lake-type controls on petroleum source rock potential ...carroll/publications/pdf/carroll and...

AAPG Bulletin, v. 85, no. 6 (June 2001), pp. 1033–1053 1033

Lake-type controls onpetroleum source rock potentialin nonmarine basinsAlan R. Carroll and Kevin M. Bohacs

ABSTRACT

Based on numerous empirical observations of lacustrine basin strata,we propose a three-fold classification of lacustrine facies associa-tions that accounts for the most important features of lacustrinepetroleum source rocks and provides a predictive framework forexploration in nonmarine basins where lacustrine facies are incom-pletely delineated.

(1) The fluvial-lacustrine facies association is characterized byfreshwater lacustrine mudstones interbedded with fluvial-deltaicdeposits, commonly including coal. Shoreline progradation domi-nates basin fill, resulting in the stacking of indistinctly expressedcycles up to 10 m thick. In map view, the deposits may be regionallywidespread but laterally discontinuous and contain strong faciescontrasts. Transported terrestrial organic matter contributes tomixed type I–III kerogens that generate waxy oil (type I kerogen ishydrogen rich and oil prone; type III kerogen is hydrogen poor andmainly gas prone). The Luman Tongue of the Green River For-mation (Wyoming) and the Honyanchi Formation (Junggar basin,China) provide examples of this facies association, which is alsopresent in the Songliao basin of northeastern China, the CentralSumatra basin, and the Cretaceous Doba/Doseo basins in west-central Africa.

(2) The fluctuating profundal facies association represents acombination of progradational and aggradational basin fill and in-cludes some of the world’s richest source rocks. Deposits are re-gionally extensive in map view, having relatively homogenoussource facies containing oil-prone, type I kerogen. Examples includethe Laney Member of the Green River Formation (Wyoming), theLucaogou Formation (Junggar basin, China), the Bucomazi For-mation (offshore west Africa), and the Lagoa Feia Formation (Cam-pos basin, Brazil).

(3) The evaporative facies association represents dominantlyaggradational fill related to desiccation cycles in saline to hyper-saline lakes and may include evaporite and eolianite deposits. Sub-littoral organic-rich mudstone facies are relatively thin but may be

Copyright !2001. The American Association of Petroleum Geologists. All rights reserved.Manuscript received December 15, 1998; revised manuscript received June 28, 2000; final acceptanceAugust 31, 2000.

AUTHORS

Alan R. Carroll ! Department ofGeological and Geophysical Sciences,University of Wisconsin, 1215 W. Dayton St.,Madison, Wisconsin, 53706;[email protected]

Alan R. Carroll has been an assistant professorat the University of Wisconsin, Madison, since1996, specializing in sedimentary basins inwestern China and the western United States.He received geology degrees from CarletonCollege (B.A. degree, 1980), the University ofMichigan, Ann Arbor (M.Sc. degree, 1983),and Stanford University (Ph.D., 1991). Heworked as an exploration geologist for Sohio(1983–1986) and a research geologist forExxon Production Research Company (1991–1995). He is an associate editor of the AAPGBulletin.

Kevin M. Bohacs ! ExxonMobil UpstreamResearch Co., 2189 Buffalo Speedway,Houston, Texas, 77252;[email protected]

Kevin M. Bohacs is a sedimentologist andstratigrapher with the Petroleum Geochemistrysection of ExxonMobil Upstream ResearchCompany (URC) in Houston, Texas. Hereceived his B.Sc. (honors) degree in geologyfrom the University of Connecticut in 1976and his Sc.D. degree in experimentalsedimentology from M.I.T. in 1981. At URC, heleads the application of sequence stratigraphyand sedimentology to organic-rich rocks fromdeep sea to swamps and lakes in basinsaround the world. As a research associate, hisprimary focus is to keep the geo- ingeochemistry, integrating field work,subsurface investigation, and laboratoryanalyses. He has written numerous articles onthe stratigraphy and sedimentology ofmudrocks, hydrocarbon source rocks, andlake systems. He was co-recipient of the AAPGJules Braunstein Memorial Award for bestposter session paper in 1995 for work on coalsequence stratigraphy and the AAPGInternational Paper Award in 1998 and wasan AAPG Distinguished Lecturer for 1999–2000.

1034 Lacustrine Petroleum Source Rocks

quite rich and widespread. The highest organic enrichment co-incides with the deepest lake stages. Low input of land plant organicmatter results in minimal lateral contrasts in organic content. Insome cases a distinctive type I-S (sulfur-rich) kerogen may generateoil at thermal maturities as low as 0.45% vitrinite reflectance equiv-alent. Examples include the Wilkins Peak Member of the GreenRiver Formation (Wyoming), the Jingjingzigou Formation (Junggarbasin, China), the Jianghan and Qaidam basins (China), and theBlanca Lila Formation (Argentina).

INTRODUCTION

The deposits of nonmarine sedimentary basins account for a grow-ing segment of current petroleum exploration and exploitation op-portunities, especially in areas of rapid market growth such asChina, southeast Asia, and western Africa. The specific techniquesneeded for locating, assessing, and developing hydrocarbon reserveswithin lacustrine basins remain relatively undeveloped comparedto those for marine systems, and they constitute a significant sourceof financial risk (cf. Sladen, 1994). This uncertainty stems not froma lack of data on lacustrine systems, but instead from their greatsedimentologic complexity, as evident from numerous studies ofmodern and ancient lakes. Furthermore, most lacustrine source rockmodels have focused on inferred climatic controls (e.g., Eugster andKelts, 1983; Talbot, 1988), although paleoclimate modeling hashad only limited success in predicting the actual occurrence oforganic-rich lacustrine facies (e.g., Barron, 1990).

Fortunately, some of the complexity evident in modern lakesresolves itself in ancient lacustrine deposits, resulting in the ex-pression of three commonly recurring motifs in the lithology andstratigraphy of lacustrine deposits. For example, Bradley (1925) wasable to generalize a trend within the heterogeneous Eocene GreenRiver Formation (Utah, Colorado, and Wyoming) from “relativelyshallow fresh-water lakes,” to “depositional basins that alternatelyflooded and evaporated either partially or completely,” and finallyto a period when “the lakes became playa-like” and evaporites pre-cipitated. A similar division, based on geography rather than ongeologic time, was proposed over 60 years later by Olsen (1990)for the widespread lacustrine deposits of the Triassic–Jurassic New-ark Supergroup (eastern United States). Olsen idealized lacustrinefacies associations as either Richmond-type, Newark-type, orFundy-type and proposed that these associations reflect differencesin climatically controlled basin hydrology. Mello et al. (1988) de-veloped similar concepts involving organic facies, subdividing Bra-zilian source rocks into freshwater and brackish-saline lacustrinefacies based on relative concentrations of biological marker com-pounds that are sensitive to lake salinity and organic matter input.Others have characterized biomarker distributions indicative ofhypersaline lacustrine source rocks (e.g., Jiang and Fowler, 1986).Carroll and Bohacs (1995, 1999) proposed a classification of ancient

