Showing papers in "AAPG Bulletin in 1984"

Stable Carbon Isotope Compositions of Crude Oils: Application to Source Depositional Environments and Petroleum Alteration

Abstract: The carbon isotopic composition of the C15+ saturate and aromatic hydrocarbon fractions and the gas chromatographs of the C15+ saturate fraction of 339 oils were geochemically and statistically evaluated. Results of this study show that the average isotopic compositions of the C15+ aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical. The average isotopic composition of the C15+ saturate fraction of oils derived from terrigenous organic sources is slightly more negative (0.9^pmil) than the average for marine oils, but this difference is insufficient to be used as a reliable source indicator. Reports in the literature stating that marine oils are isotopically more positive than te rigenous oils, and that the difference can be utilized to distinguish between the two are, therefore, not supported by this study. The isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions. The isotopic relationship is the following: Oils of terrigenous organic source: ^dgr13Caro = 1.12^dgr13Csat + 5.45; Oils of marine organic source: ^dgr13Caro = 1.10^dgr13Csat + 3.75. The difference between the two equations was evaluated statistically and a statistical parameter, CV (the canonical variable), CV = -2.53^dgr13Csat + 2.22^dgr13Caro - 11.65, was used to distinguish between marine and terrigenous oils. CV values larger than 0.47 indicate predominantly a terrigenous organic source for the oil, whereas CV values smaller than 0.47 indicate mostly a marine organic source. Thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2^pmil variations in the isotopic composition of one family of oils. Finally, stable carbon isotopes can, in some situations, help correlate biodegraded oils to their nondegraded counterparts.

Smectite Dehydration--Its Relation to Structural Development and Hydrocarbon Accumulation in Northern Gulf of Mexico Basin

Abstract: A comparison of clay diagenesis data obtained from a study of Tertiary shales from the Brazos-Colorado River system of Texas, the Mississippi River system of Louisiana, and the Niger River system of Nigeria illustrates significant differences in temperature intervals over which smectite diagenesis occurs. The threshold temperature required to initiate diagenesis ranges from about 160°F (71°C) in Mississippi River sediments to more than 300°F (150°C) in the Niger delta. Water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early or may be totally or partially trapped and released slowly through time. In either situation, the water can act as a vehicle for hydrocarbon migration p ovided hydrocarbons are present in a form and in sufficient quantities to be transported. Observations from the northern Gulf of Mexico basin indicate a close relation between buildup of high fluid pressure and the smectite-illite transformation process. Abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow, and overpressuring partly controls the geometry of growth faults and related folds in basins where shale structures are the dominant type formed. The depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting. Dips of some faults in Texas have been observed to change abruptly within the interval of smectite diagenesis, and some faults formed in the overpressured Miocene and younger sections become beddingplane types at depths where the temperature is near that required for thermal generation of petroleum. Although these faults may be important for fluid redistribution in shallow sandstone-shale sections, they are a minor factor in moving hydrocarbons out of shale below the faults in much of the Texas offshore area. Fluid movement upward through microfracture systems in the deeply buried overpressured section overlying and extending upward from fault trends in the sub-Tertiary section is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern Gulf of Mexico basin. This flushing process would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing process to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present. Shale tectonism is also the primary mechanism for structural development in the Tertiary section of the Niger delta; however, seismic data indicate that the rate of dip change of seaward-dipping listric growth faults is commonly less than that observed in Texas where dips as low as 10°-15° can occur at depths as shallow as 10,000-15,000 ft (3,048-4,572 m). Syndepositional faults in Nigeria are formed in sandstone-shale sections where the clay composition of shale is primarily kaolinite and where little water of smectite diagenesis has been added to the pore system of the host sedimentary section. Subtle differences in structural styles in the Tertiary sections of Texas and Nigeria are probably the result of differences in clay composition of the shaly sections being deformed

Microbial Alteration of Subsurface Natural Gas Accumulations

Abstract: Bacterial alteration of subsurface reservoired hydrocarbon accumulations extends into the range of the C1-C4 gaseous hydrocarbons, preferentially attacking the wet gas components in most instances. During the initial stages of bacterial gas alteration, propane seems to be selectively attacked, resulting in anomalous propane carbon isotopic compositions as well as unusual carbon isotopic distributions among the wet gas components. Extensive bacterial alteration appears to be capable of removing all but traces of the wet gas components, resulting in a dry gas that could be confused with overmature dry gas whose wet components have undergone extensive thermal cracking. Recognition of microbial alteration in subsurface reservoired hydrocarbon gas accumul tions is important for both correlation purposes and maturity estimates.

Evaluating Seals for Hydrocarbon Accumulations

Abstract: Seals are an important and commonly overlooked component in the evaluation of a potential hydrocarbon accumulation. Effective seals for hydrocarbon accumulations are typically thick, laterally continuous, ductile rocks with high capillary entry pressures. Seals need to be evaluated at two differing scales: a "micro" scale and a "mega" or prospect scale. Quantitative "micro" data measured on hand specimens of seal rock are difficult to extrapolate a billion-fold to the scale of the sealing surface for a hydrocarbon accumulation. Fortunately, each class of exploration prospects has a different set of seal problems. Geologic work can be focused on the characteristic seal problems that plague classes of prospects. Anticlines have relatively little seal risk, because any layer serving as a top seal will also be a lateral seal. Stratigraphic traps and faulted prospects have substantial seal risks. When attention is focused toward the expected sealing surface of a potential hydrocarbon accumulation, it is possible to assess the relative risk that a seal is present. Improvements in assessing seal risk for an exploration prospect directly affect the estimation of exploration success.

