Tertiary Oils Research Articles

Geochemical characteristics of Tertiary oils derived from siliceous sources in Japan, Russia and U.S.A

Siliceous sourced Tertiary oils from the Circum-Pacific area of Japan, Russia and the U.S.A. have a heavy carbon isotope composition, monomodal n-alkane distributions, and nearly identical regular sterane compositions with a predominance of C 27 homologues. These are consistent with open marine depositional environments dominated by diatomaceous organic matter. However, a number of alkane and biomarker parameters such as Pr/Ph, CPI, relative concentration of 28,30-bisnorhopane, and the C 35/C 34 homohopane ratio indicate more oxic depositional environments for the source rocks of Japan and Russia. In contrast to the California Monterey Formation sourced oils, petroleums with low maturity levels from the North Sakhalin basin, Russia and the Akita basin, Japan have lower concentrations of asphaltenes and sulphur and are characterized by higher API gravities. A correlation of extractable organic matter from source rocks vs the least matured petroleums demonstrates that oil expulsion in siliceous shales of the Akita basin occurs at a maturity level corresponding to R o≥0.65%, which is in the range of the conventional oil window (R o=0.6–1.1%).

Bicadinanes and related compounds as maturity indicators for oils and sediments

Biomarker maturity indices have proven useful in petroleum exploration to assess the thermal maturity of oils and sediments. However, their application to deltaic petroleum systems is often hindered by source and diagenetic interferences. Recently, we have described several new maturity indices based on the catagenetic products of the plant biopolymer polycadinene. These indices should be largely immune to biochemical effects during diagenesis, since little microbial activity will remain at temperatures sufficient to dissociate the polymer. Here we report changes in five polycadinene-related maturity indices through a marine-fluvio-deltaic depositional sequence in the South Sumatra Basin, Indonesia. The results are compared with the maturity required for oil generation as estimated by kinetic modelling and with conventional maturity indicators such as sterane epimerisation and the methyl phenanthrene index. The data confirm that the polycadinene indices are less subject to source and diagenetic interferences than those based on the sterane and aromatic hydrocarbon distributions. Values for several of the indices have also been measured for 17 Tertiary oils from Southeast Asia, Papua New Guinea, New Zealand and Australia. Contrary to an earlier report, the bicadinane maturity indicator (BMI-1) continues to change into the oil window and hence may be useful in ranking the relative maturity of oils as well as sediments. The BMI-1 values for two oils having unusually low sterane maturities are normal, leading us to question whether the low sterane maturity of many Tertiary-age oils is due to their source rocks having been heated more rapidly than those of older oils.

Sterane and triterpane biomarker characteristics from oils and sediment extracts of the Middle-Upper Miocene sequences, Northern Sabah basin

Biomarker studies using oil samples and sediment extracts from wells located in the northern part of the Sabah Basin show that the organic matter source for the basin's hydrocarbon to be dominantly terrigenous, indicated by high triterpane/sterane ratios, the presence of compounds diagnostic of land­ derived organic matter such as oleanane, and resins W, T and R, and the general predominance of C29 regular steranes over C27 and C28 components. The consistently similar mass fragmentograms of both mJz 191 and mJz 217 suggests that the oils and sediment extracts originated either from the same source rock or, more likely, from different source rocks with similar chemical compositions. The similarity of biomarker distributions in samples from different environments suggests that detrital plant components from similar higher land plant assemblages were redistributed by sedimentary/transport processes to various depositional settings. Petrographic studies of kerogen concentrates confirm the dominance of land-plant contribution and the general absence of marine organic matter even in the marine sediments. For the oils, the 20S/(20S+20R)-sterane biomarker maturity ratios are lower than the equilibrium value of 0.55 and moretane/hopane ratios > 0.10. This supports the apparent retardation of the isomerisation process previously reported by Grantham (1986) to occur in Far Eastern Tertiary oils. The triterpane 22S/(22S+22R) ratio, on the other hand, appears to reflect the expected maturity levels of the oils. The majority of the extracted sediments are shown to be immature by pyrolysis Tmax, vitrinite reflectance and kerogen elemental composition, indicating that the extracted hydrocarbons have migrated into the sediments from elsewhere. Therefore, biomarker maturity ratios from the extracts cannot be used to determine the true maturity levels of the sediments as the in situ biomarkers will have been mixed with those already present in the migrating hydrocarbons. Variations of biomarker maturity ratios with depth resemble those of the Production Index in all the studied wells, thus confirming the influence of non-indigenous residual hydrocarbons on the biomarker maturity ratios.

