Showing papers in "AAPG Bulletin in 1996"

Kinetic Modeling of Quartz Cementation and Porosity Loss in Deeply Buried Sandstone Reservoirs

Abstract: A mathematically simple kinetic model simulates quartz cementation and the resulting porosity loss in quartzose sandstones as a function of temperature history. Dissolved silica is considered to be sourced from quartz dissolution at stylolites or individual quartz grain contacts containing clay or mica, and diffuses short distances to sites of precipitation on clean quartz surfaces. The modeled sandstone volume is located between stylolites, and no quartz dissolution or grain interpenetration takes place within this volume. After quartz cementation starts, compactional porosity loss is typically minor, and porosity loss within the modeled sandstone volume is therefore considered to be equal to the volume of precipitated quartz cement. The quartz cementation process is mod led as a precipitation rate-controlled reaction where quartz precipitation rate per unit time and surface area can be expressed by an empirically determined logarithmic function of temperature. When the sandstone's temperature history is known, precipitation rate per unit time and surface area can be expressed as a function of time, and the amount of quartz cement precipitated within a certain time interval can be calculated by multiplying the precipitation rate function with the surface area available for quartz precipitation and integrating with respect to time. Because quartz surface area will change as quartz cement precipitation proceeds, the calculations are performed for short time steps, and quartz surface area is adjusted after each time step. The total amount of quartz cement p ecipitated during a sandstone's burial history and the corresponding porosity loss are found by taking the sum of the increments of quartz cement precipitated during each time step. The effect of variation in parameters such as grain size, detrital quartz content, abundance of clay or other grain coatings, prequartz cementation porosities, and temperature history is easily simulated with the presented algorithm. This flexibility is illustrated by presenting calculated histories of quartz cementation and porosity loss for sandstones with a range of grain sizes, framework grain compositions, degree of clay coat development, prequartz cementation porosities, and temperature histories.

Eustatic Curve for the Middle Jurassic--Cretaceous Based on Russian Platform and Siberian Stratigraphy: Zonal Resolution

Abstract: We have used the stratigraphy of the central part of the Russian platform and surrounding regions to construct a calibrated eustatic curve for the Bajocian through the Santonian. The study area is centrally located in the large Eurasian continental craton, and was covered by shallow seas during much of the Jurassic and Cretaceous. The geographic setting was a very low-gradient ramp that was repeatedly flooded and exposed. Analysis of stratal geometry of the region suggests tectonic stability throughout most of Mesozoic marine deposition. The paleogeography of the region led to extremely low rates of sediment influx. As a result, accommodation potential was limited and is interpreted to have been determined primarily by eustatic variations. The central part of the Russian latform thus provides a useful frame of reference for the quantification of eustatic variations throughout the Jurassic and Cretaceous. The biostratigraphy of the Russian platform provides the basis for reliably determining age and eustatic events. The Mesozoic section of the central part of the Russian platform is characterized by numerous hiatuses. In this study, we filled the sediment gaps left by unconformities in the central part of the Russian platform with data from stratigraphic information from the more continuous stratigraphy of the neighboring subsiding regions, such as northern Siberia. Although these sections reflect subsidence, the time scale of variations in subsidence rate is probably long relative to the duration of the stratigraphic gaps to be filled, so the subsidence rate can be calculated and filtered from the stratigraphic data. We thus have compiled a more complete eustatic curve than would be p ssible on the basis of Russian platform stratigraphy alone. Relative sea level curves were generated by backstripping stratigraphic data from well and outcrop sections distributed throughout the central part of the Russian platform. For determining paleowater depth, we developed a model specifically designed for this region based on paleoecology, sedimentology, geochemistry, and paleogeography. The curve describes a series of high-frequency eustatic events superimposed on longer term trends. Many of the events identified from our study can be correlated to those found by Haq et al. (1988) and other sea level studies from other parts of the world, but there are significant differences in the relative magnitudes of events. Because the eustatic curve resulting from this study is based on a stable reference frame, the curve can be used in sedimentary basin modeling and as a tool for quantifying subsidence history from the stratigraphy of passive margins, basins, and other active regions.

Architectural Elements and Growth Patterns of Submarine Channels: Application to Hydrocarbon Exploration

Abstract: Modern and ancient submarine channels show a wide range of architectural styles. Architectural element analysis is a useful descriptive means to characterize the type of channel fill, show the interconnectivity and lateral continuity of sand bodies, and interpret the causal sedimentary processes. This paper combines a review of the literature on submarine channels with new observations and analysis, and proposes architectural element models for submarine channels, in particular, demonstrating how these models can be applied to interpreting the sequential fill of ancient submarine channels. Data for the dimensions and degrees of lateral continuity and vertical connectivity of channel elements, such as those giving rise to reservoir heterogeneities in hydrocarbon exploratio , are presented for a variety of examples of architectural elements, providing quantitative information for reservoir analog models.

Elastic Anisotropy of Source Rocks: Implications for Hydrocarbon Generation and Primary Migration

Abstract: We combined acoustic velocities and anisotropy measurements with x-ray diffraction mineralogy, Rock-Eval organic geochemical analyses, vitrinite reflectance, and scanning electron microscope observations on 69 cores of hydrocarbon source rocks collected from eight sedimentary basins. The velocity data indicate the lamellar distribution of solid organic matter (kerogen) in compacted black shales and show a strong correlation between their elastic anisotropy and organic richness. The experimental results are consistent with the two-component anisotropic model of black shales with a continuous kerogen network in both bedding-parallel and bedding-normal directions beginning from relatively low TOC values. The strong elastic anisotropy of tight, thermally mature source rocks i further enhanced by overpressure-induced bedding-subparallel microcrack development during the main stage of hydrocarbon (HC) generation and primary migration at R0 greater than 0.75%. In agreement with published petrographic observations, these microcracks significantly aid primary HC migration, particularly horizontally. Elastic anisotropy of medium- to high-porosity illite/smectite shales is weaker than in lower porosity, predominantly illite shales; however, the overall positive trend of elastic anisotropy with kerogen content remains unchanged.