ACKNOWLEDGEMENTS

This article benefited from discussions withmany individuals, including S. C. Brassell, Y. Y.Chen, R. Cunningham, D. J. Curry, G. Genik,K. S. Glaser, G. J. Grabowski Jr., G. B. Hiesh-ima, G. H. Isaksen, B. J. Katz, M. R. Mello, K.Miskell-Gerhardt, J. E. Neal, P. Olsen, D. J.Reynolds, C. Scholz, and K. O. Stanley. G. J.Grabowski and H. B. Lo collected some of thedata used in this article. M. Wartes provideduseful comments on an early draft of themanuscript. Field studies in the Junggar basinin 1987, 1988, and 1992 were funded by theStanford-China Industrial Affiliates, a group ofcompanies that included AGIP, Amoco, An-schutz, BHP Petroleum, Chevron, Conoco, Elf-Aquitaine, Enterprise Oil, Exxon, Mobil, Occi-dental, Pecten, Phillips, Sun, Texaco,Transworld Energy International, and Unocal.Conoco provided additional support for thisarticle. The Donors of The Petroleum ResearchFund, administered by the American ChemicalSociety, also provided support for this re-search. We thank Exxon Production ResearchCo. for permission to publish this article. J. A.Curiale, B. J. Katz, B. Wiggins, and an anony-mous reviewer all provided very helpfulreviews.

Carroll and Bohacs 1035

lake basins into three end-member types that corre-spond to commonly recurring lacustrine facies associ-ations. The terms “overfilled,” “balanced-fill,” and “un-derfilled” lake basins refer to the balance betweentectonically controlled potential accommodation andclimatically controlled water plus sediment supply. Aunique advantage of this model is that it permits broadcharacteristics of hydrocarbon source rock richness andgas vs. oil generation to be predicted from limited geo-logic observations of nonsource sedimentary facies(typically encountered on structure and along basinmargins). This information can also be incorporatedinto basin thermal history models to estimate the tim-ing of generation.

Several previous studies have presented overviewsof lacustrine petroleum source rocks (e.g., Powell,1986; Katz, 1988, 1990; Kelts, 1988; Mello et al.,1988; Mello and Maxwell, 1990; Williams et al.,1995). This article is the first to classify petroleumsource rocks in accordance with the lake-type classifi-cation of Carroll and Bohacs (1995, 1999), and one ofvery few to systematically integrate detailed observa-tions of lacustrine sedimentary facies with geochemicaldata on organic facies. We emphasize detailed, non-biased sampling of carefully described stratigraphic in-tervals as the basis for interpreting the geologic controlson organic enrichment. In contrast to previous work,the model we present suggests specific, practical tech-niques for predicting source rock richness, distribution,oil vs. gas potential, and generation timing based onsparse geochemical or geologic data. Conversely, ourmodel permits broad aspects of the tectonic and cli-matic history of nonmarine basins to be predicted inpart from the character of petroleum and of sourcerock extracts. Such data may be very useful in assessingother exploration play elements, such as reservoir dis-tribution and quality, migration pathways, fault tim-ing, and basin thermal history.

Two important intervals of lacustrine strata areused for illustrative purposes in this article: thecarbonate-rich Eocene Green River Formation of theWashakie basin (Wyoming) and the predominantly si-licic Upper Permian lacustrine formations of the Jung-gar basin (northwest China). Despite gross differencesin geologic age, lithology, and paleogeography, thesame three idealized facies associations are recognizedin both cases and in many other lake basins. The pe-troleum generative characteristics of mudrocks withineach of these facies associations differ significantlyfrom each other and must be considered for successfulexploration of nonmarine basins.

LACUSTRINE SEDIMENTARY FACIESASSOCIAT IONS

Lacustrine deposits may be characterized according to(1) their sedimentary facies, fauna, and flora, (2) theirinternal stratigraphic relations (such as parasequencestacking), and (3) the character of associated deposits.In particular, evidence for open vs. closed basin hy-drology and the presence and nature of depositionalcyclicity provide the fundamental bases for categoriz-ing ancient lake systems according to types. Note thatthese types represent end-member ideals and as suchneed not be 100% representative of any one occur-rence. Furthermore, the interpretation of depositionalcycles necessarily implies simultaneous observation ofseveral different subfacies, resulting in time averagingof diachronous but evolutionary depositional environ-ments. The concept of lake type as applied to ancientdeposits therefore may not be directly applicable tomodern lakes, for which only a synoptic or snapshotview is available.

The Green River Formation in Wyoming consistsof approximately 2500 m of Eocene lacustrine andassociated alluvial strata deposited in several forelandbasins adjacent to Laramide-style uplifts (Sullivan,1985; Roehler, 1992) (Figure 1). Late Permian basinsubsidence resulted in the preservation of more than5000 m of nonmarine facies in the southern Junggarbasin, including more than 1000 m of organic-rich la-custrine mudstone (Liao et al., 1987; Carroll et al.,1990, 1992, 1995; Wartes et al., 1998) (Figure 2).Three lacustrine facies associations may be identifiedwithin both deposits, as detailed subsequently: thefluvial-lacustrine facies association, the fluctuatingprofundal facies association, and the evaporative faciesassociation.

Fluvial-Lacustrine Facies Association

The Luman Tongue consists of freshwater to mildlyalkaline facies deposited in the Washakie basin as theearliest lacustrine phase of the Green River Forma-tion (Roehler, 1992; Horsfield et al., 1994) (Figure1). As such, it resembles the Black Shale facies of theGreen River Formation in the Uinta basin, a sourceof petroleum in Altamont-Bluebell and other fields(Fouch, 1975; Ruble and Philp, 1998). The LumanTongue contains generally poorly expressed, shallow-ing-upward packages (parasequences) that were de-posited by processes of shoreline progradation (Figure3). Where evident, the base of these parasequences

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Figure 1. Location, sedimentary facies, and isopach maps of the Green River Formation in Wyoming, modified from Roehler (1992).


is typically a sharp surface overlain by organic-richcalcareous mudstone and shale or organic-poor bio-turbated mudstone and siltstone (Horsfield et al.,1994; Bohacs, 1998). These facies grade upward intolittoral coquinas, small sandy deltas, and occasionalthin coals. Flood-plain and fluvial facies also are com-mon. Cyclicity caused by lake-level fluctuation issubtle or absent, consistent with relatively open lake-basin hydrology. The laterally equivalent and super-jacent Niland Tongue of the Wasatch Formation con-tains predominantly lake-plain to alluvial deposits(including coals) and is closely related to the LumanTongue (Figure 1).

The Hongyanchi Formation in the southern Jung-gar basin consists of interbedded profundal mudstone,siltstone, limestone, and fluvial sandstone and con-glomerate (Carroll et al., 1992, 1995) (Figures 2, 4). Ittoo lacks any strongly expressed depositional cyclicity,although indistinct deepening and shoaling deposi-tional trends on the scale of 10 m or more are evident.Mudstone units are commonly slightly calcareous andlaminated in places but are commonly homogenous.Amalgamated fluvial sandstone and conglomerate bedsmay reach thicknesses up to tens of meters, recordingchanneled flow within well-developed stream systems.Desiccation cracks and other evidence of prolongedsubaerial exposure are absent. Limestone beds 20 to50 cm thick occur in places and generally compriseargillaceous micrite without visible fauna; they may bealgal in origin. Coals are not present in the HongyanchiFormation, but overlying fluvial units do contain frag-ments of petrified wood that have coaly residues (Car-roll et al., 1992).

