Oil Source Beds Research Articles

Lithology and geochemical characterization of low-permeability shale strata: A-case-study of Khadum Formation in Ciscaucasia

According to classical concepts, the primary function of oil and gas source beds is generation of hydrocarbons. The oil and gas source beds in favorable conditions generate oil and gas, while oil-andgas-bearing reservoirs accumulate hydrocarbons and donate them in the course of mining. This research aimed to prove forecast criteria for the occurrence of hydrocarbons in oil and gas source beds with a view to enhancing efficiency of prospect and exploration of adsorbed oil in terrigenous deposits or matrix oil in carbonate deposits. In terms of Khadum Formation in Ciscaucasia, the authors examine productive strata which are both the source and reservoir of hydrocarbons. The research findings on the organic porosity, as well as the analyses of adsorbed oil and matrix oil in terrigenous and carbonate deposits can help essentially modify the oil and gas exploration strategies. The obtained results prove the scientific idea that oil and gas source beds can generate, accumulate and cluster hydrocarbons, which calls for the criteria for oil exploration in oil and gas source beds. The results of this research can be used for the analysis and assessment of the hydrocarbon stock potential, as well as for the exploration and recovery of adsorbed and matrix oil in terrigenous and carbonate deposits.

Geochemistry of organic matter of the Neoproterozoic strata within the Berezovka depression (Seralakh Formation, Siberian Platform)

The results of study of geochemistry of the Neoproterozoic and Lower Cambrian deposits within the Berezovka depression are presented. A mudstone member of the Seralakh Formation is assumed to be an oil source bed. We compiled a series of geochemical maps for this formation and estimated the hydrocarbon generation potential and the scales of oil and gas formation in its mudstone member, using the volumetric-genetic method. The migrated liquid hydrocarbons amount to about 8 billion tons.

Study of the organic matter in the rocks of the crystalline basement and weathering mantle of Tatarstan

This paper is dedicated to study of the rocks of crystalline basement and weathering mantle taken from several parametric boreholes at the territory of Tatarstan Republic. Optical-microscopy in combination with methods of organic geochemistry, including isotope analysis, revealed a migration nature of the organic matter of the studied rocks from oil-source beds of the Devonian domanikoid-facies deposits.

Oil and gas accumulations in the Ordovician carbonates in the Tazhong Uplift of Tarim Basin, west China

Oil and gas fields of the Paleozoic cratonic basin in Tarim are mainly distributed in the Tabei and the Tazhong Uplifts. In the Tazhong Uplift, there are three sets of oil and gas stratum systems, of which the Ordovician traps are the largest in size and close to oil source beds with the most favorable conditions to form large-scale oil and gas fields. The Ordovician sediments in the Tazhong Uplift have experienced a stable platform and tableland margin development period. The Ordovician is divided into mudstone interval, interbedded mudstone and limestone interval, limestone interval, limestone and dolomite transitional interval and dolomite interval. Of these intervals, both interbedded mudstone and limestone interval and limestone interval contain source rocks, while limestone interval and dolomite interval are reservoir beds. Fault activities and denudation have an obvious impact on carbonate rock reservoirs. The major Ordovician trap types of the Tazhong Uplift include compressional fault block, fault anticline and buried hill, and regionally. The Tazhong Uplift can be generally divided into central fault horst trap belt, northern slope trap belt and southern slope trap belt. In view of the in-situ oil source in the Tazhong, oil and gas migration in the Tazhong Uplift is characterized by a short distance migration through sand bodies and unconformity surfaces laterally and along faults vertically. Based on this study, it is concluded that the favorable oil and gas accumulation zone should be the northern slope and the northwestern pitchout end of the uplift because these areas are located in the hydrocarbon migration pathway with weak tectonic activities and favorable preservation conditions.