Geochemistry and accumulation of carbon dioxide gases in China

Abstract: We propose the following identification indicators for CO2 of different origins based on data obtained from analyzing the molecular composition of 1198 gas samples, the CO2 carbon isotope values of 390 samples, and the helium isotope composition of 256 samples collected from wells of 16 oil- and gas-bearing basins and gas seepages in China, and reviewing literature on organic and inorganic CO2 throughout the world. CO2 is of organic origin if CO2 content 13CCO22 is of inorganic origin if ^dgr13CCO2 > -8^pmil or CO2 > 60%; and CO2 is of either organic or inorganic origin or a mixtur of organic and inorganic origins if -10^pmil 13CCO213CCO2 values of organic CO2 mainly range from -10 to -22^pmil. In tectonically stable oil- and gas-bearing basins where deep faults and magmatism are undeveloped, organic CO2 occurs almost exclusively. In tectonically active oil- and gas-bearing basins where active faults and magmatism are present or in areas of modern volcanic activity, the ^dgr13CCO2 values of inorganic CO2 range from -2 to -8^pmil and are distributed along faults. The inorganic CO2 with a ^dgr13CCO2 value of 0 ±3^pmil is of metamorphic origin, and that with ^dgr13CCO2 values of -6 ±2^pmil is mantle derived and of magmatic origin. In China, the ^dgr13CCO2 values of inorganic CO2 gas pools range from -3.41 to -8.44^pmil and the helium ratio (R:Ra) is >1, which indicates that these gases are typically derived from the mantle.

Hydrocarbon Accumulations in the Tarim Basin, China

Abstract: The Tarim basin is the largest and least explored inland basin in China. The areal extent of the basin reaches 560,000 km2. The interior of the basin is mostly covered by the Takla Makan Desert, which is about 330,000 km2 in areal extent. The basin has become the object of special attention since China set aside first- and third-round onshore bidding blocks in the Tarim basin for foreign oil firms to explore. The Tarim basin is a polyhistory superimposed basin that has experienced seven evolutionary stages: (1) Sinian-Cambrian-Ordovician aulacogen stage, (2) Silurian-Devonian intracratonic depression stage, (3) Carboniferous marginal sea stage, (4) Permian rift basin stage, (5) Triassic-Jurassic foreland basin stage, (6) Cretaceous-Paleogene Neo-Tethys bay stage, and (7) Neogene-Pleistocene foreland and inland basin stage. Both the basin's Paleozoic marine platform sequences and the Mesozoic-Cenozoic terrestrial fills are believed to contain substantial volumes of hydrocarbons. After recent years of exploration, nine oil and gas fields have been proven and 23 discoveries have been made in the Tabei, Tazhong, and Southwest areas. Kekeya, Lunnan, Sangtamu, Jiefangqudong, Donghetang, and Tazhong 4 oil fields have been put into production. Output of crude oil was 2.6 million t (metric tons) (52,000 BOPD) in 1995. The production will increase to 5 million t (100,000 BOPD) in 1997. Giant oil and gas traps probably will be discovered in the Tarim basin. The prospect is promising.

Geometry and Kinematics of Single-Layer Detachment Folds

Abstract: Detachment folds form above, below, or above and below bed-parallel thrusts where thrust displacement is transferred into folding. Geometric and kinematic models are developed for individual single-layer detachment folds formed above the tip line of a thrust. We analyze three different detachment fold shapes (kink bands, chevron, and box detachment folds) and consider three principal detachment fold models: (1) constant limb dip where the detachment fold grows by limb lengthening, (2) constant limb length where the detachment fold grows by limb rotation, and (3) variable limb length and variable limb dip where the detachment fold grows by both limb rotation and limb lengthening. The allowance for variable forelimb thickness and excess layer-parallel shear enables us to de elop a wide range of possible detachment fold geometry. The geometric validity fields for folds with excess parallel shear are reduced when forelimb thinning takes place. Folds formed with constant limb length are likely to lock up in the initial amplification stages, whereas larger amounts of shortening can be taken up by folds formed with variable limb length. The manner in which a fold evolves affects the fracture and porosity features of the folded strata. Therefore, determining which mechanism operated to form a specific detachment fold is of particular importance in hydrocarbon exploration. The models we present are aimed at permitting more accurate geometric and kinematic interpretations of individual detachment folds. Examples of detachment folds are analyzed to illustrate the po ential benefits and limitations of the geometric model presented.

Microbial Origin of Australian Coalbed Methane

Abstract: Bituminous coal seam gases from the Permian Sydney and Bowen basins, Australia, are characterized by (1) methane/ethane ratios greater than or equal to 1000, (2) ^dgr13C and ^dgrD values for methane of -60 ±10^pmil PDB and -217 ±17^pmil SMOW, respectively, (3) carbon dioxide contents of less than 5%, and (4) ^dgr13C(CO<2-CH4) values of 55 ±10^pmil PDB. These data suggest that microbial reduction of CO2 rather than traditional thermogenic reactions is mainly responsible for gas composition. Invasions by CO2 of deep-seated external origins are readily recognized by their isotopic compositions (-7 ±2^pmil PDB).