Biomarker and n-alkane isotope profiles for Tertiary oils: relationship to source rock depositional setting

Biomarker and n-alkane isotope profiles have been measured for 29 Late Cretaceous/Tertiary oils from SE Asia, China, Papua New Guinea, New Zealand and the U.S.A. The results are interpreted with respect to six kinds of source rock depositional setting: fluvio-deltaic (FD), freshwater transitional (TR), lacustrine (LAC), saline lacustrine (SAL), marine deltaic (MD) and marine carbonate (MC). A comparison of biomarker and n-alkane isotope results suggests that parameters such as the oleanane/hopane ratio may overestimate the higher plant contribution to marine oils. The abundance of bicadinanes is much more variable than that of oleanane, probably because of a specific association with the highly resinous, dipterocarp hardwoods of SE Asia. However, traces do occur in an oil from outside the paleogeographic range of the dipterocarp family, confirming that there is also a more general angiosperm source. The bicadinane isomeric distribution is shown to depend on maturity, leading to new maturity parameters which are resistant to even very heavy biodegradation. Retention indices based on the regular hopane series are provided for the main bicadinane and methyl bicadinane isomers and for a group of oleanane-related triterpanes of unknown structure. The presence of the latter compounds may indicate locally deposited as opposed to transported higher plant matter in the source. Source rock depositional setting is the primary control on the shape of the n-alkane isotope profile, with negatively sloping curves being characteristic of FD and TR oils and flat or positively sloping curves typical of marine oils. The difference is probably related to the bacterial reworking of higher plant matter in the FD environment. A slight isotopic anomaly at n-C 17 correlates with the abundance of algal-derived steranes and may indicate a minor marine contribution to the source of an FD oil.

An Integrated Geochemical and Geological Approach for Determining Hydrocarbon Generation-Migration Patterns: Central Gulf Coast Basin: ABSTRACT

ABSTRACT New geochemical, geological, and well-production data indicate a Mesozoic hydrocarbon source for many younger reservoirs in central Louisiana and southwest Mississippi. A Jurassic source has been determined for several Cretaceous reservoirs in oil and gas fields of northeast Louisiana. The characteristic Wilcox, Sparta, and Frio biomarker, Bisnorhopane, identified in Austin Chalk oils from southwest Mississippi, links these Mesozoic and Tertiary oils. Cross-plots of 13CAROM versus 13CSAT indicate an Upper Jurassic (Smackover) and Lower Cretaceous source for certain Upper Cretaceous and Tertiary oils. However, the pristane/nC17 ratio versus 13CSAT cross-plot indicates Wilcox and Sparta oils are heavily biodegraded Mesozoic (Lower Cretaceous) oils. There is an absence of gas accumulations in central Louisiana and southwest Mississippi in Midway, Wilcox, and Sparta reservoirs updip from coeval gas-prone source rocks. This is an indication of (1) a sealing regional fault system and/or stratigraphic migration barrier paralleling the Paleocene-Eocene shelf edge or (2) an earlier selective escape via vertical migration pathways. Vertical migration of hydrocarbons in northeast Louisiana, central Louisiana, and southwest Mississippi was accomplished primarily through fracture systems caused by wrench faulting; to the south and west, salt tectonics and related normal faulting played significant roles in the process. In onshore and offshore south Louisiana, normal faulting and salt-related tectonics created primary migration pathways. Regional hydrocarbon generation-migration patterns differ conceptually from those illustrated in previous models employing long-range migration based primarily on geochemical considerations. This new interpretation integrates geochemical, geological, and production data useful in identifying new exploration plays in north and central Louisiana and in southwest Mississippi.