Organic geochemical studies of some early cretaceous sediments, Abu Gharadig Basin, Western Desert, Egypt

A total of 81 shale samples from the Early Cretaceous sediments of seven wells in the Abu Gharadig Basin, were analysed for organic matter quantity, type and maturation. The total organic carbon content of analysed samples ranges between 0.5 and 2.67 wt%, indicating a fair to good organic content. The location of investigated samples on the van Krevelen diagram reflects the presence of kerogens type IV (without hydrocarbon potential), type III (gas-prone) and type II–III (oil and gas-prone). The estimated temperature of maximum pyrolytic hydrocarbon generation of analysed shales ranges between 426 and 447°C, indicating immature to thermally mature organic matter. The shale beds of Kharita Member from the northcentral part of the study area, at depths more than 2164 m, represent gas-prone source beds, while those of Alam El-Bueib Member, at depths more than 2545 m, can be considered potentially as active oil and gas-prone source beds. The present study reveals that the Early Cretaceous Alam El-Bueib Member is more enriched in thermally mature type III and II–III organic-rich shale beds, and consequently can be regarded as the potential source for the oil and gas accumulations in the carbonate reservoirs of the Alamein Member and sandstone reservoirs of the Kharita Member in the Abu El-Gharadig Basin.

Petroleum Geochemistry of the Zala Basin, Hungary

The Zala basin is a subbasin within the Pannonian basin of Hungary. Oil and smaller amounts of gas are produced from Upper Triassic through Miocene reservoirs. Our geochemical study of oils and rocks in the basin indicate that two, and possibly three, genetic oil types are present in the basin. Miocene source rocks, previously believed by explorationists to be the predominant source rock, have expelled minor amounts of hydrocarbons. The main source rock is the Upper Triassic (Rhaetian) Kossen Marl Formation or its stratigraphic equivalent. Oils derived from the Triassic source rock are recognizable by their isotopic and biological marker composition, and high content of metals. In other areas of Europe, Upper Triassic source rocks have been correlated with large oil accum lations (e.g., Molassa and Villafortuna fields, Po basin, and other fields in Italy) or are postulated to be good potential source rocks (e.g., Bristol Channel Trough). Knowledge of the geochemical characteristics of oils derived from these Upper Triassic source rocks and understanding of the source rock distribution and maturation history are important for recognizing Triassic oil-source bed relationships and for further exploration in other basins in Hungary and other parts of Europe where Triassic source rocks are present.

Oil and Gas Basins in the Former Soviet Union

The Pripyat basin is a Late Devonian rift characterized by a typical fault-block structure. Two synrift salt formations separate the Devonian stratigraphic succession into the subsalt, intersalt, and postsalt sections. Oil is produced from carbonate reservoirs of the subsalt and intersalt sections. Traps are controlled by crests of tilted fault blocks. We analyzed 276 shale and carbonate-rock samples and 21 oils to determine oil-source bed relationships in the basin. Maturities of the oils are from very immature, heavy (9[degrees] API), to very mature, light (42[degrees] API). All fields are in a narrow band on the north side of the basin, and only shows of immature, heavy oil have been obtained from the rest of the basin. Three genetic oil types are identified. Oil type A has high pristane/phytane ratios (>1.0), high amounts of C[sub 29] 18[alpha] (H) trisnorneohopane, and [delta]13C of hydrocarbons in the range of -31 to -27%. Oil types B and C contain very high amounts of gammacerane, which suggests that the oils were derived from carbonate-evaporite source facies. Type B oils are isotopically similar to type A, whereas type C oils are isotopically light (about -33%). Organic carbon content is as much as 5%, and kerogen typesmore » range from I to IV. Our data indicate that rocks within the intersalt carbonate formation are the source of the type B oils of low maturity. Thermally mature rocks that might be the source for the mature oils have not been found. Such rocks may occur in depressions adjacent to tilted fault blocks. Higher levels of thermal maturity on the north part of the basin in the vicinity of the most mature oils may be related to higher heat flow during and soon after rifting or to a suspected recently formed magmatic body in the crust below the northern zone. Present-day high temperatures in parts of the northern zone may support the latter alternative.« less

Sedimentology and carbon-sulphur geochemistry of the Velkerri Formation, a mid-Proterozoic potential oil source in northern Australia