The Effect of Grain-Coating Microquartz on Preservation of Reservoir Porosity

Abstract: Clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones, but few workers have realized that similar effects can be produced by microcrystalline quartz coatings. This phenomenon can be expected only under special circumstances, but in such cases it can have profound consequences for exploration. In the Central Graben area of the southern North Sea, unusually high porosity (20-27%) and permeability (100-1000 md) are found in certain zones in Upper Jurassic sandstones at depths of 3.4-4.4 km. The porosity in these zones is 5-15% higher than expected based on average porosity-depth trends from Brent and Haltenbanken sandstones. We propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0.1-2 µm thick. The distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules (Rhaxella), which implies a sedimentological control on the reservoir quality. We present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites. High porosities in parts of various Upper Jurassic oil fields (Ula and Gyda) have previously been explained by inhibition of quartz cementation by early hydrocarbon charge. We suggest that the microcrystalline quartz coatings provide a more plausible explanation.

Late Paleozoic Deformation of Interior North America: The Greater Ancestral Rocky Mountains

Abstract: Late Paleozoic deformation within interior North America has produced a series of north-northwest- to northwest-trending elongate basins that cover much of Oklahoma, Texas, New Mexico, Colorado, and Utah. Each basin thickens asymmetrically toward an adjacent region of coeval basement uplift from which it is separated by synsedimentary faults with great vertical relief. The remarkable coincidence in timing, geometry, and apparent structural style throughout the region of late Paleozoic deformation strongly suggests that these paired regions of basin subsidence and basement uplift form a unified system of regional deformation, the greater Ancestral Rocky Mountains. Over this region, basin subsidence and basement uplift were approximately synchronous, beginning in the Chesterian-Morrowan, continuing through the Pennsylvanian, and ending in the Wolfcampian (although minor post-Wolfcampian deformation occurs locally). The basement uplifts show evidence for folding and faulting in the Pennsylvanian and Early Permian. Reverse faults and thrust faults have been drilled over many of the uplifts, but only in the Anadarko region has thrusting of the basement uplifts over the adjacent basin been clearly documented. Extensive basement-involved thrusting also occurs along the margins of the Delaware and Midland basins, and suggests that the entire greater Ancestral Rocky Mountains region probably formed as the result of northeast-southwest-directed-intraplate shortening. Deformation within the greater Ancestral Rocky Mountains was coeval with late Paleozoic subduction along much of the North American plate margin, and has traditionally been related to emplacement of thrust sheets within the Ouachita-Marathon orogenic belt. The nature, timing, and orientation of events along the Ouachita-Marathon belt make it difficult to drive the deformation of the greater Ancestral Rocky Mountains by emplacement of the Ouachita-Marathon belt along the southern margin of North America. We speculate the deformation was driven instead by events within a late Paleozoic Andean margin along the southwestern margin of North America. Evidence for the existence of this previously unrecognized Andean margin comes from east-central Mexico, where a Pennsylvanian and Permian volcanic arc indicates that a northeast-dipping subduction boundary lay to the south and west. The interpretation that deformation throughout the greater Ancestral Rocky Mountains occurs by basement-involved overthrusting on gently to moderately dipping thrust faults suggests that potential hydrocarbon reserves beneath crystalline thrust sheets may be much greater than is generally supposed.

Carbon Isotope Stratigraphy of the Upper Kharaib and Shuaiba Formations: Implications for the Early Cretaceous Evolution of the Arabian Gulf Region

Abstract: The carbon isotope profiles of shallow-marine carbonates from the Barremian-Aptian Kharaib and Shuaiba formations of the Arabian Gulf region range between 0.5 and 7^pmil ^dgr13C PDB (Peedee belemnite). Systematic variations can be correlated with isotope profiles reported from Tethyan pelagic limestone sequences. The detailed correspondence between the isotopic signature of the relatively well-dated pelagic limestones and the poorly dated shallow-water limestones from the Arabian Gulf region suggests that global marine carbon isotope changes apparently affected deep-sea and shallow-water carbonate sediments similarly and at a similar time resolution. Although oxygen isotopes have been reset during diagenesis, carbon isotopes appear to have maintained their primary marine signature through time. No evidence has been found to connect carbon isotope trends to subaerial exposure or later meteoric diagenesis. In combination with other data, the investigated carbon isotope profiles can be used for basin-to-platform and regional correlations beyond the current resolution of biostratigraphy in shallow-water limestones. Carbon isotope stratigraphy confirms significant hiatuses in the investigated shallow-water carbonate sequences. Using carbon isotope trends as a proxy for sea level fluctuations, the carbon isotope cycles of the late Early Cretaceous of the Arabian Gulf region may represent four cycles of rising and falling sea level with a duration corresponding to that of third-order sea level fluctuations. Regional correlations derived from isotope trends provide a scenario for the larger scale stratigraphic evolution of the Arabian peninsula during the end of the Early Cretaceous. Differential rates of aggradation between realms influenced by siliciclastics and those characterized by siliciclastic-poor ("clean") carbonate deposition apparently caused the development of a pronounced basin-platform topography during the early Aptian (Shuaiba intrashelf basin and platform).

Geopressuring Mechanism of Organic Matter Cracking: Numerical Modeling

Abstract: Since the 1950s, hydrocarbon generation has been considered as one of the most important abnormal pressuring mechanisms. Hydrocarbon generation effects on pore-pressure evolution: (1) it increases the pore fluid volume through the conversion of solid kerogen into liquid or gaseous hydrocarbon, and (2) it decreases the sediment permeability because hydrocarbons remain an immiscible phase in pore water. In this paper, the two products of organic matter decomposition, liquid hydrocarbon (oil) and gaseous hydrocarbon (methane), are assumed to occur in sequence as a supplementary phase in pores as a basis for evaluating the effect of this pressuring mechanism. A simplified three-stage first-order kinetic reaction model is applied to simulate the generation of oil and gas. We u e a numerical basin model with a two-phase hydrodynamic formulation to calculate the effect of organic maturation on geopressure evolution. Oil generation does not play an important role except when the organic matter content of rocks is relatively large (>5%); however, the generation of gas has a large influence. The results show that beneath a certain depth, oil-to-gas cracking strongly influences overpressuring. The effect of this mechanism becomes more important with increasing organic matter content of rocks. Several environmental conditions, such as kerogen type, temperature gradient, and lithological characteristics, influence to various degrees the effect of organic matter maturation on overpressure development.

