes little of its production to hydrothermally dolomitized carbonates; instead, most of the reservoir pore volume can be attributed to microintercrystalline porosity within a pervasive replacement microquartz (chert) that occurs at the dolomite-limestone interface. The thickness of this chert zone in the 6-29 well is 35 m and occurs near the top of the Wabamun. The origin of the chert is problematic. It postdates matrix dolomitization and crosscuts the early Tournaisian (Mississippian) Exshaw Ash but is in turn truncated by later saddle dolomite and calcite veins. The replacement chert is composed of a microporous (up to 30%) meshwork of microquartz crystals averaging 5-10 µm in size and is texturally distinct from early diagenetic chert nodules also found in the Wabamun. The two cherts are also distinguishable isotopically, with the replacement microquartz showing significantly more depleted oxygen values (22 vs. 25 d18O standard mean ocean water [SMOW]), consistent with precipitation from hot fluids having a temperature range from about 140 to 200°C. Silicification has had the greatest impact on slightly dolomitized limestone at the outer margin of the hydrothermal dolostone pod. Entrapped floating euhedral crystals of dolomite within the replacement chert show little evidence of microdissolution, suggesting that the silicic (Begin page 52) acid-charged fluid, although undersaturated with respect to the microspar lime matrix, was saturated with respect to dolomite. We postulate that hydrothermal fluids of common parentage, only marginally evolved in composition--if at all, were responsible for both dolomitization and silicification. Both replacement processes occurred in relatively rapid succession at shallow burial depths and were related to an early Tournaisian period of hydrothermal activity associated with both wrench and minor extension structuring linked with the nascent development of the Fort St. John graben. The source of the silica is thought to be the immediately subjacent Granite Wash or possibly the Precambrian basement. Stratigraphic proximity to the Granite Wash is likely a prerequisite condition for chert reservoir development within the Wabamun and serves to explain, along with significantly different hydrothermal fluid temperatures (much hotter to the west), why Parkland is such a distinct field compared to Tangent and Teepee. Hydrocarbon reservoirs that are dominantly chert-hosted are relatively uncommon, but where recognized (e.g., Monterey Formation, California) they are related to the redistribution and transformation of biogenic opal-A silica derived from diatoms, spicules, and/or radiolarians in deeper water sediments. To our knowledge, Parkland is the only hydrothermal chert reservoir that has been reported anywhere in the world.

Integrating 3-D seismic data, field analogs, and mechanical models in the analysis of segmented normal faults in the Wytch Farm oil field, southern England, United Kingdom

Abstract: We propose a methodology for the analysis of normal fault geometries in three-dimensional (3-D) seismic data sets to provide insights into the evolution of segmented normal fault systems and to improve recovery efforts in fault-controlled oil fields. Limited seismic resolution can obscure subtle fault characteristics such as segmentation and gaps in fault continuity that are significant for oil migration and thus accurate reservoir characterization. Detailed seismic data analyses that incorporate principles of normal fault mechanics, however, can reveal evidence of fault segmentation. We integrate seismic attribute analyses, outcrop analog observations, and numerical models of fault slip and displacement fields to augment the use of 3-D seismic data for fault interpretation. We applied these techniques to the Wytch Farm oil field in southern England, resulting in the recognition of significant lateral and, to a lesser extent, vertical segmentation of reservoir-scale faults. Slip maxima on fault surfaces indicate two unambiguous segment nucleation depths, controlled by the lithological heterogeneity of the faulted section. Faults initiated preferentially in brittle sandstone and limestone units. Subsequent growth and linkage of segments, predominantly in the lateral direction, resulted in composite fault surfaces that have long lateral dimensions and multiple slip maxima. Reservoir compartmentalization is greatest at the level of prevalent segment linkages, which corresponds at Wytch Farm with the predominant hydrocarbon-producing unit, the Sherwood Sandstone. At relatively shallower depths, fault segments are younger and less evolved, resulting in a greater degree of segmentation with intact relay zones.

Simulation of potassium feldspar dissolution and illitization in the Statfjord Formation, North Sea

Abstract: Prior work has suggested that potassium mobility is an important control on the formation of late-stage illite in deep reservoirs of the North Sea. We developed a one-dimensional finite-difference reactive transport model to simulate the concurrent dissolution of K-feldspar and illitization of kaolinite in Statfjord Formation reservoirs. The kinetics of K-feldspar dissolution is set to be faster than diffusion, consistent with the observed behavior and the proposition that the reaction was transport-limited. K-feldspar dissolution produced dissolved potassium and silica that diffused and then precipitated to form illite and quartz. Areas that had higher initial reactant (K-feldspar) produced higher solute concentrations and formed chemical gradients having areas that had lower initial feldspar content. In typical North Sea cases this means mass transfer would be from sandstone to shale. The diffusive transport moved significant amounts of potassium and silica over distances of several meters and created observable diagenetic alteration. Simulations of an oil-filled reservoir showed that oil emplacement did not stop illitization but did limit potassium mobility and maximized illite formation within the sandstone. Preservation of preillitization permeability was favored in water-filled reservoirs that had slower illite formation where diffusion could export potassium from the sandstone before precipitation. Similar patterns of transport and precipitation were predicted for silica from K-feldspar dissolution. The results of the model showed good agreement with field data from other North Sea reservoirs.

Gas hydrate distribution and volume in Canada

Abstract: Gas hydrate, a solid form of natural gas and water, is inferred to widely occur in Canadian polar and continental shelf regions and in sediment of outer continental margins. Although direct indications of hydrate are few and widely separated, conditions potentially favorable for gas hydrate formation and stability, especially low to moderate temperatures under permafrost or the deep sea, combined with favorable geological conditions for gas generation and storage, cover vast areas and indicate an immense potential for natural hydrocarbon gas in the upper 2 km of many Canadian sedimentary basins. We have analyzed the potential of gas hydrates for the vast continental shelves and Arctic permafrost regions of Canada (Mackenzie delta-Beaufort Sea and Arctic Archipelago in the north and Davis Strait, the Labrador Shelf, Scotian Shelf, and Grand Banks of Newfoundland along the Canadian Atlantic margin and Canadian Pacific margin). Our conservative calculation suggests 1010-1012 m3 of gas hydrates in these regions has an associated methane gas potential estimated to be in the range of 1012-1014 m3. The volume of methane in hydrates in Canada are geographically distributed in the following regions: 0.24-8.7 x 1013 m3 in the Mackenzie delta-Beaufort Sea, 0.19-6.2 x 1014 m3 in the Arctic Archipelago, 1.9-7.8 x 1013 m3 on the Atlantic margin, and 0.32-2.4 x 1013 m3 on the Pacific margin. The total in-situ amount of methane in hydrates of Canada is estimated to be 0.44-8.1 x 1014 m3, as compared to a conventional Canadian in-situ hydrocarbon gas potential of approximately 0.27 x 1014 m3. This comparison suggests that gas hydrates represent a possible future assurance of North American energy supply if the gas can be recovered and separated from the hydrate form.