Section I Selected Topics - UW–Madisoncarroll/publications/pdf... · of physical, chemical, and biological processes and responses, and (4) studies of modern lakes and closely associated

Section I

Selected Topics

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Chapter 1�

Lake-Basin Type, Source Potential, andHydrocarbon Character: an IntegratedSequence-Stratigraphic–Geochemical

FrameworkKevin M. BohacsAlan R. Carroll1

John E. NealPaul J. Mankiewicz

Exxon Production Research Company, Houston, Texas, U.S.A.

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Bohacs, K. M., A. R. Carroll, J. E. Neal, P. J. Mankiewicz,2000, Lake-basin type, source potential, and hydrocar-bon character: an integrated-sequence-stratigraphic–geochemical framework, in E. H. Gierlowski-Kordesch and K. R. Kelts, eds., Lake basins throughspace and time: AAPG Studies in Geology 46, p. 3–34.

INTRODUCTION

Rocks associated with lakes probably account for morethan 20% of current worldwide hydrocarbon production(Kulke, 1995; Calhoun, 1999), and lacustrine organic-richrocks are significant sources of these hydrocarbons. Lacus-trine sources and reservoirs are important in many areasof current and future exploration opportunities: Africa,South America, southeast Asia, China (Hedberg, 1968;Powell, 1986; Smith, 1990; Katz, 1995).

The years since the last AAPG lake Memoir (Katz,1990) have seen both an expansion of work on mod-ern and ancient lake systems and a focusing on theirhydrocarbon potential. Through the efforts of individ-ual workers and teams in academia and industry,along with collaborative efforts (e.g., IGCP-GLOPALS,International Association of Limnogeology), we have signif-icantly increased our knowledge of lake systems on twofronts: key processes and sedimentary response-record(e.g., Anadon et al., 1991; Gierlowski-Kordesch and Kelts,1994; Katz, 1995).

Particularly enlightening have been the increase in (1)basin-scale studies of ancient systems that integratestratigraphy, sedimentology, biofacies, and inorganicand organic geochemistry, (2) the use of reflection seis-mic data to gain large-scale 3-D perspectives on basin-fillhistory, (3) studies by petroleum-industry scientists thatbenefit from this large-scale perspective and integrationof physical, chemical, and biological processes andresponses, and (4) studies of modern lakes and closelyassociated Quaternary deposits focused on key elementsof sediment delivery and dispersal, organic production

and preservation, and temporal evolution of lakehydrology (again aided by seismic-scale perspective,especially in east Africa) (e.g., Johnson et al., 1987;Scholz, 1995). Analytical advances and a broader expe-rience base integrated into geological context have alsocontributed; geochemists have better tools for difficultnonmarine organic-matter mixtures (e.g., GC-MS/MS,isotope-ratio-monitoring GC-MS), stable isotoperecords are more widely available, and more sophisti-cated studies of the interactions among inorganic andorganic geochemistry and sedimentation have been com-pleted (e.g., Horsfield et al., 1994; Renaut and Last, 1994).

These studies allow construction of process-basedmodels strongly conditioned by the geological record andprovide a solid foundation for extending hypotheses intopredictive realms. We can thus better and more naturallygroup many and disparate observations to reveal geneticrelations and construct better predictive models based onour increased understanding of the links and feedbackamong process, response, and record of lake systems.This paper presents a summary of key observations andan overview of a framework for integrating process,response, and record we find useful for understandingand predicting source character and hydrocarbon distrib-ution in lake basins.

LEADING FACTORS OF LACUSTRINEBASIN-FILL EVOLUTION

Numerous modern studies and ancient observa-tions reveal that lakes are not just small oceans. Lakes

1Present address: University of Wisconsin-Madison, 1215 W. Dayton Avenue, Madison, WI 53706

4 Bohacs et al.

differ from oceans in several significant ways (e.g.,discussions in Kelts, 1988; Sladen, 1994). These differ-ences strongly influence the occurrence, distribution,and character of hydrocarbon source, reservoir, andseal play elements. Recognizing these differences isessential to successful exploration and exploitation inlake basins. Major differences between lake andmarine depositional systems include the following.

• Lakes contain much smaller volumes of sedimentand water; hence, lake systems are much moresensitive to changing accommodation and cli-mate. Lake levels vary more widely and rapidlythan sea level—300 m in 15,000 yr is not uncom-mon (e.g., Currey and Oviatt, 1985; Manspeizer,1985; Hayberan and Hecky, 1987; Johnson et al.,1987). In systems with very low relief over wideareas, small short-term lake level changes (weeksto months) can move shorelines large distances;the shoreline of Lake Chad recently retreated upto 18 km in 9 months due to a 3-m fall and lakearea fluctuated 92% between 1966 and 1985(Mohler et al., 1995). The result in the ancientrecord is shoreline strata that are commonlypoorly developed and relatively thin (see discus-sions in Smoot, 1983 and Sladen, 1994). Waterchemistry and lake ecology can also vary greatlyover short stratigraphic intervals (e.g., Gier-lowski-Kordesch and Kelts, 1994) with greatimpact on source and seal character.

• Lake level and sediment supply are directlylinked in lake systems (e.g., Schumm, 1977; Perl-mutter and Matthews, 1990). Lake level riseswhen river discharge is high and falls when dis-charge drops. The strength of this linkage variesaccording to lake-basin type, with the strongest inclosed-hydrology basins and the weakest in open-hydrology basins. This contrasts greatly withmarine systems, where sea level and sedimentsupply are only weakly linked at most and mostmodels assume no linkage (e.g., Posamentier andVail, 1988). This linkage is the prime cause of thevariety of expression of depositional sequencesamong lake-basin types and of their contrast withmarine sequences. For example, significant thick-nesses of strongly progradational lowstand strataare unlikely to form in most lake basins.