Thrust-front zone of the Precordillera, Argentina; a thick-skinned triangle zone

Abstract: The frontal thrust zone of the Precordillera thrust belt of western Argentina only superficially resembles a classical Alberta-style triangle zone. In this thick-skinned triangle zone, the thrust front is the result of interaction between the eastward-verging, thin-skinned Central Precordillera and the westward-verging, thick-skinned Eastern Precordillera system. The western boundary, formed by the Niquivil thrust plate, has been refolded and faulted by thrust faults from the Eastern Precordillera at several localities. The deformation is still active. We divide the thick-skinned triangle zone into three segments based on the system's activity: independent, late interfering, and collision zones. Balanced cross sections across a thick-skinned triangle zone show a uniform s ortening for the Eastern Precordillera (~17 km). The structural geometry suggests multiple reactivation of the main faults. The thick-skinned triangle zone began to grow after 2.6 Ma and is still active. Episodic and alternating opposing movement is controlled by buttressing of the thin-skinned thrust system against the basement structures of the Eastern Precordillera.

Intracontinental rifting and inversion; Missour Basin and Atlas Mountains, Morocco

Abstract: An edited version of this paper was published by the American Association of Petroleum Geologists (AAPG). Copyright 1996, AAPG. See also: http://www.aapg.org/bulletin/index.cfm; http://atlas.geo.cornell.edu/morocco/publications/beauchamp1996.htm

Estimating Average Fracture Spacing in Subsurface Rock

Abstract: Knowledge of the spacing of fractures in reservoir rocks (i.e., the distance between parallel fractures in a subsurface joint set) can lead to a better understanding of the production characteristics of a reservoir and serve to quantify the relative degree of deformation in subsurface rocks. In this paper, I present a new method for estimating the spacing of subsurface fractures; this new method is easy to use from the standpoint of both data collection and data analysis. The average fracture spacing method can be applied with boreholes of any orientation relative to a fracture set. The method is especially powerful when it is used for the relatively common case of a borehole nearly parallel to a fracture set (e.g., vertical borehole intersecting vertical fractures). Average fracture spacing is estimated from an analytical solution based on observed borehole-fracture intersections and observed fracture porosity; the only data required are the dimensions of the core (or imaged borehole) and the total height of all sampled fractures. Because the likelihood of intersecting fractures increases when a well is deviated perpendicular to the fractures of a set, fracture reservoirs commonly are candidates for deviated boreholes. An informed decision on borehole deviation requires predicting the fracture intersection frequency as a function of both deviation magnitude and direction. A new method, based on probabilities of borehole-fracture intersections, uses spacing and height data from subsurface joint-like fractures and the borehole diameter to predict fracture intersection frequencies for all possible well deviations. Fracture intersection frequency solutions are presented with respect to a conventional geographic reference frame, thus simplifying even the most complex three-dimentional situations.

Glacioeustatic Fluctuations: The Mechanism Linking Stable Isotope Events and Sequence Stratigraphy from the Early Oligocene to Middle Miocene

Abstract: One of the most difficult challenges of sequence stratigraphy is the establishment of synchronism between events observed in widely-separated basins. Problems arise because the resolution of the best stratigraphic methods is not good enough to establish the synchronism of similar-aged events on a global scale. Unless a common mechanism affecting global eustasy is assumed, such as variations in the ice volume, there is no a priori reason to expect that sequences of similar age in widely-separated basins are indeed synchronous. The stable oxygen isotope composition of marine carbonates is a proxy for sea level which has been underutilized in sequence stratigraphy. Identification of isotope events is based on d{sup 18}O data from DSDP and ODP sites 522, 529, 563, 608, and 747 drilled in the Atlantic and Indian oceans. These records were used to define Oligocene and Miocene oxygen isotope zones. In addition, we present isotope data from PETROBRAS Well A drilled in the Campos Basin (Brazil). Ages of isotope events correspond well with the ages of sequence boundaries and maximum flooding surfaces. Because of the good correlation between the isotope and sequence stratigraphic records, we reconfirm that ice-volume change is the common mechanism driving sea-level fluctuations from the Oligocenemore » to present.« less

Regional-Scale Flow of Formation Waters in the Williston Basin

Abstract: The Williston basin is a structurally simple intracratonic sedimentary basin that straddles the United States-Canada border east of the Rocky Mountains and that contains an almost continuous stratigraphic record since the Middle Cambrian. Based on the wealth of data generated by the oil industry, the regional-scale characteristics of the flow of formation waters were analyzed for the Canadian side of the basin, and integrated with previous studies performed on the American side. Several aquifers and aquifer systems identified in the basin were separated by intervening aquitards and aquicludes. The Basal, Devonian, and Mannville (Dakota) aquifers are open systems, being exposed at the land surface in both recharge and discharge areas. Recharge takes place in the west-southwest at relatively high altitude in the Bighorn and Big Snowy mountains and at the Black Hills and Central Montana uplifts, whereas discharge takes place in the east and northeast at outcrop along the Canadian Precambrian shield in Manitoba and the Dakotas. The Mississippian and Pennsylvanian aquifer systems are semi-open, cropping out only in the west-southwest where they recharge, but discharging in the northeast into adjacent aquifers through confining aquitards. The Lower Cretaceous Vikin aquifer is a partially closed system, being confined by Cretaceous aquitards except for a small recharge area exposed at the Black Hills uplift and narrow discharge area in the Dakotas. The Upper aquifer is unconfined, with groundwater flow being driven by local topography, whereas the flow in all the other aquifers is regional in nature, being driven by basin-scale topography and characterized by normal hydraulic heads. The intervening aquitards seem to be strong, allowing little cross-formational flow. On regional and geological scales, the entire system seems to be at steady-state, although locally transient flow is present in places due to water use and hydrocarbon exploitation, and to some erosional rebound in the uppermost confining shales. Fresh meteoric water is present in the western and shallower parts of the basin, and brines are found in the Paleozoic aquifers in the central and eastern parts of the basin. This shows that the basin has not been completely flushed by meteoric water. Some cross-basinal inflow from the Alberta basin is apparent along the northwestern margin of the Williston basin, particularly for the Devonian and Viking aquifers. Hydrocarbons generated in the deeper, thermally mature part of the Williston basin have generally migrated within the same units updip north and northeastward, their buoyancy-driven secondary migration being enhanced by the northeastward flow of formation waters. On the western flank of the basin, the interplay between the northeastward structural downdip direction and the n rtheastward flow of formation waters creates conditions favorable for hydrodynamic oil entrapment.