Development of a Comprehensive Oxygen-Deficient Marine Biofacies Model: Evidence from Santa Monica, San Pedro, and Santa Barbara Basins, California Continental Borderland

Abstract: An extensive data base of box core x-radiographs, bottom photographs, and box core faunal data has been employed to establish the relationships between dissolved oxygen content and (1) sediment fabric, (2) biogenic structure distribution and size trends, and (3) faunal composition and species richness in three "anoxic silled basins" of the California continental borderland. Sediments of anaerobic environments are generally well laminated, whereas those of aerobic and dysaerobic environments are completely bioturbated. Intermediate, partially bioturbated sediments are produced near the present dysaerobic-anaerobic boundary by short-term temporal fluctuations in the position of the dysaerobic-anaerobic boundary. Although distribution of burrow types observed in x-radiographs does not show significant control by variations in dissolved oxygen, average size of burrows decreases markedly with decreasing oxygen. In terms of surface biogenic structures, most aerobic and upper dysaerobic sea floors are dominated by tracks and trails, but lower dysaerobic sea floors are characterized by burrow openings. No surface biogenic structures were observed on anaerobic sea floors. Regular and irregular echinoids dominate sea-floor surfaces in aerobic and upper dysaerobic environments, whereas polychaetes, ophiuroids, holothurians, small arthropods, and gastropods are common on lower dysaerobic sea floors. No surface-dwelling organisms were visible on anaerobic bottoms. Box core faunal analysis indicates that species richness generally decreases with increased depth and decreasing oxygen until it reaches near zero at the dysaerobic-anaerobic boundary. The most abrupt decrease in species richness occurs at the shelf-slope transition. These observations are highly supportive of criteria cited by Rhoads and Morse (1971) and Byers (1977) for distinguishing anaerobic facies from more oxygenated facies in ancient strata. However, their criteria for distinguishing between ancient aerobic and dysaerobic facies are not as definitive. Paleontologic criteria, biogenic structure size trends, and other rock characteristics are useful for establishing paleo-oxygen gradients but not for determining specific paleo-oxygen concentrations as there are no abrupt oxygen-controlled changes in these parameters above the anaerobic-dysaerobic boundary.

Influence of Paleoenvironmental Factors on Preservation of Organic Matter in Middle Cretaceous Greenhorn Formation, Pueblo, Colorado

Abstract: Sedimentological and geochemical studies of the Upper Cretaceous Hartland Shale and overlying Bridge Creek Limestone Members of the Greenhorn Formation reveal close associations between abundance of current-induced sedimentary structures, extent of bioturbation, and type of preserved organic matter. Abundant hydrogen-rich organic matter (4-5 wt. %) and low sulfur to organic carbon ratios are characteristic of the laminated facies, which lack current-induced sedimentary structures. Sparse hydrogen-poor organic matter (0.1-0.5 wt. %) and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies, which contain numerous sedimentary structures indicative of currents. The concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively (by currents) rather than by the rate of oxygen consumption (by decomposition). Thus, paleoclimatic and paleo-oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units. It is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen-depleted bottom water and high preservation of organic matter, thus accounting for the intermittent widespread deposition of organic-rich strata during maximum transgression and depth of the Greenhorn sea. The lateral association of voluminous deltaic deposits with dee -water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the Hartland Shale and Bridge Creek Limestone Members.

Identification of Source Rocks on Wireline Logs by Density/Resistivity and Sonic Transit Time/Resistivity Crossplots

Abstract: Source rock formations generally show a lower density, a lower sonic transit time, and a higher resistivity than other sediments of equal compaction and comparable mineralogy. This phenomenon can be used to identify source rocks on wireline logs provided the source rocks have a minimum thickness within the resolution of the sondes used and are sufficiently rich in organic matter. Classification rules have been established to assist in the recognition of source rocks on a combination of logs. Because of the low density contrast between water and organic matter, the method becomes inaccurate at high water saturations (i.e., low compaction). Within limits, the amount of organic matter contained in a sediment can be estimated from log anomalies. When source rocks become mature, free oil is present in addition to kerogen, and the resistivity increases by a factor of 10 or more.

Recent Advances in Petroleum Geochemistry Applied to Hydrocarbon Exploration

Abstract: This paper presents a critical review of recent advances in the study of generation, maturation, and migration of petroleum. The different source potentials for generating hydrocarbons depend on the type of kerogen. In addition to the classic types I, II, and III, a residual, oxidized organic matter is also observed, which has no potential for hydrocarbon generation. Coal is widely considered as a potential source for gas, but some coals may generate sizeable amounts of oil as well. The development of routine analytical tools, particularly pyrolysis, makes possible the preparation of geochemical logs that are the basis for screening samples and for interpreting other analyses. Among optical techniques, the major subjects of discussion concern the nature of amorphous organ c matter, which may encompass different chemical types of kerogen, and the applicability of vitrinite reflectance techniques to type I and II kerogens. Following important advances in identification of biological markers in sediments and crude oils, these markers are used for oil-source rock correlation, and proposed for reconstruction of depositional environment and subsequent thermal evolution. Migration of hydrocarbons is now better understood, and the importance of a hydrocarbon-phase migration is widely recognized. Overpressuring of pore fluids is mainly responsible for expulsion of petroleum. Geologic models of basin evolution (subsidence, compaction, thermal history) and geochemical models (hydrocarbon generation and migration) become progressively available. Their integration int a comprehensive model should be one of the major developments in future petroleum exploration.