The aforementioned features are common to manyother hydrologically open lake basins, such as the Rich-mond and several other basins filled by elements of theNewark Supergroup. Olsen (1990) characterizedRichmond-type basins as containing significant coalsand bioturbated shallow-water and fluvial facies andthick intervals of laminated siltstone. Evidence forsubaerial exposure is absent, consistent with highprecipitation/evaporation ratios. Other examples ofthis fluvial-lacustrine facies association include the Pa-leocene of the Fort Union Formation in Wyoming (Liroand Pardus, 1990), some Cretaceous units in the Son-gliao basin of northeastern China (Yang et al., 1985;Xue and Galloway, 1993; D. Li et al., 1995), somePaleocene intervals in the Central Sumatra basin (Kel-ley et al., 1995), and the Cretaceous strata of theDoba/Doseo basins in west-central Africa (Genik,1993).

Fluctuating Profundal Facies Association

The lower LaClede Bed of the Laney Member of theGreen River Formation consists of alkaline lake depos-its that are widely distributed over the greater GreenRiver basin (Roehler, 1992; Horsfield et al., 1994) (Fig-ures 1, 3). It resembles similar facies in the ParachuteCreek Member of the Green River Formation in Utahand Colorado, which, although immature with respectto petroleum generation, constitutes the well-knownGreen River oil shale deposits. In contrast to the Lu-man Tongue, the lower LaClede Bed contains well-defined parasequences that record major lake desicca-tion and flooding (Horsfield et al., 1994). Theparasequences typically begin with algal and oolitic fa-cies, followed by deposition of finely laminatedorganic-rich calcareous shale. Subsequent evaporativecontraction of the lake resulted in a gradual verticalincrease in dolomicrite, magadii-type chert, and ef-florescent crusts of saline minerals, typically culminat-ing in mud-crack horizons. During uncommonly hu-mid phases, limited progradation of fluvial clastic faciesoccurred at the lake margins, resulting in mixedaggradational/progradational stratal geometries.

The Lucaogou Formation (Junggar basin, China)consists primarily of profundal laminated mudstoneand occasional siltstone interbeds and includes themost organic-rich facies in the southern Junggar basin(Carroll et al., 1992; Carroll, 1998) (Figures 2, 4). TheLucaogou Formation is predominantly siliciclastic,with the exception of occasional nodular dolomite.Distinct 1–4 m scale cycles occur near its base and arepunctuated by mud-cracked desiccation surfaces.These cycles grade upward into the most organic-richinterval of the Lucaogou Formation, where there areno obvious signs of subaerial exposure. Cyclicity con-tinues to be expressed, however, as variations in thethickness and preservation of lamina. Flooding surfaceshave not been identified within this interval; therefore,lamina thicknesses appear to record varying input ofclay and silt in an area distant from any shoreline (Car-roll, 1998). These variations in detrital grainsize mayin turn be linked to fluctuating lake level. Occasionalcentimeter-scale graded beds record fine-grained tur-bidite deposition, and minor soft-sediment slumping iscommon.

Fluctuating profundal facies were also described byVan Houten (1962) in the Newark and related basins.Olsen (1990) attributed these cycles to lake levelchanges driven by Milankovitch-scale climate changesin basins where long-term water inflows were closely


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Figure 2. Location of Junggar basin Permian lacustrine facies (modified from Carroll et al., 1992, and Carroll, 1998). Master sectionsfrom Tianchi and Urumqi are aligned according to an approximate lithostratigraphic correlation between these localities. Permianisopach contours represent the Jingjingzigou, Lucaogou, and Hongyanchi Formations but do not include the entire Upper Permianinterval.

balanced by outflows. Van Houten cycles possess ex-treme lateral continuity, except where they intersectbasin-marginal alluvial fans. The ratio of organic-richto organic-poor units in the Newark basin, however, isrelatively low compared to the lower LaClede Bed orLucaogou Formation. The preservation of fine laminaeimplies the lack of burrowing in fauna or resuspensionof bottom sediments by currents. Laminated intervals,therefore, are interpreted to have been deposited inwater depths below storm wave base, and anoxic con-ditions prevail below the sediment-water interface(Olsen, 1990). Similar profundal facies also occur inthe Bucomazi Formation offshore west Africa (Bur-wood et al., 1995) and in the Lagoa Feia Formationoffshore Brazil (Campos basin) (Trinidade et al.,1995).

Evaporative Facies Association

The Wilkins Peak Member of the Green River For-mation (Figures 1, 3) encompasses a wide variety offacies, ranging from alluvial fan and sheetflood sand-stone to laminated oil shale (e.g., Bradley and Eugs-ter, 1969; Smoot, 1983). This member is bestknown, however, for its bedded trona and halite,which are interpreted to have been deposited inevaporative playas (Eugster and Surdam, 1973; Eugs-ter and Hardie, 1975). Basin-center deposits recorddominantly aggradational stacking of organic-richmudstone facies during lake flooding and carbonateand evaporite facies during lake desiccation. Thebasin-center organic-rich mudstone facies grade ver-tically and laterally into littoral and supralittoral de-posits of dolomitic and siliciclastic mudstone andgrainstone facies that have desiccation cracks andevaporitic minerals or their replacements (Bohacs,1998). The lake-margin lithofacies intertongue withcarbonate and siliciclastic sandstone and mudstonefacies probably deposited in alluvial-fan and fluvialsystems. The basin-center evaporite beds lap margin-ward onto subaerially exposed lake and lake-marginstrata. Laterally equivalent eolian reworked grain-stone and mudstone facies are uncommon. Figure 3Cillustrates the distribution of lithologies in a repre-sentative vertical section. Stratigraphic relationshipsamong littoral and lake-plain facies are complex be-

cause of interactions between sheetfloods and risinglake levels (Smoot, 1983).

The Jingjingzigou Formation (Junggar basin,China) (Figures 2, 4) consists of dolomitic gray andgray-green mudstone, medium gray to dark gray mud-stone, wave-rippled dolomitic siltstone, and fine-grained sandstone (Carroll et al., 1992; Carroll, 1998).Although evaporite minerals are absent from basin-marginal outcrop exposures, possible evaporitepseudomorphs occur. Displacive dolomite nodulesranging in size from 1 to 10 cm are common, particu-larly within the upper parts of siltstone beds. Shoaling-upward lacustrine cycles 10 to 2 m thick occurthroughout the measured section. Tabular siltstoneand sandstone beds ranging from 20 to 50 cm in thick-ness appear at the base of many of the cycles. Minorscour at the base of siltstone and sandstone beds iscommon, as is soft-sediment loading into underlyingmudstone. The sandstone beds are interpreted to rep-resent relatively unconfined, episodic flows across alake plain during onset of flooding and lacustrine trans-gression. Mudstone facies typically possess relativelythick, discontinuous, wavy laminae (#1 mm) and com-monly grade upward into poorly laminated or homog-enous intervals. Thin intervals ($10 cm) of submilli-meter scale laminae occur rarely. Abundant mudcracks, commonly sand or silt-filled, occur near cycletops and attest to frequent subaerial exposure anddesiccation.