Development of Sediment Overpressure and Its Effect on Thermal Maturation: Application to the Gulf of Mexico Basin

Abstract: High sedimentation rates can potentially lead to overpressuring and sediment undercompaction within basins. Sediments with anomalously high porosity, in turn, induce low thermal conductivities and so tend to act as a thermal insulator to the flow of heat. In the Gulf of Mexico basin (Gulf basin), the generation of overpressure is caused mainly by the inability of pore pressure fluids to escape at a rate commensurate with sedimentation. We modeled the generation and dissipation of abnormal sediment pore pressure due to variations in sedimentation rate, facies, formation porosity, and permeability within the Gulf basin using finite-element techniques to solve the differential equations of both heat and fluid transport within compacting sediments. We assume that the porosity effective stress relationship within the sediment follows a negative exponential steady-state form when the pore pressure is hydrostatic. An important feature of our modeling approach is the assumption that sediments are incapable of significant expansion in response to increasing pore pressure. Sediments are assumed to hydrofracture when the pore pressure approaches the lithostatic pressure, rather than a common assumption of porosity expansion even in lithified sediments. From our modeling, we conclude that significant overpressures have been created (and dissipated) at various times within the Gulf basin and track, in general, the west to east migration of sediment loads deposited since the Cretaceous. Although predicted overpressures of more than 0.75 kpsi (i.e., an equivalent excess hydraulic head of 500 m) of Campanian-Maastrichtian age remain to the present day, the main phase of overpressure development in the Gulf basin is predicted to have occured during the Miocene-Holocene. Maximum overpressures (~13.6 kpsi; excess hydraulic head of 9.4 km) are predicted for the present day. Overpressure development during the Miocene-Quaternary, a consequence of rapid sediment deposition associated with the Mississippi delta system, is also predicted to be associated with undercompaction. This undercompaction led to increased temperature gradients during the Miocene and Quaternary despite the fact that the anomalous basal heat flow engendered by extension had practically dissipated. We further predict that by the end of the Neogene, temperatures would have been approaching s eady state over broad regions of the Gulf basin implying that the highest temperatures occur in the deepest parts of the basin. In contrast, during the Quaternary, the rapid progradation of overpressured and undercompacted sediments resulted in a thick section that has yet to reach thermal equilibrium and thus is anomalously cold with respect to its present depth. The predicted vitrinite reflectance indicates that for most of the Gulf basin history, the depth to the top of the oil window remained at approximately 2.5±0.5 km below sea floor (bsf). Similarly, the depth to the base of the oil window ranged from 3.5 to 6.5 km bsf. This relatively constant position of the top of the oil window defines a maturation "front" that propagated from the offshore into the End_Page 1367------------------------------ onshore regions of the northern Gulf basin as a function of time. As such, hydrocarbon generation is predicted to have occurred continuously within the Jurassic and Cretaceous sections of the onshore region during the entire Cenozoic. Prior to this, maturation fronts within each of the onshore basins resulted in maturation of Upper Jurassic source rocks during the Early Cretaceous. In the offshore Gulf Coast area, pre-Tertiary source rocks are predicted to be overmature for liquid hydrocarbons at present. In the offshore regions affected by Quaternary sedimentation, the depth to the top of the oil window has been significantly depressed in response to sediment loading and subsidence.

Sequence Stratigraphy and Composition of Late Quaternary Shelf-Margin Deltas, Northern Gulf of Mexico

Abstract: High-resolution seismic profiles and foundation borings from the northwestern Gulf of Mexico record the physical attributes and depositional histories of several late Quaternary sequences that were deposited by wave-modified, river-dominated shelf-margin deltas during successive periods of lowered sea level. Each progressively younger deltaic sequence is thinner and exhibits a systematic decrease in the abundance and concentration of sand, which is attributed to a shift in the axes of trunk streams and greater structural influence through time. Our study shows that (1) contemporaneous structural deformation controlled the thickness of each sequence, the oblique directions of delta progradation, the axes of major fluvial channels, and the geometries of delta lobes at the shelf margin; (2) sedimentation was rapid in response to rapid eustatic fluctuations and structural influence; (3) boundaries of these high-frequency sequences are the correlative conformities of updip fluvial incision and coincide with downlap surfaces at the shelf margin; (4) the downlap surfaces are not true surfaces, but zones of parallel reflections that become progressively higher and younger in the direction of progradation; (5) the downlap zones are composed of marine muds that do not contain the high concentrations of shell debris expected in condens d sections; (6) possible paleosols capping the two oldest sequences are regressive surfaces of subaerial exposure that were preserved during transgressions; and (7) no incised valleys or submarine canyons breach the paleoshelf margin, even though incised drainages were present updip and sea level curves indicate several periods of rapid fall.