Abstract The Velkerri Formation is a flat-lying, 400 m thick shale unit of mid-Proterozoic age that occurs in the McArthur Basin of northern Australia. It underlies an area approximately equivalent to that of the Netherlands and contains intervals tens of metres thick, highly rich in organic matter. TOC values are commonly 1 to 3% but exceed 5% at three levels: the maximum value measured was 8.7% TOC. The region has not been buried deeper than about 2500 m and the organic matter is mature; two of these intervals represent impressive oil source beds. Jackson et al. reported the discovery of “live” oil from the lower interval, thereby raising considerable interest in the petroleum potential of the sequence. Initial reconnaissance geochemical studies of material from one drillhole indicated deposition in a large lake or barred bay (Donnelly and Crick). Detailed facies analysis, however, indicates deposition in a variety of sub-environments on an epicratonic marine shelf. In most of these sub-environments the depositional interface was beyond the effects of normal wave and tidal current activity. The stratigraphic relationships between the Velkerri Formation and associated wave- and tide-deposited shoreface sandstones of the Bessie Creek and Moroak Sandstones, and the widespread presence of glauconite throughout the Velkerri Formation are incompatible with the lacustrine-barred bay setting suggested by Donnelly and Crick. The results of a subsequent systematic geochemical sampling program of a second drillhole, presented here, provide some support for a marine origin. Although C/S plots (Berner and Raiswell) have been successful in differentiating between marine and freshwater environments in Phanerozoic rocks their use in the Proterozoic should be approached with caution, and preferably only within the constraints of a full consideration of depositional and diagenetic influences on CSFe systematics. Degree of pyritiation of iron (DOP, Raiswell et al.) and C/Fe/S plots (Aller et al.), as used in this study, are additional tools suitable for classifying the depositional environment. Due to the limited regional information it is difficult to classify the Velkerri Formation into one of the published models for anoxic basin sediments. It shows some similarities to several European Mesozoic oil prone beds which developed during major marine transgressions (e.g. Hallam).

Oil source bed recognition and crude oil correlation, Trinidad, West Indies

Potential source rocks capable of generating hydrocarbons in Trinidad were identified using routine geochemical parameters. Tertiary formations have a poor oil source rating due to low organic carbon contents (TOC < 1.0%) and because they contain immature, gas prone type III kerogens. Oil prone organic facies, characterised by relatively high TOCs (2.2% average) and amorphous marine type II kerogens, in the Upper Cretaceous Naparima Hill and Gautier Formations, suggest good to excellent oil source potential. Produced oils were classified into genetic types on the basis of gas chromatography and gas chromatography-mass spectrometry fingerprinting. Only one genetic oil type, sourced from predominantly marine organic matter, was recognised. Biodegradation is evident in several oils analysed. Chromatographic characteristics and gross geochemical properties of produced oils correlate best with rock extracts from the Naparima Hill and Gautier Formations, indicating a probable genetic association.

Source Rock Potential of Upper Devonian-Lower Mississippian Formations in Oklahoma and Western Arkansas: ABSTRACT

In central and southern Oklahoma, Upper Devonian-Lower Mississippian strata are rich oil source beds (mean = 7% TOC) at an early stage of generation. The major accumulations of oil in this region have been documented as coming from the Woodford formation. Equivalent-age source rocks in the deep part of the Anadarko basin have matured to or beyond the gas generation stage and significant generation is still occurring within parts of the basin. Around the Ozark uplift, age-equivalent rocks contain less organic matter (mean = 3.5% TOC) and more terrigenous clastic sediment than in central and southern Oklahoma. Both oil- and gas-generating types of kerogen are present and the organic matter has reached the early to main stages of oil generation. Upper Devonian-Lower Mississippian source rocks in the Ouachita core area are metamorphosed and no longer have significant hydrocarbon-generating capability. Age-equivalent rocks in the frontal zone are rich oil source beds at an early stage of generation. Significant hydrocarbon accumulations are possible at depth within the frontal zone and from frontal zone rocks that have been overthrust by metamorphosed Ouachita core-area rocks, particularly in the northwestern part of the Ouachita province.