Flexure of Lithosphere Beneath Apennine and Carpathian Foredeep Basins: Evidence for an Insufficient Topographic Load

Abstract: The Pliocene foredeep basin external to the Apennine thrust belt and the late Miocene foredeep basin external to the outer East Carpathians are considered typical molassic foredeeps that formed contemporaneously with thrusting in the adjacent mountain belts. A simple mechanical model in which the response of the lithosphere to loading was approximately by a semi-infinite (broken) elastic plate overlying a weak fluid was used to analyze the deflection of the foreland lithosphere beneath the Apennine and outer East Carpathian foredeep basins along three transects perpendicular to the thrust belts. Comparison between the calculated and observed depth to basement suggests that the emplacement of thrust sheets onto the foreland lithosphere during mountain building failed to pr duce a sufficient load to create the observed foredeep basins and observed topography regardless of the position of the slab end or the flexural rigidity assumed for the foreland lithosphere. We infer that an additional subsurface load must be present and that its magnitude must be roughly equal to the load created by that part of the thrust belt that is above sea level. For the two Apennine transects, an acceptable fit to the shape and size of the observed foredeep basin was generated by assuming effective elastic plate thicknesses of 8 and 15 km (4.9 and 9.3 mi) and applied vertical forces of 2 × 1015 dynes/cm and 1.3 × 1015 dynes/cm at the slab end. For the Carpathian transect, an effective plate thickness of 30 km (18.6 mi) and an applied force of 1.5 #215; 1015 dynes/cm generated an acceptable fit. This study suggests a method whereby an estimate of the depth to autochthonous basement rocks beneath overthrust belts can be made in some areas where this information is not available from seismic or drilling data. Such an approach may enable exploration geologists to determine if crystalline rocks encountered in drilling overthrust terranes are a part of the autochthonous basement, or if they are part of an overthrust sheet. In the latter situation, oil-bearing sequences of sedimentary rocks might lie below crystalline rocks contained in the allochthonous sheet.

Sedimentation, Tectonism, and Hydrocarbon Generation in Delaware Basin, West Texas and Southeastern New Mexico

Abstract: The Delaware basin is the western major structural subdivision of the Permian basin of west Texas and New Mexico, with an area of more than 13,000 mi2 (33,500 km2) and containing a volume of 40,000 mi3 (170,000 km3) of Phanerozoic sedimentary rocks. Its history as a separate basin dates from the Early Carboniferous, but its roots began in the late Proterozoic Era, probably as a north-south-trending aulacogen. Through much of the Paleozoic Era, the basin and its predecessors formed a confined depression surrounded by carbonate shelves. In this depression, organic debris was preserved. Burial converted this material to kerogen and then to hydrocarbons. Deposition of thick evaporite strata during the Late Permian formed a seal that preserved the hydrocarbons and facilitated their migration to porous carbonate reservoirs in the surrounding shelves. In the basin, deeper burial caused conversion of heavier hydrocarbons into gas in the older rocks, whereas paraffinic oils in younger strata accumulated in smaller reservoirs. Apparently, there were three chief intervals of hydrocarbon generation in the basin: (1) Middle Ordovician, Late Devonian, and Mississippian; (2) Middle Pennsylvanian; and (3) Early and middle Permian. The accumulations of gas and oil in the basin have been largely undisturbed by the mild tectonic activity since the end of the Permian. The region was practically unaffected by the Laramide deformation. Thus, it remains an important oil- and gas-producing province.

Geothermal Gradients, Hydrodynamics, and Hydrocarbon Occurrences, Alberta, Canada

Abstract: Using coal rank data and the demonstrated genetic link between the present hydrodynamic regime and geothermal gradient pattern, it is possible to reconstruct the geothermal history of Alberta over the past 60 m.y. Palinspastic adjustment for tectonic compression caused by the major Laramide thrusting shows that the predeformation isoreflectance lines increased logarithmically with depth. In the late Paleocene, the geothermal gradient was about 23°C/km (1.25°F/100 ft) in the eastern Alberta Plains, compared to about 30°C/km (1.65°F/100 ft) in the western Alberta Plains, a regional trend opposite to the post-Laramide trends. Reconstruction of the early Eocene surface indicates western uplands with geothermal gradients as low as 21°C/km (1.15°F/ 00 ft) and eastern lowlands with geothermal gradients of 27°C/km (1.5°F/100 ft). Compared to the present situation, this represents an enhanced topographic surface and a subdued geothermal gradient pattern. The genetic relations of topography (water-table elevation), hydrodynamic regime, and geothermal gradient pattern in both the early Eocene and the present conform to a model developed for any compacted sedimentary basin with subaerial relief. In this model, on a regional scale, high topographic areas have high water-table elevations with correspondingly high potentiometric surfaces; these areas control the regional recharge of cold meteoric water and hence have low geothermal gradients. Areas of medium elevation exhibit regional lateral flow and intermediate geothermal gradients. Regional topographic lows correspond with low potentiometric surfaces, and the regional discharge of warm formation waters from deep in the basin results in high geothermal gradients. Local topographic featur s cause perturbations in this picture. Study of both the regional pattern and fine detail of the early Eocene and present topographic surfaces and geothermal gradient patterns shows the validity of the model. The result of the second Laramide orogenic pulse was, therefore, to reverse the geothermal gradient pattern resulting mainly from compaction flow, and to impose a topographically controlled hydrodynamic regime with flow in the same direction but with a 9°C/km (0.5°F/100 ft) decreased geothermal gradient in western Alberta and a 4°C/km (0.2°F/100 ft) increased geothermal gradient in eastern Alberta. As erosion proceeded during the Tertiary, the entire Alberta basin warmed up until the present temperature regime was attained. Study of some major hydrocarbon occurrences shows a genetic link between the hydrocarbon occurrence and the present fluid regime which itself is controlled by the present topography. For at least the significant Upper Cretaceous shallow gas reserves in southeastern Alberta, however, their position may already have been determined as early as the beginning of the Eocene by the topographic surface at that time. Therefore, the extent to which the present topographic surface is responsible for hydrocarbon accumulation through gravity-induced cross-formational flow remains unknown because this process was operative as long ago as the early Eocene. What does not ap ear to be in doubt is the fact that gravity-induced cross-formational flow does control the hydrodynamic regime, which in turn influences the geothermal gradient pattern and the accumulation of hydrocarbons.