Although the composition of Wilkins Peak evap-orites is unusual, evaporative lake facies occur in manyother lacustrine basins. For example, desiccation cyclesin parts of the Fundy and similar basins of the NewarkSupergroup include shallow lake and playa facies andare associated with gypsum nodules, salt-collapsestructures, and eolian dunes (Hubert and Mertz, 1984;Smoot and Olsen, 1988). Other examples include theBlanca Lila Formation (Pleistocene of Argentina)(Vandervoort, 1997) and deposits within several Ter-tiary basins in China (e.g., Fu et al., 1986).

LACUSTRINE ORGANIC FACIES

Each of the sedimentary facies associations describedpreviously correspond to distinctive organic facies as

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Figure 3. Green River Formation measured sections from (A) outcrop of Luman Tongue at Hiawatha locality (21-T12N-R100W), (B)outcrop of lower LaClede Bed of the Laney Member at Trail Dugway locality (18-T14N-R99W), and (C) core of Wilkins Peak Memberfrom the UPR 41–43 well (23-T17N-R109W; see Figure 1 for locations). TOC " total organic carbon (%); HI " Rock-Eval hydrogenindex (mg/g).

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A. Hongyanchi Formation B. Lucaogou Formation C. Jingjingzigou Formation

Figure 4. Measured sections from (A) Hongyanchi Formation at Urumqi, (B) Lucaogou Formation at Tianchi, and (C) Jingjingzigou Formation at Tianchi (modified from Carroll,1998; see Figure 2 for locations).


determined from such features as total organic carbon(% TOC), Rock-Eval hydrogen indices (HIs), visualker-ogen descriptions, pyrolysis products, and biomarkergeochemistry. Biomarkers, or biological marker com-pounds, are complex molecular fossils derived fromonce-living organisms (Peters and Moldowan, 1993).They have the capacity to provide detailed qualitativeinformation on organic matter input, water column ox-ygenation, and thermal maturity. A unique advantageof biomarkers over other geochemical measurements isthat the same compounds are found in both rock ex-tracts and expelled oils, making it possible to interpretsource rock depositional environments in cases wherethe rocks themselves are inaccessible. Biomarkers arealso commonly used to assess the relative salinity ofsource rock depositional environments, based on thepartial record they provide of the types of organismsthat were present. This information can be correlatedwith physical characteristics that suggest a particular sa-linity range, such as abundant mud cracks or the pres-ence of evaporites. It should be noted, however, that inthis context, terms such as “fresh”, “brackish”, “saline”,or “hypersaline” do not normally imply specific soluteconcentrations, but instead represent interpreted rela-tive salinity levels. The following discussion summa-rizes the application of several biomarker parameters tothe interpretation of lacustrine depositional environ-ments and organic matter sources; see Peters and Mol-dowan (1993) for more detailed documentation.

Table 1 summarizes bulk organic matter and bio-marker parameters for Green River Formation andJunggar basin samples, as well as data reported for otherlacustrine units. These occurrences are subdivided intoalgal-terrestrial, algal, and algal-hypersaline organic fa-cies. Note that for each facies, considerable variationmay occur in a given biomarker parameter, both withinone unit and among different units. This variation re-sults from differing lake ecologies, differing levels ofthermal maturity, heterogeneous depositional environ-ments, and possibly different analytical methods usedby different workers. Because only limited stratigraphicdata are available in most of the studies cited, some re-ported occurrences probably also include samples frommore than one organic facies. Despite these uncertain-ties, however, each organic facies exhibits a distinct pat-tern when all the parameters are considered together.

Algal-Terrestrial Organic Facies

The fluvial-lacustrine facies association corresponds toalgal-terrestrial organic facies in the Luman and Niland

tongues and in the Hongyanchi Formation. The ma-jority of TOC values in Luman Tongue and NilandTongue mudstone facies lie between 2 and 8% (Figures3A, 5A), but some coaly samples exceed 50% TOC(Figure 6). Rock-Eval hydrogen indices (HIs) for indi-vidual samples from a continuous section of the Lumanand Niland tongues are highly variable (Table 1). Theiraverage HI is 310, in sharp contrast to type I kerogenHI values that range between 600 and 900 and canexceed 1000 (e.g., Graham et al., 1990). Closer ex-amination suggests the presence of two distinct organicmatter populations within these units; one populationaverages 4.87% TOC and HI # 500, and the otheraverages 9.34% TOC and HI $ 500 (Figure 6). Visualkerogen descriptions indicate that organic maceralslikewise contain two populations; they are dominatedby alginite but also contain varying proportions of vi-trinite (Horsfield et al., 1994). Luman Tongue and Ni-land Tongue source rocks, therefore, record mixedaquatic and terrestrial organic matter input. Population1 samples are tightly clustered around a linear regres-sion line representing HI 805 (Figure 6), attesting tothe constancy of organic matter type within these sam-ples. Variation in their TOC content likely resulted inpart from higher dilution of organic matter by silt andclay. Population 2 samples are much more widely scat-tered around a line representing HI 324, suggesting amore heterogenous mixture of aquatic and terrestrialorganic matter. Biomarker concentrations in extractsfrom the Luman Tongue and Niland Tongue are indic-ative of relatively oxic freshwater environments thathave mixed input of aquatic and terrestrial organicmatter (Table 1). Analysis of kerogen structure viapyrolysis–gas chromatography techniques indicatesthat at higher thermal maturities, Luman Tonguemudstone would generate waxy oil (Horsfield et al.,1994).

The TOC values in the Hongyanchi Formationare lower on average than the Luman and Nilandtongues (Figures 4A, 5B), although values up to 7.7%have been reported (Carroll et al., 1992). The TOCand HI values are also lower in part because of sig-nificantly greater thermal maturity of the HongyanchiFormation (Figure 5A, B). Overall organic mattercomposition appears similar to the Luman and Ni-land tongues, although too few samples have beencollected for a full assessment. HI ranges from 58 to444. The highest organic enrichments correspond tomore profundal intervals, whereas the lowest enrich-ments correspond to massive limestone and mud-stone (Figure 4A). Vitrinite and inertinite combine


0

100

200

A. Luman and Niland tongues

n = 171mean %TOC = 7.09

std. dev. = 8.63Ro = 0.35-0.45%

0

100

200

0 255 10 15 20

0

5

10

15

20

25

30

35

40

0

5

10

15

0

3

6

9

12

15

0

1

2

3

4

5

6

7

8

0

3

6

9

12

15

S 2 (m

g/g)

S 2 (m

g/g)