Recognizing and Quantifying Expulsion of Oil from the Woodford Formation and Age-Equivalent Rocks in Oklahoma and Arkansas

Accumulations of oil in fractures, stylolites, burrows, sandstone lenses, and chert nodules, deeply embedded within organic-rich, mature oil source beds of Late Devonian-Early Mississippian age are considered prima facie evidence of internal migration and expulsion. Most oil migrated internally and was expelled as a separate phase. Within a given section in which chert and black shale are interbedded, contain the same type of organic matter, and have reached the same level of thermal maturity, black shales typically contain less hydrocarbons per weight percent TOC than cherts. The observed differences represent the amount of oil expelled from the black shales and range from 10 to 15 mg/g TOC for total bitumen, 7 to 8 mg/g TOC for saturated hydrocarbons, and 11 mg/g TOC for volatile hydrocarbons. Approximately 27-33% of the oil generated in these source rocks was calculated to have been expelled. In the main oil-producing region of central and southern Oklahoma, 22 billion bbl of bitumen and 16 billion bbl of saturated hydrocarbons were estimated to have been expelled from the Woodford formation. Relatively efficient, separate-phase oil expulsion may be characteristic of very rich oil source rocks like those reported here. Such rocks would reach effective oil saturation and begin to expel oil as a separate phase at a relatively early stage of generation. Timing and efficiency of oil expulsion must then be influenced by the concentration and type of organic matter in the source rock, because these factors determine the volume of oil generated and, hence, the time when a source rock becomes oil saturated.

THE ISOTOPIC METHOD OF IDENTIFYING OIL-SOURCE BEDS APPLIED TO SEVERAL REGIONS OF THE USSR

Recognition of the source beds for the petroleum in a given sedimentary basin is of the highest practical value in an exploration program. Geological and geochemical analysis, which is largely supported by organic geochemistry, is required to forecast petroleum-producing properties in sediments. Methods for determining the type of kerogen, the degree of catagenesis, and the oil-generating potential are well developed. The most difficult step in identifying oil-source beds is to establish direct genetic relationships between the oils in the section and particular source beds. If such correlations can be established, one can solve various problems ranging from the fundamental question of the origin of oil, through those relating to the conditions and facies favorable to oil formation to practical problems in evaluating reserves and efficient surveying. The author demonstrates that carbon isotope ratios can be used reliably to correlate oils with source beds and to identify the original type of organic matter, even where the oil has been affected by secondary processes. 58 references.

Oil Source Bed Distribution in Upper Tertiary of Gulf Coast: ABSTRACT

Effective oil source beds have not been reported in Miocene and younger Gulf Coast sediments and the organic matter present is invariably immature and oxidized. Crude oil composition, however, indicates origin from mature source beds containing reduced kerogen. Oil distribution suggests extensive vertical migration through fracture systems from localized sources in deeply buried, geopressured shales. A model is proposed in which oil source beds were deposited in intraslope basins that formed behind salt ridges. The combination of silled basin topography, rapid sedimentation, and enhanced oxygen-minimum zones during global warmups resulted in periodic anoxic environments and preservation of oil-generating organic matter. Anoxia was most widespread during the middle Miocene and Pliocene transgressions and rare during regressive cycles when anoxia occurred primarily in hypersaline conditions such as exist today in the Orca basin. End_of_Article - Last_Page 251------------