Effect of Water Washing on C15+ Hydrocarbon Fraction of Crude Oils from Northwest Palawan, Philippines

Abstract: Recognition and subsequent correlation of biodegraded and/or water-washed oils with their unaltered counterparts has been a long-recognized problem in petroleum geochemistry. Because water washing and biodegradation can occur concurrently, it is difficult to delineate the individual effects of these two oil-altering processes, especially on the C15+ hydrocarbon fraction. Both processes can cause an oil to become enriched in sulfur and to have a lower API gravity owing to loss of the more volatile components. Thus, the altered oil may appear to be a high-sulfur crude of low maturity. Altered and unaltered oil fractions extracted from cores of a water-washed carbonate reservoir offshore northwest Palawan were studied to observe the effect of alteration on correla ion parameters. Water washing resulted in the following changes: (1) depletion of 13C in the C15+ branched-cyclic alkane fraction, whereas virtually no change occurred in isotope composition of the aromatic fraction; (2) selective loss of dibenzothiophenes followed by loss of aromatic hydrocarbons; and (3) virtually no change in steranes or tricyclic and pentacyclic terpanes, with the exception of enhancement of 17^agr(H) norhopane and decrease of 18^agr(H) trisnorhopane. In situations of severe water washing, mild biodegradation also occurred, as indicated by the loss of n-alkanes (and possible selective loss of n-alkanes having less than 25 carbon atoms) relative to branched alkanes, cycloalkanes, and aromatic components. Because cycloalkanes and aromatic hydrocarbons are not generally preferentially utilized by bacteria as long as n-alkanes are present, the changes observed in these two fractions most likely result from water washing. Because of the potential for extreme compositional changes in the n-alkane and aromatic fractions, the most useful correlation parameters between the altered and unaltered oils are sterane and terpane distributions. Stable carbon isotope ratios are also useful; however, more studies of oils in which water washing is known to have occurred need to be made to explain the general response (or various responses) of the isotopic composition of oils to water washing.

Listric Normal Faults: An Illustrated Summary

Abstract: Movement along listric normal faults is instrumental in formation of several types of structural traps (e.g., rollover anticlines and upthrown-fault-block closures). This type of fault may occur where brittle rocks overlie ductile rocks in an extensional regime. The extensional regime may be locally derived within a broader stress regime of another type, as evidenced by transtension associated with strike-slip movement and arched strata in a compressive setting. The flattening of the fault reflects an increase in ductility of the rocks with depth and, in some cases, deformation of the fault due to compaction or tilting of the upthrown block. The dip angle may vary along the strike of the fault in response to changes in throw. In cross section, a listric fault may consist of relatively short, en echelon fault segments. This geometry may be particularly characteristic of growth faults. Sedimentary faults may sole in ductile strata, or they may represent the brittle part of a fault-flow system. Fault patterns commonly are characterized by bifurcation, some of which may occur near the ends of individual faults comprising a zone. Although unequivocal recognition of listric normal faults requires unusually extensive outcrop data, close subsurface control, or high-quality seismic data, their presence is suggested indirectly by such features as increasing dip either depth toward the controlling fault ("reverse drag"), thick progradational sandstone overlying ductile strata, and in some cases arcuate fault patterns, basins, or uplifts. Listric normal faults form during rifting, drifting, and evolution of passive continental margins with concomitant basinal development. The basement is offset by listric faults as a fundamental element in the development of other types of basins, including those which formed during postorogenic extension. They also occur as secondary extensional features in an overall compressive stress regime due to plate convergence and during transform or strike-slip faulting.--Modified journal abstract.

Petroleum Source-Bed Evaluation of Tertiary Niger Delta: REPLY

Abstract: A total of 63 sidewall and drill-cutting shale samples obtained from a depth slice of 800 to 3650 m in two onshore and two offshore wells located respectively in the eastern and western Niger delta basin, Nigeria, were analyzed for organic-matter type, concentration, and thermal maturity. Dominant sedimentary kerogens were the humic and mixed types. Organic carbon contents of the sediments ranged from 0.4 to 4.4% and the degree of thermal evolution as evidenced by vitrinite reflectance (R/sub 0/%), odd-even ratios (OER, 2C/sub 29//(C/sub 28/ + C/sub 30/)), and variations of the ratios of the concentration of soluble organic matter to total organic carbon content (SOM/TOC) were within the ranges 0.2 to 0.7, 4.0 to 1.0, and 10 to 240 respectively. On the basis of the variation of the maturation parameters, the onset of mature source beds in the onshore and offshore delta were delimited at approximately 3375 and 2900 m respectively. The inferred threshold temperature was about 95/sup 0/C. The results of the present study support the hypothesis that the main source beds of the Niger delta basin are the deeply buried paralic or paralic-marine facies.