Preliminary investigation of oil and source rock organic geochemistry from selected Tertiary basins of Thailand

Selected samples of crude oils and extracts from source rocks obtained from six Thailand Tertiary basins of the central plain and of the Gulf of Thailand regions were examined for geochemical properties and molecular compositions. Analyses were performed using GC, CGCMS and carbon isotope mass spectrometry. Though these results should be viewed as preliminary, the results are significant in terms of a regional understanding of the petroleum geochemistry of Thailand. Results from bulk geochemical properties and biomarker assemblages characterize derivatives of organic sources deposited in lacustrine environments. The organic matter is mainly derived from algae with varying amounts of higher plant material. However, an observed variation in the pristane/phytane ratios among the samples may imply differences in depositional oxicity. On the other hand, basinal differences in sedimentation rates, or in the oxygen concentration of the varying waters and/or sediment pore-waters resulted in spatial heterogeneities in the quantity and degree of preservation of the organic matter. In addition, a degree of physical separation between these paleo-lacustrine environments is indicated by differences in paleosalinity, e.g. the hypersaline biomarker, gammacerane, which is restricted to samples from the offshore Gulf of Thailand basins. Maturity parameters for these Tertiary oils and source rock extracts were determined using biomarker analyses of T s/T m, 22S/22S + 22R C 31 hopane, C 30 moretane/hopane, 20R/20S + 20R C 29 sterane, and aromatic compounds. Though the samples demonstrate an overall relatively low level of maturity as specified by the biomarker index, a degree of individual basinal variability is also distinguishable. The observed differences in the maturity values indicate regional heterogeneity among the basin thermal histories, suggesting differences in geothermal gradients and/or in the basin subsidence rates.

Secular and environmental constraints on the occurrence of dinosterane in sediments

The distribution patterns of sedimentary A-ring methylated steranes have changed markedly over geological time. Although dinosterane and its isomer 24-ethyl-4α-methylcholestane have been tentatively identified in three Proterozoic rock units, they are either not detectable or occur in low abundance relative to 3-methyl steranes throughout most of the Palaeozoic. Between Permian and middle Triassic times (260-220 Ma ago), 4-methyl sterane abundances in marine sediments increased markedly. The presence of dinosterane in some middle Triassic marine sediments is contemporaneous with the appearance of fossil cysts of uncontested dinoflagellate affinity. 4-Methyl steranes, including dinosterane or their precursor sterenes and sterols, then show a continuous presence, often in high abundance, in marine sediments from the late Triassic through to the present day. Assemblages of 4-methyl steranes and their precursors, but with dinosterane absent or in low relative abundance, are often the predominant steroids in lacustrine sediments in the Cainozoic. Dinosterane appears to arise predominantly from marine dinoflagellates and, as a consequence, is a useful biological marker for Mesozoic and Cainozoic marine organic matter. The isomer 24-ethyl-4α-methylcholestane is likely to have multiple origins although its very high abundance in Tertiary lacustrine sediments and oils, compared to older materials, suggests that dinoflagellates could also be the source in these cases.

Investigation of the terrestrial occurrence and biological source of the petroleum geochemical biomarker oleanane

Oleanane is a triterpane that is commonly found in Late Cretaceous (Campanian) through Cenozoic marine and deltaic rocks and related oils. Based on its affinity to the β-Amyrin group of natural products and its abundance in Tertiary deltaic sediments and oils, such as in the Niger Delta, oleanane is thought to be a geochemical fossil of terrestrial flowering plants. The β-Amyrin group forms the basis of many angiosperm triterpenoids and triterpenoid saponins. These compounds appear in moderately advanced flowering plant lineages and are often used as a defense against herbivores. This group of compounds has not been reported from any other seed plant group, including conifers.We examined a suite of middle to early Cretaceous terrestrial sediments and plant fossils for the presence of oleanane. The existence of oleanane was documented by Gas Chromatography - Mass Spectrometry (GC-MS) and confirmed with Metastable Reaction Monitoring (MRM) GC-MS.Oleanane was identified from fossil bennettitaleans, an extinct group of seed plants. Cycadeoidea paynei and C. superba are permineralized fossil stems and are from the Early Cretaceous Inyan Kara Group of the Black Hills, SD. Oleanane was also found in carbonaceous compressed leaf fossils of Dioonites from the Barremian - early Aptian (Zone I) of the Potomac Group from Richmond, VA. This increases the number of seed plant groups known to have oleanane, and is of additional interest as the bennettitaleans are thought to be closely related to angiosperms.Oleanane was not found in contemporaneous fossil conifers. Our sample consisted of fossil conifer wood collected from the Campanian Magothy Beds of Martha's Vineyard, MA; the Cenomanian Raritan Formation of Sayreville, NJ; and the Barremian - early Aptian Patuxent Formation of Greenbelt, MD.Confirmed reports of oleanane have been from Late Cretaceous and younger marine sediments. We now report oleanane from Early Cretaceous terrestrial strata of the Inyan Kara Group. In addition we systematically sampled the sequence of Cretaceous terrestrial sediments of the coastal plain of the eastern U.S.A, and found oleanane in the Barremian - early Aptian (Zone I) Potomac Group. Preliminary data indicate variation in the occurrence of oleanane in contemporaneous Early Cretaceous sites from the Potomac Group. The localities accumulated in different depositional environments and the occurrence may be controlled by lithofacies.