A SOURCE BED STUDY OF THE OLIGO‐MIOCENE ASMARI LIMESTONE IN SW IRAN

Detailed geological and geochemical investigations in the folded belt of the Zagros geosyncline reveal that the source rocks for the very large Asmari oil accumulations may not be the PabdehGurpi and/or Kazhdumi formations as has been suggested; the hydrocarbons are more likely to be indigenous to the Asmari. Most previous investigators have assumed that only organic‐rich marls and shales serve as effective source rocks. However, geochemical analysis shows that the organic content of the Asmari carbonate, although not very rich, compares favourably with the source beds proposed by earlier workers.A rock should not be considered an oil source bed if it does not currently contain at least traces of oil. If the oil had been generated in the Pabdeh‐Gurpi and/or Kazhdumi formations, some hydrocarbons should still be present within these rocks.For all source rocks, there must be a positive relationship between thermal history and oil generation. Consequently, temperature and depth of burial are essential factors in petroleum genesis. The temperature history of the Asmari Formation is related to the thickness of the overlying Teritiary rocks. Maximum burial of the oligo‐Miocene Asmari, throughout most of the folded belt, is assumed to have occured during Early Pliocene time before the last phase of Zagros orogeny.Asmari limestone, therefore, qualifies as a source rock where it is buried deep enough to reach the optimum temperature needed to release its hydrocarbons. Under conditions of lower temprature and shallower burial, the limestones serve only as reservoir rocks.

Oil Source Beds and Oil Prospect Definition in Upper Tertiary of Gulf Coast: ABSTRACT

Data on the origin and migration of oil in the highly productive Gulf Coast Tertiary are sparse. What is known with some certainty is that crude oil (1) is most commonly found in old salt-related structures, (2) probably migrated vertically a considerable distance through fault-associated fracture systems, and (3) most likely originated in deeply buried high-pressure marine-slope shales. All other conclusions are highly speculative. Oil compositions suggest multiple sources with mixed type II and type III kerogens. Although many migration modes have been suggested, most oil movement probably took place as a continuous oil phase or as a solute in a supercritical gas phase. Accumulation occurred most readily in structural closures within or just above the soft geopressure zone where pressure gradients are high and seals are effective. Because of their location in the hard geopressure zone, oil source beds are rarely penetrated, and the lack of adequate samples has made practical application of geochemical data in the Gulf Coast difficult if not impossible. Nearly all published information defines only thermally immature, organic lean, gas-only source beds. Consequently, until appropriate samples can be collected and analyzed, it is important to develop conceptual models based on geologic history and seismic-stratigraphic methods to predict distribution of oil source beds. A model for oil source bed deposition in anoxic, salt-controlled intraslope basins has been developed. Most modern, silled basin analogs, however, are oxic and do not contain oil-generating kerogens. Reduced bottom circulation and resulting anoxia could have existed better during periods of global warmup and high sea level stands that occurred during the Pliocene and middle Miocene. Definition of these oil source bed sites, either directly by geochemical analysis or indirectly through seismic investigations, can greatly enhance our ability to predict oil occurrences and to separate oil from gas prospects. It should also stimulate collaboration among Gulf Coast geologists, geophysicists, geochemists, biostratigraphers, and petroleum engineers, which will result in improved geologic mod ls and exploration successes, quite possibly including the discovery of oil in obscure and subtle traps. End_of_Article - Last_Page 1211------------

Organic Geochemistry of Pennsylvanian-Permian Oils and Black Shales, Northern Denver Basin: ABSTRACT

We performed organic geochemical analyses of Paleozoic shales and oils from the northern Denver basin to determine oil-source bed relationships. We recognized two general oil types: oil produced from reservoirs of Virgilian and Wolfcampian age in northeastern Colorado and Nebraska, and oil produced form the Lower Permian Lyons Sandstone near the basin axis in Colorado. Low-gravity oil (20° API) produced from the Virgilian-age reservoir at the Amazon field (Nebraska) and a higher gravity oil (37° API) produced from a well near the Amazon field (Wexpro 1-23 Lyngholm) can be distinguished geochemically from the other Virgilian-Wolfcampian oils studied and may be genetically unrelated to them. For comparison, we analyzed oils from the Minnelusa Formation (Permian-Pe nsylvanian) in the Powder River basin. These oils are geochemically unlike any Paleozoic oils analyzed in this study in southeastern Wyoming and Colorado. We evaluated shale samples of Desmoinesian, Wolfcampian, and Leonardian age for source rock potential. Shales of Leonardian and Wolfcampian age in southwestern Nebraska contain as much as 6% organic carbon and 4,000-7,000 ppm extractable hydrocarbons. However, the Wolfcampian samples analyzed contain hydrocarbons that are enriched in carbon-13 by 3-5 per mil compared to nearby oil occurrences and are, therefore, improbable source rocks for current oil discoveries. Hydrocarbons extracted from one sample of Leonardian shale are isotopically similar to the Virgilian-Wolfcampian oils. Thermally mature shales of Desmoinesian age in northwestern Nebraska containing as much as 18% organic carbon and about 800-8,000 ppm hydrocarbons are considered excellent potential source rocks. End_of_Article - Last_Page 463------------