A Novel Approach for Recognition and Quantification of Hydrocarbon Migration Effects in Shale-Sandstone Sequences

Abstract: A detailed organic geochemical study of over 150 samples from two cores with a total combined length of 320 m (1,050 ft) through sequences of interbedded source rock-type shales (0.84% Rm maturity) and reservoir sandstones allowed recognition and quantitation of a number of migration effects. Detailed gas chromatography-mass spectrometry of steranes and triterpanes was used to insure that samples being compared to investigate migration effects contain organic matter of a similar type. Thin shales interbedded in sands and the edges of thick shale units are depleted in petroleum-range hydrocarbons to a much higher degree than the centers of thick shale units. For the alkanes, expulsion occurs with pronounced compositional fractionation effects: shorter chain length n-alkanes are expelled preferentially, and isoprenoid alkanes are expelled to a lesser degree than their straight-chain isomers. Based on material balance calculations, expulsion efficiencies were determined and found to be very high in certain instances. For thin interbedded shales, they decrease from about 80% around C15 to near zero in the C25+ region. There is no evidence for significant redistribution of steranes and triterpanes in the two sequences. Compared to C15 to C25 n-alkanes, they appear relatively immobile. The composition of the hydrocarbons impregnating parts of the reservoir sandstones is in agreement with expulsion occurring with pronounced fractionation based on molecular chain length. Hence, consideration of bulk expulsion efficiencies gives an unrealistic picture. Furthermore, the impregnation of a siltstone cap rock from an underlying hydrocarbon accumulation seems to have occurred by bulk-oil migration and without significant fractionation. The degree of hydrocarbon depletion of some of the shales of both sequences appears to be controlled by compaction, and the primary migration process appears to have occurred with chromatographic separation. The migration phenomena observed in both sequences lead us to propose that the main phase of expulsion can be preceded by an earlier sta e, during which the edges of thick shales and thin interbedded shales appear to be slowly and continuously depleted by the chromatographic processes. The composition of the hydrocarbon product accumulating in the reservoirs at this stage appears to be controlled primarily by physical processes rather than by the type and maturity of the organic matter in the generating source rock. By this mechanism, the origin of accumulations of light oils and gas-condensates in low mature sequences bearing predominantly terrestrial-derived organic matter can be explained. Finally, the migration effects documented in this study have some consequences for interpretation of geochemical data (e.g., the pristane/n-C17 ratio, a commonly accepted maturity parameter, has been shown to be also controlled by the degree of hydrocarbon expulsion).

Geochemical Surface Exploration for Hydrocarbons in North Sea

Abstract: Approximately 350 sediment samples were taken in the central part of the North Sea, and hydrocarbon composition, methane (C1) and ethane (C2) yields, and carbon isotope composition of the methane (^dgr13C1) adsorbed in the sediment were determined. Methane generation or oxidation by bacterial activity can not be deduced from the analytical data. This fact is explained by the assumption of a threshold of methane concentration in sediments below which methane oxidizing bacteria are inactive, and that this threshold was not reached in the area investigated. Background gases in the surface sediments, characterized by low C1 yields and ^dgr13C1 > -37 ppt can be distinguished from thermal sediment g ses which are obviously derived from a hydrocarbon generating source rock in the subsurface. Source rock data, obtained from several wells situated on a cross section through the investigated area, were compared to the geochemical patterns of the sediment gases and used to define profile sections with "good" (^dgr13C1 ^approx -40 ppt, yield C1 ^approx 217 ppb) and "poor" source rock properties (^dgr13C1 ^approx -33 ppt, yield C1 ^approx 24 ppb). An assessment of the source rock potential of the whole area under investigation has been carried out using the geochemical data of the surface sediment gases and taking into account that the C1 and C2 yields are generally higher in clay sediments than in sand samples. The number of producing wells was related to the number of all wells (271) in the areas characterized by surface geochemistry as good or poor source rock areas. The comparison showed that the relative number of producing wells is higher in areas geochemically characterized as good. Hydrocarbons in surface sediments of "good" North Sea areas are not generated by bacteria at shallow depth; they are formed thermally from the organic carbon of deep source rocks and migrate to the surface. Therefore they are indicative of thermal hydrocarbon generation in the subsurface.

Graben Hydrocarbon Occurrences and Structural Style

Abstract: Major hydrocarbon occurrences, types of traps, and structural styles have been synthesized from the Sirte basin, the Suez and Viking grabens, and from other normal faulted regions Hydrocarbons occur in a stacked succession of one or more basins: pregraben, graben fill, and interior sag Preservation of pregraben accumulations depends on late initiation of crustal arching and limitation of uplift to the graben shoulders Furthermore, a stable pregraben tectonic environment is required to ensure internal continuity of subsequent fault-block closures Hydrocarbon traps in the pregraben and graben-fill sediments are primarily dependent on the multidirectional orientation of normal faults, tilting of fault blocks, and, in many cases, flexing or erosion parallel with block edges The fault pattern is dominated by longitudinal faults subparallel with the graben axis and, secondarily, by oblique faults The direction and degree of block rotation are influenced by fault profile (planar or listric), degree of extension, fault pattern (doglegs, junctions, terminations), downwarping of the subsequent sag basin, and isostatic adjustments between large blocks Fold closures result from the upward termination of faults into forced folds, which may be accentuated by drag during renewed faulting Passive drape and differential compaction extend he folds to shallower depths Because they form along block edges, the flexures have trends that mimic the multidirectional fault patterns Truncation traps are usually developed from the erosional retreat of fault scarps and they can also mimic the fault patterns Where the sedimentary succession is complete, an interior sag basin is superimposed across the graben Its lower part is structured mostly by drape and differential compaction across the underlying graben fault blocks and fault-block topography The upper part is usually an unsegmented syncline, and hydrocarbon traps at this level include depositional buildups (reefs, turbidite mounds), and salt diapirs or other structures caused by superimposed deformation unrelated to graben tectonics

Determination of Paleoheat Flux from Vitrinite Reflectance Data

Abstract: A method is presented for determining paleoheat flux from the reflectance of vitrinite collected from wells. The maturation of vitrinite is modeled using a time-temperature integral that assumes pseudo-first-order kinetics. In testing the numerical algorithm, it was necessary to introduce a "critical temperature" of 295° K, below which vitrinite does not mature; this result is consistent with the latest findings of independent researchers. The degree of consistency obtained from a number of wells is not perfect; much of the inconsistency can be attributed to large uncertainties in the vitrinite reflectance readings. To test the method, heat-flow history was determined for the North Sea basin and the National Petroleum Reserve of Alaska. Quantitative results were obtained detailing the increase and/or decrease of heat flux with time, and were consistent with the qualitative behavior inferred for paleoheat flow from current geologic models.