Ephemeral-Fluvial Deposits: Integrated Outcrop and Simulation Studies Reveal Complexity

Abstract: Ephemeral-fluvial sandstones form the main reservoirs in many oil and gas fields. Production histories and development difficulties suggest these deposits are more complex than is commonly supposed in the petroleum industry. There is inadequate published information to account for this observation. Relevant sedimentological literature on such deposits is qualitative and unsuitable for detailed reservoir characterization studies; therefore, integrated sedimentological, petrophysical, and fluid-flow simulation studies have been conducted on outcrop analogs in the southwestern United States. Our key objectives were to define flow units and to find areas of sensitivity inherent both in the sediments and in the analytical and statistical procedures used to describe and model s ch systems. We report results of integrated studies on the Lower Jurassic Kayenta Formation in southeastern Utah. The Kayenta Formation is a relatively proximal, sand-rich succession dominated by channel-fill sandstones. Sedimentological analysis identified eight main genetic units. In contrast, reservoir simulations, based on probe-permeameter data collected at outcrop, show that six of the main genetic units have uniform flow behavior. Intraformational mudclast conglomerates are the dominant permeability barriers, both because of the clasts themselves and because they act as sites of preferential cementation. Stochastic permeability models conditioned to the outcrop data produce results little different from models based on averaged values for each facies. The most sensitive feature of the mode s concerns the assumptions made about the mudclast conglomerates. The model results show that only by integrating analytical approaches can we understand the full character of an outcrop analog in such a way that we can use it predictively.

Thermal and Tectonic History of the Ordos Basin, China: Evidence from Apatite Fission Track Analysis, Vitrinite Reflectance, and K-Ar Dating

Abstract: Apatite fission track analysis, vitrinite reflectance data, and K-Ar dating of Permian-Carboniferous and Mesozoic core samples have been successfully integrated to reconstruct the thermal and tectonic history of the Ordos basin, China. Apatite fission track ages of Carboniferous-Jurassic sedimentary rocks range between 3 and 137 Ma, and are significantly younger than the stratigraphic ages. Confined fission track lengths demonstrate exclusively mixed length distribution, indicating complex thermal history. The data suggest that the samples must have all experienced higher paleotemperatures in the past. Mean vitrinite reflectance values (R0) of the Triassic rocks range from 0.61 to 1.06%, giving a high coalification gradient of 0.36%/km and suggesting a high paleothermal gradient of 57°C/km. Permian-Carboniferous rocks have Ro values on the order of 1.0-3.0%, and locally up to 4.0-6.0%. Some high R0 values coincide with positive gravity and magnetic anomalies. K-Ar dating on Permian-Triassi samples reveals distinct illitization at 170-160 Ma, during which a thermal event occurred due to subsurface magmatic intrusion related to the early Yanshanian movement. The petroleum source rocks of the Upper Triassic experienced peak temperatures ranging from 90 to 160°C, corresponding to the oil window, and Permian-Carboniferous source rocks were heated to more than 150°C, passing through and out of the gas window. Due to rapid uplift and erosion in response to the rise of the Qinghai-Tibet plateau associated with the Asia-India collision and the Himalayan orogeny, cooling has taken place at least since approximately 23 Ma. The difference in the rate and amount of uplift between the eastern and western parts of the basin resulted in differential uplift and the present-day s ructural pattern of the basin.

A Two-Dimensional Regional Basin Model of Williston Basin Hydrocarbon Systems

Abstract: Institut Francais du Petrole's two-dimensional model, temispack, is used to discuss the functioning of petroleum systems in the Williston basin along a 330-km-long section, focusing on four regional source intervals: Ordovician Yeoman formation, Lower Devonian Winnipegosis Formation, Upper DevonianuLower Mississippian Bakken Formation, and Mississippian Lodgepole formation. Thermal history calibration against present temperature and source rock maturity profiles suggests that the Williston basin can be divided into a region of constant heat flow of about 55 mW/m2 away from the Nesson anticline, and a region of higher heat flow and enhanced thermal maturity in the vicinity of the Nesson anticline. Original kinetic parameters used in the calibration were derived or each of the four source rocks from Rock-Eval yield curves. Bakken overpressures are entirely due to oil generation, not compaction disequilibrium. Very low Bakken vertical permeabilities range from 0.01 to 0.001 nd are matched against observed overpressures, whereas Bakken porosities based on the model and confirmed by measurements are inferred to be also unusually low, around 3%. Mature Bakken shales do not seem to have reached hydraulic fractionation thresholds, except perhaps locally in regions of extensional tectonic stress. Hydraulic fracturing cannot be viewed as a pervasive mechanism driving Bakken oil expulsion. Our expulsion model confirms the high residual Bakken oil saturations and explains the low residual S1/TOC by the low Bakken shale porosities. Approximately 85% of the oil generated in the Bakken shales is predicted to have been expelled out of Bakken strata, which agrees with observed S1/TOC trends. Madison Group accumulations at the subcrop edge are found to be fed by Lodgepole-sourced oil only, in particular by the rich middle Lodgepole interval. These accumulations probably result from a three-dimensional migration pattern. Madison accumulations in the Nesson anticline are found to be fed mostly by Lodgepole-sourced oil mixed with minor amounts of Bakken-sourced oil. The vast majority of the expelled Bakken oils are lost in the Madison Group at very low saturations. This explains the low conventional oil resource associated with Bakken sources by recent geochemical studies. Expulsion and migration occurred no earlier than the atest Cretaceous-Paleocene in the Yeoman and no earlier than the Eocene in other source rocks, consistent with temporal controls on oil migration. Flow models show no restriction to expulsion and migration of Yeoman oil. This suggests a potential undiscovered oil resource in Ordovician and Silurian strata of Williston basin, northeast of the Nesson anticline.