Oil source bed hydrocarbon analysis: some methods and interpretations

Oil source bed hydrocarbon analysis: some methods and interpretations

Phosphate-rich sedimentary rocks: Significance for organic facies and petroleum exploration

Abstract Phosphorus-bearing rocks and sediments can be divided into two genetically distinct classes: phosphatic shales or limestones and phosphorites. Phosphatic shales are primary sediments in which phosphate nodules or micronodules have formed diagenetically by precipitation of calcium phosphates derived mainly from organic phosphorus. The nodules form in reducing environments at shallow depths within the sediments, where loss of phosphate by diffusion to the overlying water column is minimized. Highly biogenic sediments containing large amounts of organic matter and some fine clastic debris provide ideal environments for the formation of phosphate nodules. Phosphorites, in contrast, represent concentrated accumulations of reworked phosphate nodules which originated in phosphatic shales or limestones. Currents, wave action, recrystallization, and erosion and resedimentation are important mechanisms in the concentration process. Phosphatic shales and limestones may become excellent oil source rocks if thermal maturity is achieved. They are useful facies indicators for anoxic or nearly anoxic depositional environments, and are often associated with restricted basins, or, during certain geologic periods, with broad shelves developed during transgressions. Phosphorites, in contrast, are often correlated with sea-level regressions or uplifts. They are modest source rocks because of their low organic carbon contents and the fact that they were reworked under oxidizing conditions. Nevertheless, because phosphorites are derived from, and often grade into, phosphatic shales, they also are of potential utility in the search for oil source beds.

Vitrinite Reflectance of Woodford Shale in Anadarko Basin, Oklahoma: ABSTRACT

The Woodford Shale (Upper Devonian-Lower Mississippian) is a black shale thought to be an important oil-source bed in the Anadarko basin of western Oklahoma. The reflectance in immersion oil (Ro) of first-generation vitrinite particles found in this stratigraphic unit is related to temperature history and thus End_Page 591------------------------------ hydrocarbon-generating potential. Samples of the Woodford Shale were obtained from 18 wells drilled in the Anadarko basin by various operators, and polished sections of the shale were prepared and interpreted by the author while working at the Oklahoma Geological Survey. A minimum of 60 vitrinite reflectance measurements were recorded for each well. The Woodford was sampled at depths of from 5,060 ft (1,542 m) in the northeastern shelf to 20,308 ft (6,190 m) in the deepest part of the basin in Beckham County, southwestern Oklahoma. A systematic increase in mean vitrinite reflectance (mean Ro) with depth was observed. From northeast to southwest across the Anadarko basin mean Ro increased from 0.51 to 2.60%. An isoreflectance map for the Woodford Shale in the Anad rko basin was prepared using data collected during this study. The Woodford Shale should have generated commercial quantities of oil in those areas of the basin where the shale has a mean Ro of from 0.60 to 1.35%. In Kiowa County, Oklahoma, the Woodford Shale was sampled in a fault block bordering the Wichita uplift on the southern boundary of the basin. It has, in this well, an anomalously low mean Ro of 0.48%, possibly due to a shallow depth of burial throughout its history. End_of_Article - Last_Page 592------------