Coal quality, coal rank variation and its relation to reconstructed overburden, Upper Cretaceous and Tertiary plains coals, Alberta, Canada

Abstract: Near-surface coals in the Alberta plains occur in Upper Cretaceous and Tertiary sediments, the former represented by the Belly River Group, Horseshoe Canyon Formation, and Wapiti Formation, and the latter by the Scollard Member of the Paskapoo Formation. Statistical techniques were used to document both the distribution of and the interrelationships among the components of proximate and ultimate analysis and calorific value determinations. The coals discussed range in rank from subbituminous C to high-volatile bituminous C. These coals are variable in ash content and low in sulfur content. The relationships among calorific value (dry basis, in kJ/kg) and ash (dry basis, in %) and calorific value (moist mineral-matter-free basis, in kJ/kg) and equilibrium moisture (MEQ, in %) were determined to be CVD = 29,677 - 315.17 ashD, and CVMMMF = 31,816 - 442.05 MEQ, respectively. The maximum depth of coal seam burial (DOB, in m) was reconstructed on the basis of previously published graphs, relating the equilibrium moisture loss to depth of coal seam burial, using the relationship: log10MEQ = 1.865 - 0.000416 DOB. A least-squares regression analysis of calorific value (MMMF basis, in k /kg) on the calculated depth of burial (DOB, in m) yielded the equation, CVMMMF = 14,748 + 6.25 DOB, indicating a coalification gradient, determined on the reconstructed overburden, of 6.25 kJ/kg/m (0.82 Btu/lb/ft) of depth (or overburden). Near-surface coals in the Alberta plains increase in rank toward the west-southwest (toward the foothills and mountains region) because of progressively greater amounts of overburden that existed in that direction during the Tertiary. Erosion has since removed between 900 and 1,900 m (3,000 and 6,200 ft) of sediment, with the greatest amount of removal occurring in a west-southwest direction.

Crustal Profile of Mountain Belt: COCORP Deep Seismic Reflection Profiling in New England Appalachians and Implications for Architecture of Convergent Mountain Chains

Abstract: Relatively shallow reflections from beneath the Taconic Allochthon and Green Mountain anticlinorium are suggested to mark a zone of detachment that carries Precambrian Grenville basement of the Green Mountains in its upper plate. The data suggest that tectonic thickening of the shelf sedimentary and synorogenic flysch sequence beneath the Taconic Allochthon has occurred, and that major thrust-fault systems (Champlain thrust of western Vermont), which imbricate shelf lithologies, project downplunge into the seismic profile. Eastward-dipping reflections beneath the east flank of the Green Mountains appear to project across the Connecticut River valley into New Hampshire. Because some of these reflections can be projected into an exposed zone of probable thrust-imbricated Precambrian Grenville basement and younger metasediments, there is a strong implication that similar rock types exist in the deeper crust beneath New Hampshire. The gross geometry of the eastward-dipping zone is a large thrust ramp that facilitated faulting of continental basement in the Green Mountains to higher structural levels, thus suggesting the anticlinorium is a large hanging-wall structure or ramp anticlinorium. Highly deformed satellitic massifs of Grenville basement, such as the Chester dome, and sub-Silurian and Silurian-Devonian rocks of the Bronson Hill Anticlinorium and Merrimack Synclinorium are probably transported along intracrustal detachments as well. Data suggest the possiblity of a tectonically buried continental margin transition zone, much like that interpreted for the southern Appalachians. This zone would have been preserved from prior rifting of the proto-Atlantic ocean and its inherited geometry could have controlled subsequent convergent deformation during Appalachian orogenesis. Although regional deep seismic reflection profiling has not been carried out across the Alps, existing data imply lateral changes in crustal reflection character similar to those observed in the Appalachians. Deep seismic reflection data from convergent mountain belts raises the possibility of such preserved transition zones beneath transported rocks, and provides an alternative interpretation to lithosphere-penetrating "sutures" in explaining lateral geologic and geophysical contrasts in the crust.--Modified journal abstract.

Regional variations of porosity and cement; St. Peter and Mount Simon sandstones in Illinois Basin