Influence of Lithofacies and Diagenesis on Norwegian North Sea Chalk Reservoirs

Abstract: The depositional mechanism of chalk is a key influence in the chalkAEs ultimate reservoir quality. Classically, the depositional mechanism is interpreted from core descriptions. Where core data are lacking, dipmeter and borehole imagery logs have proven useful in making lithofacies assessments. Criteria for recognition of three chalk categories are established. Category III chalks correspond to those chalks that have been deposited by gravity flows or slumping and tend to have the best reservoir parameters. Category I chalks are most often affiliated with pelagic deposition and tend to have the poorest reservoir parameters. Category II chalks are intermediate between I and III. Anomalously high primary porosities have been maintained in Norwegian North Sea chalks where the effects of mechanical and chemical compaction have been limited. The diagenetic pathway of a chalk reflects changes brought about by mechanical and chemical compaction. Five factors most heavily influence the diagenetic pathway: (1) burial depth, (2) chalk type, (3) overpressuring, (4) presence of hydrocarbons, and (5) original grain size. Assessments of the sedimentological model, diagenetic pathway, and resultant reservoir quality are provided in case studies of Edda, Tor, and Eldfisk fields. Because the distribution of chalk is largely independent of existing structures, most fields have a component of stratigraphic/diagenetic trapping. Each case study shows unique examples of how petrophysical and reservoir engineering data can be incorporated in assessments of chalk type and the diagenetic pathway and how they may affect reservoir parameters and productivity.

Exploring for Hydrocarbons Under Thrust Belts--A Challenging New Frontier in the Carpathians and Elsewhere

Abstract: New significant reserves of hydrocarbons may occur in subthrust autochthonous and parautochthonous series buried below the frontal zones of thin-skinned thrust belts. The subthrust plays have been tested in several orogenic belts of the world, the Carpathians being one of the best examples. The arcuate thin-skinned Carpathian orogenic belt, which evolved during the Mesozoic and Cenozoic, is thrust tens of kilometers over its Neogene foredeeps and the underlying European plate. Various structural and stratigraphic settings and potential hydrocarbon plays have been recognized within the buried margins of the European plate, including a late Paleozoic Hercynian compressional system, Mesozoic rifted margins of the Tethys, and a Cenozoic synorogenic foreland-type fault system. Possibly, deeper parautochthonous structures, documented on examples from the southern Apennines, may also be present below the thin-skinned frontal zone of the Carpathian thrust belt. In addition to these structural settings, large Paleogene valleys/submarine canyons have been found within the margins f the European plate. These structural and morphologic features, if combined with source rocks, reservoirs, and proper burial history, represent potential hydrocarbon plays. Generation of hydrocarbons from sources within the subthrust plate was greatly enhanced by emplacement of the wedge-shaped thrust belt, which may also provide a regional seal; therefore, the combination of the long and complex geological history of the European plate with the impact of the Alpine thrusting and foreland deformation created unique conditions for generation, entrapment, and preservation of hydrocarbons in subthrust settings.

Thermal Conductivity of Wilcox and Frio Sandstones in South Texas (Gulf of Mexico Basin)

Abstract: Thermal conductivity and petrographic data are presented for verifying mechanistic models of sandstone thermal conductivity. We measured the thermal conductivity of 83 Wilcox and Frio sandstones from south Texas in the Gulf of Mexico sedimentary basin, and correlated conductivity to petrographic variables. Thermal conductivities of water-saturated sandstones at 20°C (68°F) and 3 MPa (435 psi) were measured on core plugs using a divided-bar apparatus. Thermal conductivity ranges from 2.06 to 5.73 W/m/K over a porosity range of 2.4 to 29.6%. Because of a higher quartz content, Wilcox sandstones at a given porosity are more conductive than Frio sandstones. A grain-matrix conductivity of 5.9 W/m/K is estimated for Wilcox sandstones; matrix conductivity is adequately described with an arithmetic mixing model. Thermal conductivities of clean ( 35% of the solids) sandstones, the dependence on quartz content can be dropped and thermal conductivities can be predicted with a linear decrease in conductivity with increasing porosity. These sandstones appear isotropic with respect to thermal conductivity.

Seismic Sequence Stratigraphy and Hydrocarbon Potential of the Phu Khanh Basin, Offshore Central Vietnam, South China Sea

Abstract: The Phu Khanh Basin, offshore central Vietnam, is the only undrilled basin on the Vietnam margin of the South China Sea. Analysis of multichannel seismic data indicates that the Phu Khanh Basin follows a typical rift-margin development: faulted basement, synrift sedimentation, a breakup unconformity, and postrift sedimentation. Initial rifting probably began during the Late Cretaceous(?) or Paleogene. Rifting and uplift appear to have resumed or continued locally during the late Oligocene and early Miocene. The later part of basin development was dominated by subsidence. Postrift sedimentation evolved from a transgressive ramp phase to a regressive shelf-slope phase. During the transgressive phase, rising sea level provided favorable conditions for carbonate buildup development. The regressive interval contains a number of sequences composed of seismically resolvable lowstand, highstand, and rarely transgressive, systems tracts. These sequences are interpreted as third-order sequences superposed on second-order eustatic cycles. Principal potential source rocks are believed to be synrift lacustrine sediments. The synrift sediments may lie below the oil floor and probably are gas prone. In the postrift section, the transgressive interval and the starved, basinward portion of transgressive and highstand systems tracts within the regressive interval also may have source rock potential. Carbonate complexes, weathered basement, shallow-water sands, and basin-floor fans all have the potential to provide reservoirs. Potential traps include carbonate complexes, basement hills and associated anticlines, fault traps, and coastal sand traps. Hydrocarbon indicators, such as flat spots, amplitude anomalies, and gas chimneys with seeps, occur at a number of locations.