% TOC

Freq

uenc

y

Freq

uenc

yFr

eque

ncy

C. Laney Membern = 91

mean %TOC = 7.84std. dev. = 3.71

Ro = 0.40%

B. Hongyanchi Formationn = 27


Ro = 0.86-1.09%

D. Lucaogou Formationn = 69


Ro = 0.77-0.88%

F. Jingjingzigou Formationn = 8


Ro = 0.88-0.91%

y = 5.60x - 2.55r 2 = 0.82

y = 8.61x - 9.91r 2 = 0.98

y = 8.30x - 7.58r 2 = 0.95

y = 3.10x - 9.23r 2 = 0.78

0 255 10 15 200

100

200

0

10

20

30

40

50

60

S 2 (m

g/g)

% TOC

E. Wilkins Peak Membern = 201


Ro = 0.20-0.45%

y = 7.90x - 2.87r 2 = 0.97

y = 3.87x - 4.79r 2 = 0.57

>24%

Figure 5. Percent TOC (determined by LECO) vs. Rock-Eval S2 (points), and % TOC histograms (gray shading) for samples from sixlacustrine petroleum source rock units. The slope of the linear regression lines multiplied by 100 gives the average Rock-Eval HI foreach population, corrected for adsorption of pyrolysates on the rock matrix (cf. Langford and Blanc-Valleron, 1990). Ro " vitrinitereflectance in oil for each population. Note that vitrinite reflectances for the Junggar basin samples (B, D, and F) have been affectedby outcrop oxidation and thus may be slightly higher than equivalent unweathered rocks (Carroll, 1998).

water, suboxic depositional environments that havemixed terrestrial and aquatic organic matter input(Table 1).

to average 48% of visible kerogen; the remainder ismostly weakly fluorescent amorphous material. Bio-marker distributions include features typical of fresh-

1044Lacustrine

PetroleumSource

Rocks

Table 1. Selected Geochemical Characteristics of Lacustrine Source Rocks and Oils*

Basin Formation % Ro % TOC HI Pristane/Phytane** b-carotane SteranesTricyclicIndex**

Gamma.Index Hopane/Steranes

Algal-Terrestrial Organic Facies (Fluvial-Lacustrine Facies Association)

Wyoming, Luman Tongue(rocks)†

0.35–0.45 1.0–59.2 55–985 1.5–4.8 not detected C29 # C27 ! C28

abundant 4-methyl(not reported) (negligible) 1.0–4.0

Junggar basin (W. China),Hongyanchi Fm. (rocks)††

0.86–1.09 0–8 58–365 1.5–4.1 trace C29 # C27 # C28 48–71 4–11 2.6–7.7

Brazil basins(rocks and oils)‡

0.4–0.7 "6.5% $779 #1.3 not detected C29 ## C27 30–100 20–40 5–15

Gabon Kissenda shale‡‡ 0.65–0.72 0.6–2.2 93–458 0.8–1.2 not detected C27 # C29 " C28

abundant 4-methyl(not reported) 7–35 4.2–17.5

Phisanulok basin(Thailand; oils)§

n.a. n.a. n.a. 2.7–4.0 not detected C29 ## C28 # C27 (negligible) (negligible) 28.1–47.5

C. Sumatra basin(rocks and oils)§§

0.4–0.8 0.7–9.2 110–946 2.2–3.2 not detected? C29 # C28 % C27

abundant 4-methyl12–27 4–6 3.8–7.7

Algal Organic Facies (Fluctuating Profundal Facies Association)

Wyoming, Laney Member(rocks)†

0.40 1.5–17.1 52–1003 0.1–0.5 present C29 # C27 # C28

abundant 4-methyl(not reported) (high) 0.4–0.8

Junggar basin (W. China),Lucaogou Fm. (rocks)††

0.77–0.88 0–23 481–766 1.0–1.6 present C29 ! C28 ## C27 79–85 19–36 2.5–5.2

Brazil basins(rocks and oils)‡

0.4–0.8 "9 "900 #1.1 present C29 # C27 100–200 20–70 5–15

Bucomazi shales (Angola)organic-rich zone (rocks)||

0.56–0.81 2–24 #700 (not reported) (not reported) C27 # C29 ! C28 (not reported) 13–150 1.5–7.7

CarrollandBohacs

1045

Hypersaline-Algal Organic Facies (Evaporative Facies Association)

Wyoming, Wilkins PeakMember (rocks)|| ||

0.20–0.45 4.1–19.0 32–1001 0.1–1.1 abundant C29 # C28 ## C27 6–21 8–82 0.2–2.0

Junggar basin (W. China)Jingjingzigou Fm. (rocks)††

0.88–0.91 0.8–2.0 129–477 0.8–1.1 abundant todominant

C29 # C28 # C27 79–245 15–34 1.7–7.1

Junggar basin (W. China)Karamay trend (oils)#

n.a. n.a. n.a. 0.9–2.3 abundant todominant

C29 ! C28 # C27 260–580 31–56 0.7–2.2

Jianghan basin (E. China)(rocks and oils)##

0.45–0.55? "6.6 712–838 0.1–0.5 present C27 # C29 # C28 7–28 35–216 0.3–2.4

*% Ro " vitrinite reflectance in oil; % TOC " total organic carbon; HI " Rock-Eval Hydrogen Index; Tricyclic Index " 100(C23 tricyclic terpane/C30 17#(H), 21b(H)-hopane); Gamma. Index " gammacerane/C30 17#(H),21b(H)-hopane; Hopane/Steranes " C30 17#(H), 21b(H)-hopane/C29 20S # 20R 14#(H), 17#(H)-steranes; n.a. " not applicable.

**Ratio strongly affected by thermal maturity.†Horsfield et al. (1994).††Carroll (1998).‡Mello et al. (1988).‡‡Kuo (1994).§Kulwadee and Philp (1991).§§Kelley et al. (1995).||Burwood et al. (1992, 1995).|| ||this study.#Clayton et al. (1997).##Fu et al. (1986), Philp and Fan (1987), Peters et al. (1996).


0

100

200

0 10 20 30 40 50 60

S2

(mg/

g)

% TOC

y = 8.05x - 8.08r2 = 0.94

y = 3.24x - 1.01r2 = 0.86

Luman + Niland Tongues

HI > 500HI < 500

Population 1n = 86


Population 2n = 85

mean %TOC = 9.34std. dev. = 11.76

Figure 6. Percent TOC vs. S2 plot for Luman Tongue and Ni-land Tongue samples. The samples have been divided into twopopulations according to HI, and separate linear regression lineshave been calculated for each population.

Algal-terrestrial organic facies elsewhere typicallyhave moderately high TOC contents (!1–10%) andmixed type I–type III kerogens (Table 1) and com-monly occur in association with fluvial deposits andcoal. High molecular weight n-alkanes (waxes) are ma-jor constituents of both rock extracts and oils becauseof input of protective tissues from higher land plants(Tissot and Welte, 1984) and membrane lipids fromcertain classes of freshwater algae (Goth et al., 1988;Tegelaar et al., 1989). Selective enrichment in thesecompounds may also result from selective bacterialdegradation of other components in oxic to suboxicdepositional environments (Powell, 1986). Highpristane/phytane ratios (Table 1) result both fromhigher land plant input and oxic depositional condi-tions. High hopane/sterane ratios record bacterial deg-radation of relatively abundant terrestrial organicmatter.