Abstract: Petrographic study of over 230 thin sections of cuttings, cores, and outcrop samples from the Ordovician St. Peter and Cambrian Mount Simon Sandstones of the craton-center Illinois basin yielded a series of maps that show a fairly regular distribution of primary and secondary porosity and cements. Both primary and secondary porosity are present in the St. Peter Sandstone. Primary porosity is dominant from outcrop to 4,000 ft (1,219 m), whereas secondary porosity is dominant at depths greater than 4,000 ft. The overall porosity decline with burial is best described by an exponential equation: ^phgr = 30.8 exp (-0.00032 d), where ^phgr equals porosity and d is depth in feet. However, two linear equations provide the best fit of primary (above 4,000 ft, 1,219 m) and secondary porosity (below 4,000 ft). The chronology of porosity development appears to be closely related to the models proposed by P. J. C. Nagtegaal and includes early cementation, leaching of cement and framework, stabilized framework, and framework collapse. Cements in the St. Peter are depth-dependent. They define distinct regions, are mappable, and include calcite, dolomite, silica overgrowths, chert, and chalcedony. Virtually everywhere in the Mount Simon Sandstone, porosity is secondary and results from dissolution of authigenic and replacement cements, framework grains, and fractures. In the subsurface, porosity reaches a maximum of 18% at 5,000 ft (1,524 m) and drops rather sharply to 8% below 5,000 ft. As with the St. Peter Sandstone, the overall decline is best described by an exponential equation of the form: ^phgr = 31.08 exp (-0.00026 d). Cements in the Mount Simon include quartz and feldspar overgrowths, hematite and kaolinite, carbonate, chlorite, and microquartz (chert). Basinwide maps show that they have a fairly regular distribution pattern in the basin. We suggest that in the future, basinwide maps of cement and porosity types will be abundant. Such maps should enhance understanding of diagenesis, especially where map pattern can be related to fluid and burial history. Maps of cements and porosity types can also aid the calibration of wireline logs, help find diagenetic traps, and help the design of drilling fluid systems and well-completion procedures. The routine use of cuttings rather than cores will hasten the advent of such maps.

Temperatures and Depth-Dependent Heat Flow in Western North Sea

Abstract: Bottom-hole temperatures and lithologic information from 167 released wells between 52° and 58°N in the British sector of the North Sea have been studied and measurements of thermal conductivity have been conducted on many samples from all the main stratigraphic units. Although large errors may be associated with the temperature information, it is sufficiently reliable to establish that heat flow is not conserved with depth over much of the western North Sea. In the Central graben and Anglo-Dutch basin, heat flow in the upper 1-2 km (3,300-6,600 ft) is around 50 mW m-2 increasing downward to about 100 mW m-2. Farther west, on the Mid-North Sea high and on the East Midlands shelf, the situation is reversed with shallow heat flows of about 80 mW m-2, decreasing downward to about 50 mW m-2. These variations are not readily explained either by heat-flow refraction or by tectonic activity. They are most probably attributable to the deep and continuing circulation of water. The circulation may well be on a very large scale and must be largely fault controlled.

Thermal History of Michigan Basin

Abstract: The elevated organic maturity observed in shallowly buried units from the Michigan basin implies that higher temperatures and thicker overburdens once existed in the basin. Evidence from sediment-accumulation rates, regional dips, and maturity of Pennsylvanian-age coals suggests that up to 1,000 m (3,280 ft) of sediment were removed by erosion prior to the Late Jurassic, when the basin became stable. Geothermal gradients during Paleozoic basin subsidence probably ranged from 35° to 45°C/km (19° to 25°F/1,000 ft), in contrast to the average present value of 25°C/km (14°F/1,000 ft). Depths to the top of the oil window ranged from 1,900 to 2,300 m (6,230 to 7,540 ft) during the Paleozoic. Post-Pennsylvanian erosional uplift and further thermal m turation of the basin have combined to raise the top of the oil window to its present level of 500 m (1,640 ft).

Middle Cretaceous Carbonate Reservoirs, Fahud Field and Northwestern Oman

Abstract: Reservoir facies in Fahud field and throughout northwestern Oman are in shallow-shelf carbonates of the middle Cretaceous Mishrif and Mauddud Formations. Interparticle porosity formed in the Mishrif as sand aprons of lithoclast and skeletal grainstones surrounding fault-block islands, and less commonly in the Mauddud as biostromes of rudist packstones. Moldic porosity after fine rudist debris is more common than interparticle porosity and occurs in thicker stratigraphic units, interpreted to have formed locally in meteoric-water lenses associated with islands, and regionally during subaerial exposure associated with sea level lows.

Evolution of Oil-Generative Window and Oil and Gas Occurrence in Tertiary Niger Delta Basin

Abstract: Assuming a simple model of delta development involving progradation and uniform burial at 500 m/m.y. (1,640 ft/m.y.) to present depths, oil-genesis nomographs derived from the time-temperature index (TTI) method were constructed for geothermal gradients ranging from 2.2° to 5.1°C/100 m (1.2°-2.8°F/100 ft) of the Niger delta and used in mapping the positions (depth, temperature) of the top of the oil-generative window (OGW) at various times between 40 m.y.B.P. and the present. During the active subsidence phase, oil generation within any megasedimentary unit was initiated at a temperature of 140°-146°C (284°-294.8°F) and depth of 3,000-5,200 m (9,843-17,060 ft) within 7-11 m.y. after deposition of the potential source rocks. After cessation of subsidence, vertical upward movement of the OGW by 800-1,600 m (2,625-5,249 ft) was accompanied by a temperature lowering of 23°-54°C (41°-97°F), producing correspondingly heavier crudes. In a central belt of the delta, hydrocarbon generation and expulsion from the lower part of the Agbada Formation predate the cessation of subsidence and structural deformation, whereas, in other areas, it postdates the cessation of subsidence and structural deformation. In this central belt, the Agbada is the major oil source, with the Akata serving as a gas source. In the other areas, both the Agbada and Akata constitute oil sources, which implies that the thermal conditions rather than the kerogen type influence the oil/gas mix in the Niger delta basin.