Precambrian Basement Geology of the Permian Basin Region of West Texas and Eastern New Mexico: A Geophysical Perspective

Abstract: Because most of the Permian basin region of west Texas and southern New Mexico is covered by Phanerozoic rocks, other means must be found to examine the Precambrian upper crustal geology of the region. We have combined geologic information on the Precambrian from outcrops and wells with geophysical information from gravity and magnetic surveys in an integrated analysis of the history and structure of basement rocks in the region. Geophysical anomalies can be related to six Precambrian events: formation of the Early Proterozoic outer tectonic belt, igneous activity in the southern Granite-Rhyolite province, an episode of pre-Grenville extension, the Grenville orogeny, rifting to form the Delaware aulacogen, and Eocambrian rifting to form the early Paleozoic continental mar in. Two geophysical features were studied in detail: the Abilene gravity minimum and the Central Basin platform gravity high. The Abilene gravity minimum is shown to extend from the Delaware basin across north-central Texas and is interpreted to be caused by a granitic batholith similar in size to the Sierra Nevada batholith in California and Nevada. This batholith appears to be related to formation of the southern Granite-Rhyolite province, possibly as a continental margin arc batholith. Because of this interpretation, we have located the Grenville tectonic front southward from its commonly quoted position, closer to the Llano uplift. Middle Proterozoic mafic intrusions are found to core the Central Basin platform and the Roosevelt uplift. These intrusions formed at about 1.1 Ga and are related in time to both the Mid-Continent rift system and the Grenville orogeny in Texas. Because these features are likely to be rift related, they suggest that the concept of a Delaware aulacogen needs to be revised only to the extent that the rifting is Proterozoic in age, not Eocambrian. Precambrian basement structures and changes in lithology have influenced the structure and stratigraphy in the overlying Permian basin, and thus have potential exploration significance. Interpretation of the gravity and magnetic data with geologic information also leads us to suggest the existence of pre-Ellenburger basins, which may be extensive and of potential exploration interest.

Analysis of Sonic Well Logs Applied to Erosion Estimates in the Bighorn Basin, Wyoming

Abstract: An improved exponential model of sonic transit time data as a function of depth takes into account the physical range of rock sonic velocities. In this way, the model is more geologically realistic for predicting compaction trends when compared to linear or simple exponential functions that fail at large depth intervals. The improved model is applied to the Bighorn basin of northwestern Wyoming for calculation of erosion amounts. This basin was chosen because of extensive geomorphic research that constrains erosion models and because of the importance of quantifying erosion amounts for basin analysis and hydrocarbon maturation prediction. Thirty-six wells were analyzed using the improved exponential model. Seven of these wells, due to limited data from the Tertiary section, were excluded from the basin erosion analysis. Erosion amounts from the remaining 29 wells ranged from 0 to 5600 ft (1700 m), with an average of 2500 ft (800 m).

Quantification of Porosity and Permeability Reduction Due to Calcite Cementation Using Computer-Assisted Petrographic Image Analysis Techniques

Abstract: Calcite cementation is often an important factor in the evolution of reservoir pore systems. Although petrographically obvious, the effect that cementation has had on the petrophysical properties of a pore system may be difficult to evaluate quantitatively. To this end, a computer-assisted petrographic image analysis (PIA) technique was developed to quantify porosity and permeability reduction due to calcite cementation. With this technique, pore area and specific surface of the extant pore system are measured from digital images of core-plug thin sections. Porosity is estimated from the measurements of pore area, and an empirical equation relating pore area and specific surface to core permeability is derived using the Kozeny-Carman expression. In this manner, a permeability model is developed for the pore system in question, thus providing a means of estimating permeability from PIA measurements. To estimate the porosity and permeability of the precalcite pore system, calcite cement is discriminated from the same digital images and analyzed as pore space. This effectively backstrips calcite from the extant pore network to yield the precursor pore network. A comparison of the porosity and permeability of the extant and precalcite pore networks shows the quantitative significance of calcite cementation. This technique is demonstrated using two dolomite reservoirs that exhibit varying amounts of late-stage calcite cements: Little Sand Draw field, Wyoming, and Bindley field, Kansas. Calcite cement was found to be minor and restricted to moldic pores in Little Sand Draw dolomites, resulting in less than a twofold change in permeability. In contrast, late calcite cements are somewhat more abundant in Bindley dolomites, but more importantly, they occupy intercrystalline pores as well as moldic pores. The net effect was a 10- to 1000-fold decrease in permeability and the localized destruction of reservoir-quality rocks in Bindley field.

Textural and Compositional Variability Across Littoral Segments of Lake Tanganyika: The Effect of Asymmetric Basin Structure on Sedimentation in Large Rift Lakes

Abstract: Lake Tanganyika, part of the East African rift system, represents one of the most widely cited modern analogs for interpreting ancient rift lakes. To date, few published detailed sedimentologic studies of the modern sediments allow for comparisons to outcrop- and well-bore-scale observations within ancient strata. Four recurrent structural margin types exist along the alternating half-graben structure of the lake: hinged margins, axial margins, accommodation zone margins, and escarpment margins. The hinged margin consists of a series of structurally controlled benches over which long, continuous tracts of bioclastic lag deposits predominate; clastic sands are limited to moderate-size silty deltas and long, narrow shoreface sands. The axial margin is dominated by a wave-dominated, silt-rich delta system. Accommodation zone margins consist of bioclastic lag deposits atop structural highs, whereas carbonate and clastic mud accumulates farther offshore. Escarpment margins contain small fan-delta deposits alternating along shore with talus deposits; offshore carbonate and clastic mud is present away from active gravity-flow deposition. Total organic carbon (TOC) and pyrolysis data from fine-grained samples subtly reflect the contrasts in margin types, but these values are controlled more directly by water depth. Although facies are similar among all margin types, their spatial distribution, in particular the degree to which facies tracts trend parallel to shore, best discriminates among the different margin types. These data suggest that unique but predictable associations of reservoir, seal, and source facies exist along each of the different margin types.