Triterpanes and ring-A degraded triterpanes as biomarkers characteristic of Tertiary oils derived from predominantly higher plant sources

A number of oils, previously proposed to be derived from terrigenous source materials, have been obtained from the Taranaki Basin, New Zealand; Niger Delta; Beaufort-Mackenzie region; Taiwan and Indonesia and their C 24 tetracyclic and C 30 pentacyclic terpenoid distributions examined by GC-MS and GC-MS/MS. The C 24-tetracyclic terpanes are thought to be derived from their corresponding C 30 hopane, oleanane, ursane and lupane counterparts as previously reported. A mixture of at least nine C 30 pentacyclic terpanes are found to be present with remarkably uniform distributions in oils from these widely separated provinces. Whilst definitive structures have not been proposed for these compounds, information has been obtained as to whether these pentacyclic terpanes are all six-membered rings, or contain one or two five-membered rings. The presence of this mixture of tetracyclic and pentacyclic terpanes provides a good indicator for oils derived from terrigenous source materials and, in the case of these basins, of Tertiary age.

Maturation sequence of Tertiary crude oils in the Qaidam Basin and its significance in petroleum resource assessment

This paper deals systematically with the geochemical analysis of 54 samples of Tertiary source rocks and crude oils in the western part of the Qaidam Basin. Taking the isomerization degree of C 29 sterane biomarker as the indicator of maturity, a complete evolution sequence of crude oils from immature, low mature, mature, to high mature stages corresponding to the transformation products of organic matters in different evolutional periods, has been found in the basin. It can be seen from the data presented that the widespread existence of immature petroleum and crude oil maturation sequences in the Tertiary terrestrial sedimentary basins in China is an important geochemical characteristic, a result which goes against the limitations imposed by the theory that hydrocarbons are formed from kerogens. This new concept has practical significance in petroleum exploration and petroleum resource evaluation by maturity grading. Our study shows that most of the oils discovered recently in the Qaidam Basin are low matured. However, the prospects for oil discoveries of other hydrocarbon-forming stages, particularly mature oils at greater depth, are bright.

Lower Tertiary and Upper Cretaceous Source Rocks in Louisiana and Mississippi: Implications to Gulf of Mexico Crude Oil (1)

The Lower Tertiary Sparta Formation, Wilcox Group, and the Midway Group in southern Louisiana include organic-rich source facies that generate crude oil at relatively high thermal maturities. The Wilcox Group is an important source of Wilcox crude oil, and regional kerogen variations explain two crude oil subfamilies. Wilcox crude oils in downdip areas of southern Louisiana migrated short distances, but long-range lateral migration (about 150 km) best explains Wilcox crude oils far updip from mature source rocks. Crude oils in Oligocene and younger reservoirs in southern Louisiana migrated vertically from deep lower Tertiary source rocks. Some crude oils in Upper Cretaceous Tuscaloosa reservoirs were emplaced by long-range lateral migration from Tuscaloosa source rocks. G ven little evidence of upper Tertiary source rocks and the overmaturity problems of Mesozoic source rocks, most crude oils in upper Tertiary and Pleistocene reservoirs of the Gulf continental shelf are best explained by vertical migration from deep lower Tertiary source rocks. Even so, it is simplistic to assume an exclusive lower Tertiary origin. Many Tertiary and Pleistocene crude oils of this study probably include an overprint of high-maturity hydrocarbons from Mesozoic sources.

Oil-Source Rock Correlation Using Carbon Isotope Data and Biological Marker Compounds, Cook Inlet-Alaska Peninsula, Alaska: ABSTRACT