Petroleum Source-Rock Potential and Crude-Oil Correlation in the Great Basin

Abstract: Petroleum source beds in the Great Basin region include marine Paleozoic rocks and nonmarine upper Mesozoic and lower Cenozoic rocks. Many inactive and potential source beds have been identified in continental-rise deposits of the Ordovician Vinini and Devonian Woodruff formations in the eastern part of the Roberts Mountains allochthon (Antler orogen), in central and north-central Nevada; in flysch-trough and prodelta-basin deposits of the Mississippian Chainman Shale and equivalent rocks of the Antler foreland basin, in Nevada and western Utah: and in lake-basin deposits of the Cretaceous Newark Canyon Formation and the Paleogene Sheep Pass and Elk0 formations and equivalent rocks, in cenfral and eastern Nevada. All of the oil fields in the Great Basin are located within NeogeneQuaternary basins that formed during Neogene “Basin-Range” block faulting. Most of the oil shows and reservoir crude oils analyzed can be correlated with Mississippian or Paleogene source rocks. Regional petroleum sourcerock evaluation and crude-oillsource-rock correlation indicate that conditions necessary for crude-oil accumulation are present locally throughout this region. The Mississippian Chainman Shale is confirmed as the major petroleum source rock, having generated the oil in the Trap Spring, Bacon Flat, and Grant Canyon fields in Railroad Valley and the Blackburn field in Pine Valley. The Paleogene Sheep Pass Formation is the source of the oil in fhe Eagle Springs field and probably the Currant field in Railroad Valley. Oil occurrences in the northern Great Basin are believed to be derived from two or more other Terfiary lacustrine sequences. Geological and geochemical evidence indicate a much broader range of source-rocklreservoir-rock target combinations than would be pursued under a Tertiary sourcelreservoir exploration model. Regional evaluation indicates that the Chainman Shale, and possibly other Paleozoic units, may be effective source rocks for oilaccumulation in other valleys in the Great Basin.

A Method for Estimating Subsurface Fracture Density in Core

Abstract: Quantitative information on fracture density may be used to delineate areas, structures, or lithologies that have a relatively high density of subsurface fractures. A quantitative estimate of subsurface fracture density can be obtained from core data. Previously, no unequivocal technique existed for estimating the degree of subsurface fracturing from oil well core data. The spacing of joints at outcrops is commonly observed to be a direct, linear function of the thickness of the jointed bed. Our method for estimating subsurface fracture density is a statistical estimate based on this empirical relation. The probability of a given number of core-fracture intersections is determined for a range of possible fracture densities, after accounting for core diameter variations, bed thickness, geometric corrections necessitated by bedding dip, and variations in fracture pitch. The true fracture density is then estimated by finding the fracture density value that yields a calculated number of fractured beds equal to the observed number of fractured beds. The technique was tested by measuring fracture density in the field at 2 outcrops. Hypothetical cores were constructed on photographs of the outcrops, and estimates of fracture density from the core data yielded values in agreement with those obtained by field measurement. The core data required for estimation of fracture density are core diameter, bed thickness, number of beds (fractured and unfractured), and the angles between core axis, bedding, and fractures. The mathematics should be programmed on a computer for ease of use.

Subtidal stromatolites in Monterey Formation and other organic-rich rocks as suggested source contributors to petroleum formation

Abstract: The Miocene Monterey Formation contains thin organic-rich laminations that mimic benthic mats forming today in coastal upwelling regimes of the eastern Pacific. Modern mats are comprised dominantly of filamentous sulfur-oxidizing bacteria of the family Beggiatoaceae. The characteristic spongy texture imparted to the sediments by these bacterial mats is most readily recognized using scanning electron microscopy (SEM). The spongy network becomes progressively compressed with increasing compaction and diagenesis, but is still distinguishable. Rock-Eval pyrolysis of modern mats reveals high S1 peaks and unusually high oxygen indices. Both parameters decrease gradually with diagenesis, as evidenced by mat-laminated Monterey Formation samples of different thermal grades. In addition, anomalously light nitrogen isotopic signatures of modern mats may be retained. The ^dgrN15 values of mat-laminated Monterey Formation samples, although diagenetically enriched in 15N, are still lighter than would be expected for algally dominated organic matter in marine low-oxygen environments. On the basis of organic richness and high hydrogen indices of mat-laminated samples, and coincidence of mat-laminated rocks and oil in some fields in the San Joaquin Valley of California, the organic-matter assemblage contained within filamentous bacterial mats is suggested to be a significant contributor to petroleum formation. Additional evidence for this conclusion is provided by similar investigations of subtidal cyanobacterial mats in carbonate rocks such as the Green River Formation oil shales.

Geologic Evolution of Petroliferous Basins on Continental Shelf of China

Abstract: The coastline of southeastern China is about 18,000 km (11,200 mi) in length, and its aggregate continental shelf area within 200-m (660-ft) water depth is well over 1 million km2 (390,000 mi2). Recent geophysical exploration and petroleum drilling records aid in understanding the geologic evolution of these petroliferous basins. Two types of tectonic basins are present on the continental shelf areas: (1) Bohai Gulf, South Yellow Sea, and Beibu Gulf are intraplate polyphase rift-depression basins, and (2) East China Sea, mouth of the Pearl River, and the Yingge Sea are epicontinental rift-depressions basins. Both types are believed to be of extensional origin. The severe convergence of the Indian plate with the Eurasia plate produced east-northeast-s reading of the South China Sea basin, which resulted in two triple junctions on its northern margins. The Pacific plate was subducted by downthrust beneath the Eurasia continental crust. The extension mechanism could be the rising of an upper mantle plume to produce two weak north-northeast-trending fracture zones. A series of intraplate and epicontinental rift-depression basins was formed. The depositional models and sea level variations of these basins have been interpreted from drilling records and seismic profiles. They can be explained by the tectonoeustatic changes in sea level and Cenozoic climatic changes in China.