Petroleum Source Beds: Environment of Deposition and Stratigraphy: ABSTRACT

Measurement of organic carbon content, alone, is insufficient to identify potential oil source beds because terrestrial OM (Organic Matter), oxidixzed planktonic OM, or reworked OM from a previous sedimentary cycle can create misleadingly high levels of organic carbon in marine sediments. Consequently, the presence of an organic facies, as identified End_Page 2465------------------------------ by kerogen typing, is essential to establish oil source rock character. Kerogen is mainly dependent on the origin of the precursor plant remains (whether planktonic or terrestrial) and on the oxidizing (oxic) or reducing (anoxic) character of the early depositional environment. Oxygen depletion, sufficient to arrest or minimize bioturbation at the benthic boundary, enhances oil source bed deposition, because it leads to almost entirely anaerobic microbial reworking of planktonic remains. This of bacterial reworking favors the preservation and concentration of lipids in the residual OM, leading to the formation of oil-prone koergens (types I and II). Marine oil source bed deposition and occurrence are controlled mainly by factors relevant to qualitative organic matter preservation during early sedimentation, rather than by planktonic productivity in the shallow euphotic zone. Many areas of high planktonic productivity, today, do not correspond with zones of high organic enrichment in bottom sediments (e.g., Grand Banks of Newfoundland, Antarctica, Australia, Northwest Shelf, Northeast Brazilian Shelf) because of oxic conditions, commonly in combination with low sedimentation rates. Conversely, whenever zones of high productivity, such as those present in some coastal upwellings, are underlain by anoxic water layers, then prolific oil source bed deposition does occur. Zones where deep ventilation and thus oxic conditions prevail at sea bottom are much more common than zones of oxygen depletion. Persistent oxic conditions at the benthic boundary lead to deposition of gas-prone, type III, to non-source, type IV organic facies, depending on sedimentation rate and amount of terrestrial organic matter input. Such unfavorable organic facies, resulting from past oxic conditions, have been commonly recognized as stratigraphically widespread under continental margins and in cratonic basins, regardless of past water depth. Thus, prolific oil source beds, in terms of relative rock volumes, are the exception in most sedimentary basins. These observations are compatible with the functioning of the carbon cycle: efficient organic matter recycling through mineralization, rather than enhanced preservation, is the most common and most probable fate of dead organic matter in the environment. Geochemical-sedimentologic evidence suggests that potential oil source beds are and have been deposited in the geologic past in four main anoxic settings as follows: 1. Large anoxic lakes: Permanent stratification promotes development of anoxic bottom water, particularly in large lakes which are not subject to seasonal overturn, such as Lake Tanganyika. Warm equable climatic conditions favor lacustrine anoxia and nonmarine oil source bed deposition. 2. Anoxic silled basins: Only those landlocked silled basins with positive water balance tend to become anoxic. The Baltic and Black Seas are examples. In arid-region seas (Red and Mediterranean Seas), evaporation exceeds river inflow, causing negative water balance and well-oxygenated bottom waters. Silled basins should be prone to oil source bed deposition at times of worldwide transgression, at high and low paleolatitudes. Silled-basin geometry, however, does not automatically imply the presence of oil source beds. 3. Anoxic layers caused by upwelling: These develop only when the oxygen supply in deep water cannot match demand owing to high surface biologic productivity. Examples are the Benguela Current and Peru coastal upwelling. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply is commonly sufficient to match strongest demand. Oil source beds and phosphorites resulting from upwelling are present preferentially at low paleolatitudes and at times of worldwide transgression. 4. Open-ocean anoxic layers: These are present in the oxygen-minimum layers of the northeastern Pacific and northern Indian Oceans, far from deep, oxygenated polar water sources. They are analogous, on a smaller scale, to worldwide anoxic events which occurred at global climatic warmups and major transgressions, as in Late Jurassic and middle Cretaceous times. Known marine oil source bed systems are not randomly distributed in time but tend to coincide with periods of worldwide transgression and oceanic anoxia. Recognition of the proposed anoxic models in ancient sedimentary basins helps in regional stratigraphic mapping of oil shale and oil source beds. Furthermore, explanation and prediction of the most favorable zones for widespread and prolific oil source bed occurrence can be achieved by paleogeographic reconstructions (plate tectonics, paleoclimate, and paleo-oceanography) conducted in conjunction with seismic stratigraphy and regional geochemical studies. End_of_Article - Last_Page 2466------------