Rock and oil samples from the Cook Inlet-Alaska Peninsula area were analyzed to determine the source of the commercial hydrocarbons produced in the Cook Inlet basin from lower Tertiary nonmarine sandstone reservoirs. Rock-Eval (hydrogen index) analysis and organic carbon content were used to identify the most favorable rock samples for solvent extraction and carbon isotope, gas-chromatographic (GC), and gas-chromatrographic/mass-spectrometric (GCMS) analyses. On the basis of organic-matter richness, five nonmarine Tertiary coal and shale samples and 12 marine Mesozoic (Upper Triassic and Middle Jurassic) shale samples were selected. A total of 28 oil and condensate samples from producing wells, oil-stem tests, field separators, and seeps were used for oil-oil and oil-source rock correlation. On the basis of biomarker and carbon isotope data, four of the shallower oils and condensates are from nonmarine source rocks, and 24 of the deeper oils are sourced from marine shales. Geochemical and regional geologic considerations indicate the following conclusions. The upper Tertiary nonmarine oils and condensates associated with commercial microbial gas accumulations are geochemically similar to the immature organic matter in the Tertiary nonmarine rocks. In the upper Cook Inlet, marine oils in lower Tertiary nonmarine reservoirs originated from Middle Jurassic rocks that matured during themore » Pliocene to Holocene; in the lower Cook Inlet-Alaska Peninsula area, oils migrated from both Upper Triassic and Middle Jurassic source rocks during the Late Cretaceous to early Tertiary. Although three petroleum systems are identified, this study on the petroleum potential in a convergent-margin setting indicates that only one of these three systems was responsible for the 1.2 billion bbl of recoverable oil in the lower Tertiary nonmarine reservoirs.« less

Bicadinanes in oils from the Surma Basin, Bangladesh

Bicadinanes (C 30 pentacyclic triterpanes) have only been reported in crude oils and sediment extracts from S.E. Asia. The present investigation indicates that they are also widely present in land plant-derived Tertiary oils from the Surma Basin of N.E. Bangladesh. Oleanane is also abundant in these oils though its distribution does not correlate with those of the bicadinanes. This is consistent with separate specific land plant sources for these two classes of triterpanes, and with the formation of bicadinanes during maturation of polymerised angiosperm tree resin.

Sterane isomerisation and moretane/hopane ratios in crude oils derived from Tertiary source rocks

The extent of sterane isomerisation reactions and the moretane/hopane ratios of 234 crude oils, taken world wide, from a wide variety of source rocks of differing geological ages, have been measured. This data indicates that in 78 crude oils derived from Tertiary source rocks, sterane isomerisation reactions as determined by the 20S/(20S + 20R) ration of the C 29 5α(H), 14α(H), 17α(H) normal-steranes and the C 29 iso/(iso + normal) ratio [iso = 5 α (H), 14 β(H), 17 β(H)] are mainly incomplete and sometimes considerably so. In addition, the same crude oils have 17β(H), 21α(H)-moretane/17α(H), 21β(H)-hopane ratios which are significantly greater (predominantly in the range 0.10–0.30) than those of crude oils derived from older, mature source rocks (mainly less than 0.1). This data, for crude oils, lends support to the hypothesis, proposed by Mackenzie and McKenzie (1983) for source rock extracts, that the time/temperature constraints of sterane isomerisation reactions are such that the time available for isomerisation in Tertiary sediments is generally insufficient, despite generation of crude oil at relatively high temperatures. An alternative hypothesis is that the incomplete sterane isomerisation of Tertiary crude oils may be due to generation of these crude oils from their deltaic, land plant-containing source rocks under low heating conditions. A third hypothesis proposes that the Tertiary crude oils may have picked up the incompletely isomerised steranes from immature sediments during migration. Although possible in particular instances, such a mechanism does not appear to be generally applicable since, in that case, the phenomenon would then appear to be restricted to the Tertiary. The higher moretane/hopane ratios of the Tertiary crude oils could suggest that constraints, similar to those applying in sterane isomerisation, also operate in the conversion of moretane to 17α(H)-hopane.

Unusual biological marker geochemistry of oils and possible source rocks, offshore Beaufort-Mackenzie Delta, Canada

Tertiary oils and source rocks from the offshore Beaufort-Mackenzie Delta have been examined for their biological marker content. The triterpane and sterane distributions suggest a predominant land plant contribution to the oils. The oils range in maturity from extremely immature to moderately mature 18(α)H-Oleanne is a major triterpane of the oils together with probable bisnorlupanes, and an excellent correlation is observed in the concentration of these components in individual oils. An excellent correlation is also observed between the concentration of bisnorlupanes and oil maturity, based on sterna and triterpane maturation parameters, suggesting that the decrease in relative concentration of these components with increasing maturity is a result of progressive dilution by hydrocarbons generated by the cracking of kerogen. Only one formation in cores taken from the delta has been found to contain bisnorlupanes, and were present they are the dominant hydrocarbons of the branched/cyclic fraction.