Algal Organic Facies

The fluctuating profundal facies association typicallyincludes extremely organic-rich laminated mudstonefacies containing abundant algal-derived organic mat-ter. The TOC values in the lower LaClede Bed of theLaney Member of the Green River Formation are asgreat as 20%, and the average HI is 830 (Figures 3B,5C). Alginite is the principal organic maceral (Hors-field et al., 1994). Subsidiary vitrinite and intertinite,

however, also occur in the highest TOC bluebeds, sug-gesting that these units were deposited during times ofenhanced terrestrial runoff. Ectogenic meromixis as aresult of increased inflow of fresh surface water hasbeen proposed as a mechanism for promoting salinitystratification (Boyer, 1982), which in turn may havehelped preserve organic matter. Biomarker distribu-tions are consistent with predominantly aquatic or-ganic matter input in saline lakes with anoxic bottomwaters (Table 1).

Maximum Lucaogou Formation organic enrich-ment occurs in laminated mudstone, interpreted as an-oxic profundal facies (Graham et al., 1990; Carroll etal., 1992). Organic richness is highly variable withinthe Lucaogou Formation, ranging from less than 0.5%to greater than 20% TOC and is highest in submilli-meter laminated intervals (Carroll, 1998) (Figures 4B,5D). The lower average TOC values in the LucaogouFormation compared with the lower LaClede Bed mayin fact reflect more statistically representative samplingof the Lucaogou Formation. Average Rock-Eval HI val-ues over this interval are essentially identical with thelower LaClede Bed (Figure 5C, D), and both are con-sistent with relatively homogenous type I kerogen.Fluorescent amorphous material dominates LucaogouFormation kerogens, averaging 58% of visible organicmatter, and attests to probable algal input (Carroll,1998). Very light d13C values in extracts of the richestsamples (Carroll, 1998) are inconsistent with the hy-pothesis that organic richness is controlled by high pri-mary productivity, but correlations between produc-tivity and organic richness have been reported in otherbasins (e.g., Curiale and Gibling, 1994). The absenceof subaerial exposure surfaces, presence of fish fossils,and modest elevation of b-carotane and gammaceranein Lucagogou Formation extracts (Table 1) togetherare consistent with deposition within a salinity-stratified lake.

Algal organic facies in other basins represent someof the world’s richest lacustrine source rocks and con-tain mostly type I kerogen that has HI values that typ-ically reach maxima in the 600–900 range and mayexceed 1000 (Espitalie et al., 1977). Lower HI valuesreflect temporal variations in primary productivity(e.g., Curiale and Stout, 1993), degree of water-column anoxia (Demaison and Moore, 1980), and ex-perimental artifacts arising from Rock-Eval pyrolysis(Katz, 1983; Langford and Blanc-Valleron, 1990). Ex-tracts and oils are commonly rich in n-alkanes derivedfrom membrane lipids of aquatic organisms. The dis-tribution of n-alkanes may differ from freshwater fa-


Lake Plain/Alluvialn = 14


y = 8.67x - 3.90r 2 = 0.90

S 2 (m

g/g)

0

1

2

3

4

5

6

7

8

0

100

Freq

uenc

y

Supralittoraln = 25


y = 7.58x - 2.67r 2 = 0.96

S 2 (m

g/g)

0

2

4

6

8

10

0

100

Freq

uenc

y

S 2 (m

g/g)

Littoraln = 86


y = 8.00x - 3.27r 2 = 0.92

0

5

10

15

20

25

30

35

40

0

100

Freq

uenc

y

S 2 (m

g/g)

n = 76mean %TOC = 7.33

std. dev. = 4.22

y = 7.83x - 1.82r 2 = 0.96

0

2

4

6

8

10

00

100

5 10 15 20%TOC

Freq

uenc

y

Sublittoral

Figure 7. Percent TOC histograms for core samples from theWilkins Peak Member, subdivided according to interpreted dep-ositional environment.

cies, however, in having lower odd-carbon-numberpreference and less abundant n-C30# components.Pristane/phytane ratios are typically close to or lessthan 1.0, reflecting lower terrestrial input, anoxic dep-ositional conditions, and possibly elevated salinity.Other biomarker characteristics reflect the contribu-tions of specific classes of aquatic organisms, as evi-denced by the presence of moderate amounts ofb-carotane, elevated tricyclic terpanes, and gammacer-ane. Gammacerane is a commonly cited indicator ofsalinity (e.g., Mello et al., 1988), although more recentwork with compound-specific isotopic analysis sug-gests that gammacerane may in fact record input frombacterial communities living at or below the chemoc-line in stratified lakes (Schoell et al., 1994; SinningheDamste et al., 1995).

Hypersaline Algal Organic Facies

Oil shales in the Wilkins Peak Member of the GreenRiver Formation are thinner and slightly less areally ex-tensive than those in the lower LaClede Bed but con-tain equally high maximum organic enrichments (upto 18% TOC) (Bohacs, 1998) (Figure 5E). The TOCcontents in the Wilkins Peak Member vary widely butare generally lowest in littoral and lake-plain mudstonefacies and highest in sublittoral mudstones and shales(Figure 7). These relationships demonstrate that thehighest organic enrichments occur when the lakes weredeepest. Note that average HI values in the WilkinsPeak Member (determined from S2/TOC plots) areessentially identical for each environment, indicatingvery little lateral contrast in organic matter type.Amorphous organic matter, derived from algal or bac-terial sources, dominates visible organic matter, con-firming very low input from land plants. Some of themost organically enriched strata occur just above themajor flooding surface at the base of each sequence(Figure 3C). Biomarker distributions are consistentwith deposition under hypersaline, anoxic conditions(Table 1), although certain biomarker ratios (e.g.,pristane/phytane ratios) may also reflect low thermalmaturity.

Organic enrichment in the Jingjingzigou Forma-tion mudstone is relatively low (TOC less than 2%)(Figures 4C, 5F). Total organic carbon is highest in me-dium and dark gray laminated mudstones and drops toless than 1.0% in nonlaminated facies. Rock-Eval HIvalues reach a maximum of 477, suggesting oxidationof organic matter prior to burial. The JingjingzigouFormation contains high proportions of amorphous


kerogen, presumably of algal or bacterial origin (Car-roll, 1998). Minor vitrinite, inertinite, and alginitemake up the remainder of the visible kerogen. Bio-marker distributions are commonly dominated by b-carotane, which has been associated with shallow, hy-persaline lakes in which certain highly specializedorganisms occur (Murphy et al., 1967; Hall and Doug-las, 1983; Brassell et al., 1988; Duncan and Hamilton,1988; ten Haven et al., 1988).