Correlation studies on crude oils and source-rocks in the German Molasse Basin

Abstract Three types of oils can be distinguished in the German Molasse Basin according to their distributions of alkanes and aromatic hydrocarbons. GC-MS investigations, which till now have been restricted mainly to the analysis of steranes, have revealed the presence of only 2 oil families for which 2 different source-rocks must be assumed. We have tentatively concluded that the Tertiary oils have originated from Jurassic source-rocks of the “Germanic” facies, whereas the Triassic oils show some resemblance to Jurassic rocks of the “Helvetian” facies. The Tertiary foreland sediments have not yet been heated sufficiently to have generated oil.

Trends “Late Diagenetic Theory of Petroleum Genesis”-A Review

Recently, the theory of late diagenetic origin of petroleum, which admits that most commercial oils originated by thermal transformation of insoluble organic matter (kerogen) and water required for primary migration could he supplied by dehydration of expandable clays during burial of source sediments, has recieved much support from many workers. In this paper, a general review and the author's coments for the basic problems on this theory are presented. The important points among those are as follows. In the Japanese Tertiary oil fields, it seems that water released by such dehydration of expandable clays as suggested by Powers (1967) have not been effective for the primary migration of oil based on the depth-pattern investigation of clays and hydrocarbons. Accordingly the alternative explanation will be needed for the primary migration of the Japanese Tertiary oils. Commercial oils have not been found so much in areas consisting of oil shales principally, despite of the result that oil shales contain predominantly lipid-rich kerogen which has higher potential for oil generation by the advocators of “late diagenetic thory.” Although Powers (1967) has given only a simple explanation for this problem from the view of clay components of oil rhales, more thorough research on this problem will be required. At present, most of the studies favoured to “late diagenetic theory” are essentially carried out on the basis of “fossil” kerogen and the problems on transformation mechanism and process of “young” to “fossil” kerogen are left unsolved. However, for the formation of commercial oils, hydrocarbons generated during transformation of “young” to “fossil” kerogen seem to be much more important based on various geological and geochemical data. Further study on this problem is required.

Characteristics of Oil Provinces: A Study for Students

ABSTRACT Important differences are discernible between oil basins wholly of Tertiary origin (like some Californian basins) and basins of Mesozoic origin nonethless producing from Tertiary rocks (like the Maracaibo basin). Both sets of basins differ in multiple ways from productive Palaeozoic basins. It is suggested that the differences are due primarily to distant migration, which has enabled oil in older sediments to become pooled far out on the forelands, in shelf sediments and non-orogenic traps, whereas Tertiary oils are still in their basins of origin and in traps of orogenic generation. Combinations of Mesozoic and Palaeozoic oil are much rarer than combinations of Mesozoic and Tertiary oil. It is suggested that this has two causes: the spacing of orogenic episodes, which left the first half of Mesozoic time essentially quiescent, and the introduction of a totally new basin-creating mechanism during late Mesozoic time. This involved the creation of extensional basins consequent on continental drift. As oil geologists have been traditionally resistant to the drift hypothesis, it is very likely that there are more new oil provinces still to be found in Mesozoic rocks than in either Tertiary or Palaeozoic. It is also likely that the familiar association between Tertiary oil basins and Tertiary orogenic belts has obscured the fact that the Mesozoic extensional basins opened up by drift continued to be basinal during the early Tertiary; the Gulf Coast is the familiar exemplar. Wherever a large delta was built outwards over the Tertiary breakover between continental and oceanic crust, equally good prospects should be provided. But if the locales are far removed from the Alpine orogenic belts, they will lie well off shorelines defined by the postglacial rise of sea level. The ideal source sediment is deduced to be one that characterizes many oil provinces and is conspicuously rare or absent in other regions -- a dark-coloured, phosphatic pelite or marl, with organic remains almost entirely planktonic or nektonic and almost wholly microscopic except for fish remains.

Paleozoic oil in Uzbekistan and adjacent territories

Direct evidence of the presence of oil in the region is characteristically widespread within a broad stratigraphic span as well as territorially. This Paleozoic oil is of the naphthene-aromatic type, in contrast with the Mesozoic and Tertiary oils of Uzbekistan, suggesting a justifiable hypothesis of an independent Paleozoic cycle of oleogenesis involving accumulation of hydrocarbons. – IGR Staff.