Hypersaline organic facies commonly featurelower overall TOC values than do brackish-saline fa-cies, although relatively thin intervals of high TOC oc-cur during periods of maximum lake expansion.Mixtures of type I and type III kerogens appear to re-sult from varying degrees of preservation of aquatic or-ganic matter and from the admixture of minor terres-trial material. Maximum HI values equal or exceedthose seen in algal organic facies (Table 1), but averagevalues tend to be lower. Biomarker distributions reflectlow-diversity assemblages of highly specialized organ-isms living under harsh environmental conditions.Pristane/phytane ratios substantially below 1.0 may re-sult either from anoxic depositional conditions in astratified, hypersaline lake, or from contributions fromhalophilic bacteria (Goosens et al., 1984). b-carotaneis commonly an abundant to dominant constituent, asare elevated concentrations of tricyclic terpanes andgammacerane (Table 1). Hopane/sterane ratios aretypically low. In addition to the examples cited previ-ously, hypersaline lacustrine organic facies are knownfrom several Tertiary basins in China (Shi et al., 1982;Fu et al., 1986; Sheng et al., 1987; Brassell et al., 1988;R. Li et al., 1988, 1992; Z. Chen et al., 1994; J. Chenet al., 1996; Peters et al., 1996) and in Spain (e.g.,Salvany and Ortı, 1994; Sanz et al., 1994).

DISCUSSION: PETROLEUM GENERATIONFROM LACUSTRINE SOURCE ROCKS

Bradley (1925) noted that for certain members of theGreen River Formation, “large volumes of microscopicplants and perhaps also animals accumulated.” Studiesof oil-shale facies of the Mahogany ledge of the Para-chute Creek Member led to the recognition of type Ilacustrine kerogen, characterized by high atomic H/Cratios and HI (van Krevelen, 1961; Espitalie et al.,1977). Subsequent experience has shown, however,that kerogens in many modern and ancient lake sys-tems differ from type I, and that type I is not evenrepresentative of all members of the Green River For-

mation. For example, modern east African lakes pre-serve organic matter ranging from type I to type III(Katz, 1988, 1990; Talbot, 1988). Recent studies havedemonstrated that these distinctions are critical notonly for estimating the relative quality and quantity ofoil and gas that may be generated, but also for deter-mining the timing of hydrocarbon generation (e.g., An-ders et al., 1992; Tegelaar and Noble, 1994; Peters etal., 1996). The following discussion considers genera-tive characteristics of mixed type I–III kerogen typicalof the algal-terrestrial organic facies (fluvial-lacustrinefacies association), type I kerogen typical of algal or-ganic facies (fluctuating profundal facies association),and type I-S kerogen that has been reported in associ-ation with some hypersaline-algal organic facies (evap-orative facies association).

Oil and Gas Generation from Mixed Type I–III Kerogens

Tegelaar and Noble (1994) demonstrated through py-rolysis experiments that vitrinite-dominated (terres-trial) kerogen generally generates hydrocarbons over abroader range of temperatures than do most algal-dominated kerogens and that generation continueswell above the range of temperatures associated withgeneration from algal-dominated kerogen. This resultsuggests that given the same burial and thermal history,mixtures of aquatic and terrestrial organic mattershould generate petroleum over a broader range ofthermal maturities than purely algal source facies. Datafrom the Uinta basin in Utah are consistent with thishypothesis. Oils in the Altamont-Bluebell fields weresourced primarily from freshwater lake facies of thelower Green River and underlying formations that re-semble the Luman Tongue (Fouch, 1975; Ruble andPhilp, 1998). Based on Rock-Eval transformation ra-tios, light hydrocarbon yields, and atomic H/C ratios,hydrocarbon generation from these facies occurred inthe deep Uinta basin over a broad range of thermalmaturities, ranging from approximately 0.70 to 1.35 %Ro (Anders et al., 1992).

Uinta basin oils are solid at surface temperaturesbecause of their very high wax content. High wax con-tent and pour point are also associated with freshwaterlacustrine source rocks in the Central Sumatra basin(Kelley et al., 1995), in the Songliao basin of northChina (Yang et al., 1985), in central African rift basins(Genik, 1993), and in some Brazilian rift basins (Melloet al., 1988; Mello and Maxwell, 1990). Minoramounts of natural gas are also common in oil fieldssourced by mixed type I–III mixed kerogens (e.g.,


Clem, 1985; Rice et al., 1992; Genik, 1993; Kelley etal., 1995), but the sources of these gases have not beenclearly established.

Oil Generation from Type I Kerogen

Waples (1980) and Sweeney et al. (1987) showed thattype I kerogens generate oil over a very narrow rangeof thermal maturities, reflecting little variation inchemical bond type within these homogenous kero-gens. Tegelaar and Noble (1994) found that the onsetof oil generation from Green River type I kerogenshould occur between 0.8 and 0.9% Ro, and peak gen-eration is at 0.95–1.05% Ro. The overall oil window(defined by 10–90% kerogen transformation ratios)thus spans only about 0.3% Ro on average, or less than30&C based on a 1&C/m.y. heating rate. They foundthat the peak generation other type I kerogens also gen-erate over a very narrow range of thermal maturities,although the absolute thermal maturity at which gen-eration begins could vary widely. They calculated thatthe onset of oil generation could range between 0.54and 0.96% Ro and concluded that bulk chemicalcomposition alone was not sufficient to predict gener-ation timing. Using pyrolysis–gas chromatographytechniques to better discern kerogen chemical struc-tures, they found that two other factors influence gen-eration timing: (1) the types and mixtures of biomacro-molecules preserved, and (2) organic sulfur content.Whether absolute generation timing can be accuratelydetermined from bulk chemical measurements such asRock-Eval HIs is, therefore, currently unclear.

Type I kerogens generate paraffinic oils producedfrom the Karamay and associated fields in the Junggarbasin (Clayton et al., 1997). Karamay oils are low insulfur, and except where biodegraded, contain abun-dant n-alkanes that have molecular weights less thanC30. A similar n-alkane distribution was reported foroils sourced from type I kerogen in the Campos basinof Brazil (Mello et al., 1988; Mello and Maxwell,1990). The distribution of n-alkanes in oils generatedfrom type I kerogens may vary widely, however, andare subject to modifications related to thermal matur-ity and biodegradation.

Early Generation from Type I-S Kerogen?

Several Tertiary lacustrine basins in China containoils that appear to have been generated at lower ther-mal maturities than are normally associated with thebreakdown of typical marine or lacustrine kerogens.

Such oils have been reported from onshore exten-sions of the Bohai Bay basin (Shengli oil field) (Shiet al., 1982; Z. Chen et al., 1994), the Dongpu basin(R. Li et al., 1988, 1992), the Jianghan basin (Fu etal., 1986; Sheng et al., 1987; Brassell et al., 1988;Peters et al., 1996), and the Qaidam basin (Huanget al., 1991) and are commonly associated with salineor hypersaline source rocks. Based on source rock–oilcorrelations and biomarker maturity measurements,significant generation appears to have occurred fromthese rocks at vitrinite reflectance values as low as0.45%. Several molecular maturity measurements, in-cluding the ratios of 20S/(20S # 20R) desmethylsteranes and 22S/(22S # 22R) homohopanes, pro-vide confirmation of the low thermal maturity ofthese oils. Jianghan basin oils, for example, com-monly have 20S sterane ratios as low as 0.17–0.24,in contrast to the observation by (Mackenzie et al.,1980) that petroleum generation from marine type IIkerogen begins at values of about 0.40. Likewise,Jianghan oils have C32 22S hopane ratios as low as0.44 (Fu et al., 1986), in contrast to values of ap-proximately 0.50–0.55 that typically mark the onsetof petroleum generation from marine kerogens. Ifthese low maturities are representative of a significantfraction of the total petroleum generation, then ex-ploration models for basins containing such oils mustallow for earlier generation of oil relative to trap for-mation and other play elements.

Two explanations have been offered for these low-maturity oils. The first is that they do not representthermal degradation of kerogens at all, but instead re-sult from degradation and migration of original solubleorganic matter that was never incorporated into kero-gen. For example, certain coals and carbonaceousshales rich in higher land plant remains (particularlyfrom conifers) may contain abundant resins (solubleorganic matter), which may be expelled as liquids(Snowdon and Powell, 1982; Stout, 1995). Huang etal. (1991) invoked such an explanation for Qaidam ba-sin light oils and condensates that have 20S sterane ma-turity ratios of 0.18–0.23. X. Li et al. (1998) proposedthat two distinct pulses of petroleum generation oc-curred in the East China Sea basin, the first occurringat less than 0.55% Ro and the second between 0.55 and1.30% Ro. Resinite derived oils reported from Canadaare naphthenic (high percentage of hydrocarbons hav-ing ring structures) rather than paraffinic, but differentresin types and varying proportions of resinite relativeto other macerals may partly account for more paraf-finic oils in China.


Another important consideration is the potentialvolumetric significance of oil generated from resins,which typically comprise relatively low percentages ofthe total organic matter (Tissot and Welte, 1984). Sa-line or hypersaline lacustrine facies would be expectedto contain relatively little resinous land plant organicmatter. For example, oils in the Shengli field arethought to represent transformation of less than 10%of the total organic matter present in the source rocks(Cheng Keming, Research Institute of Petroleum Ex-ploration and Development, Beijing, 1997, personalcommunication). Further volumetric data on the Shen-gli oil field and other accumulations of low maturityoils are needed to determine whether expulsion of bio-genic bitumen is adequate to account for the knownoil reserves.

A second and possibly complementary explanationis that low-maturity oils result from thermal degrada-tion of a distinctive type I-S (sulfur-rich) kerogen. typeI-S kerogen is defined as having an HI above 600 anda S/C atomic ratio greater than 0.04 (SinningheDamste et al., 1993; Peters et al., 1996). Jianghan basinoils are generally high in sulfur (3.59–12.91%) (Shenget al., 1987), and aromatic organosulfur compoundsare abundant (Philp and Fan, 1987; Sheng et al., 1987).Peters et al. (1996) presented experimental kineticdata showing that the type I-S kerogen in Jianghanhypersaline lacustrine source rocks reacts quickly tothermal stress, in a manner similar to marine type II-Skerogen in the Monterey Formation. Type II-S kero-gens have been demonstrated to generate oil at lowerthermal exposures than other kerogens (Orr, 1986).Hydrous pyrolysis experiments using Monterey kero-gens provide evidence that this generation actually oc-curs via a two-step process (Baskin and Peters, 1992).In the first step, cleavage of relatively weak S–C bondsgenerates heavy bitumen that is retained on the kero-gen matrix. Thermal degradation of this bitumen atslightly higher temperatures results in the expulsion ofoil. Thus, this model appears to combine elements ofthe “oil from resins” model discussed previously withthermal degradation of an uncommonly reactivesulfur-rich kerogen. Low maturity Monterey Forma-tion oils, however, have low API gravities and are dom-inated by asphaltenes and resins, whereas Jianghan oilsare dominantly paraffinic (Philp and Fan, 1987). Morework is clearly needed to resolve the actual generationmechanisms responsible for low-maturity oils inChina.

Type I-S kerogens appear to result from dia-genetic sulfurization rather than from biological in-

put of organosulfur compounds. Sheng et al. (1987)identified long-chain normal alkyl-thiophenes andalkyl-thiolanes, long-chain isoprenoid-thiophenes andisoprenoid-thiolanes, and benzothiophenes in sulfur-rich oils. The distribution of these compounds mirrorsthat of the corresponding alkanes, leading Sheng et al.(1987) to conclude that the sulfur compounds origi-nated through diagenetic reactions between elementalsulfur and sulfides with phytol, fatty acids, and alco-hols. The apparent rarity of type I-S kerogen maystem from the fact that most lake waters have lowsulfate concentrations relative to marine systems andthat in siliciclastic lacustrine systems there is com-monly excess iron available to form pyrite or othersulfides. These limitations, however, may be over-come if sulfide scavengers such as Fe and Mn arescarce, especially if sulfate supply to the basin is un-usually high. For example, Sinninghe Damste et al.(1993) documented type I-S kerogen in two TertiarySpanish oil shales, which they interpreted as havingformed as a result of weathering of Triassic evaporites.Sulfate from the evaporites was microbially reducedin a freshwater lake and incorporated into previouslydeposited remains of Botryococcus braunii algae. Amore common setting for type I-S kerogen is probablyin evaporative lakes where siliciclastic sediment inputis low and evaporative concentration of sulfate occurs.In the Chinese basins, the oil source rocks are closelyassociated with gypsum and other evaporites and havebiomarker characteristics indicative of anoxic saline toanoxic hypersaline environmental conditions (Shi etal., 1982; Fu et al., 1986; Philp and Fan, 1987; Shenget al., 1987; Brassell et al., 1988; Huang et al., 1991;Z. Chen et al., 1994).

CONCLUSIONS

1. Three end-member lacustrine facies associationshave been observed to recur in widely disparategeographic settings and over a wide range of geo-logic time: fluvial lacustrine, fluctuating-profundal,and evaporative. Significant differences exist amongeach of the these associations in terms of oil vs. gasgeneration and the timing of petroleum generationrelative to basin thermal history. These associationscan be used to make first-order predictions of pe-troleum generation type (oil vs. gas) and timingfrom limited geologic data on nonsource facies.Conversely, geochemical analysis of source rocks oroils may aid in predicting lacustrine paleoenviron-


ments and the likely distribution and quality of res-ervoir and seal facies.

2. Lacustrine source rocks display a high degree ofgeochemical heterogeneity relative to marine facies,therefore their complete characterization requiresanalysis of a relatively large number of samples se-lected at fixed, regular intervals. Selective samplingof a few particularly rich beds may lead to erroneousconclusions concerning overall generative potential.

3. No typical lacustrine source rock or oil exists. Thedefault-value kerogen properties and kinetics some-times used in basin modeling are therefore of lim-ited utility. For optimal precision, detailed kineticmeasurements should be made from immatureequivalents of the suspected source facies, if possi-ble, and should be supported by molecular maturitymeasurements from source rock extracts and gen-erated oils.

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