• Lake shorelines can move basinward either byprogradation or simple withdrawal of water.Progradation deposits a distinct rock package,whereas withdrawal leaves little record other thandesiccation features on previously deposited strata.

• The nature and existence of a lake is fundamen-tally controlled by the relative rates of potential

accommodation change and supply of sedi-ment+water. Potential accommodation is thespace available for sediment accumulation belowthe basin’s outlet or spillpoint (it is a function ofbasin subsidence, outlet height, and inheritedbasin shape) (Carroll and Bohacs, 1995, 1999). Thetype of lake system is controlled by how much ofthat space is filled by some combination of sedi-ment+water and over what time span (Gilbert,1890); hence, climate (∞sediment+water) and tec-tonics/inherited topography (∞potential accom-modation), commonly exert co-equal control onthe nature and distribution of lacustrine deposi-tional systems tracts and their source, reservoir,and seal lithofacies.

The contrast among lake and marine systems alsomakes it inappropriate to directly apply one unmodifiedmarine sequence-stratigraphic model to all lake systems.The sequence-stratigraphic approach, looking at a hier-archy of rock packages bounded by various surfaces,works very well in lake strata; however, the expressionof depositional sequences varies as a function of lakedepositional system, just as shallow-marine carbonatesequences look different from shallow-marine siliciclas-tic sequences. Indeed, one lacustrine model is not applic-able to all lake-basin types. Failure to appreciate thesedifferences has led to some of the difficulties encoun-tered in exploring in lacustrine basins. It is possible,however, to understand and predict lake-basin type,as well as source and hydrocarbon distribution andcharacter, using first principles of sequence stratigra-phy and lake depositional environments.

It also became clear that there were underlyingmajor controls when we examined modern andancient lake examples with a wide variety of ages, tec-tonic settings, climates, and latitudes (Carroll andBohacs, 1995, 1999). The many good studies of modernlakes (e.g., Johnson et al., 1987; Cohen, 1989; Scholzand Rosendahl, 1990) reveal an almost bewilderingarray of process, interactions, and feedbacks that por-tend a corresponding large complexity in ancient lakedeposits. This complexity, however, is not completelyrecorded in ancient lake strata from many ages andbasins. We, along with several others (e.g., Bradley,1931; Glenn and Kelts, 1991; Gore, 1989; Olsen, 1990),see three major facies associations in lake strata at thetongue to member scale (depositional sequence tosequence-set scale; meters to hundreds of meters) anda characteristic stacking of these facies associations asa basin fills (Figure 1). (Most workers attributed thesethree motifs or lake types to endemic, generally cli-matic, causes, hindering their general applicabilityand even their recognition.)

Figure 1—Comparison of lake-basin-type facies associations in vertical section with examples from theEocene Green River Formation, Washakie basin, Wyoming. Each lake-basin type has distinctive associationsof lithologies, sedimentary structures, biota, and geochemical nature. These characteristics appear to bestrongly controlled by the interaction of potential accommodation and sediment+water supply and record the integrated history of a lake’s hydrologic state. These associations are relatively independent of overall thickness, age, and inferred water depth.

Lake-Basin Type, Source Potential, and Hydrocarbon Character: an Integrated Sequence-Stratigraphic–Geochemical Framework 5

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Current ripples

Wave ripples

Combined-flow ripples

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Planar lamination

Flaser or lenticular bedding

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Fining-upward bed

Mudcracks

Stylolites

Concretions

Oolite

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Plant fossils

Ostracods

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6 Bohacs et al.

These three end-member facies associations are rec-ognized based on objective physical, chemical, and bio-logical criteria (Carroll and Bohacs, 1999; cf. Olsen, 1990,and references therein). Specifically, each end memberhas characteristic lithologic successions, sedimentarystructures, geochemical indicators, fossils, and stratalstacking patterns. Table 1 lists the most commonattributes of each of these lake facies associations; moredetails on stratal stacking patterns are found in latersections of this paper. In summary, the three lacustrinefacies associations were named fluvial-lacustrine, fluctu-ating profundal, and evaporative by Carroll and Bohacs(1999), according to their most generally recognizablecharacteristics. The fluvial-lacustrine facies associationtends to be composed dominantly of marlstone,argillaceous coquina, and bioclastic grainstone, alongwith sandstone, carbonaceous mudstones, and coalsthat contain freshwater fauna and mixtures of aquaticand terrigenous organic matter in beds and bedsetsdominated by physical sedimentary structures (Fig-ures 1a, 2). The fluctuating-profundal lacustrinefacies association typically comprises a complexinterbedding of heterogeneous lithologies (carbon-ate, siliciclastic, argillaceous, organic-rich mudstone)containing fresh- to saline-water biota and domi-nantly aquatic organic matter in beds and bedsetswith both biogenic and physical sedimentary struc-tures (Figures 1b, 3). The evaporative lacustrinefacies association actually contains a wide variety of

lithofacies (clastic sandstone and grainstone, kerogen-ite, evaporite) that contains a low-diversity, salinity-tolerant flora in beds and bedsets with sedimentarystructures due to physical transport, biogenic precipi-tation, desiccation, and crystallization (e.g., ripples,stromatolites, mudcracks, displacive fabrics, andcumulate textures) (Figures 1c, 4). These three end-members appear to capture a large portion of the vari-ation in essential attributes of lacustrine strata and areuseful for exploration-scale summaries despite theobserved wide variation in accidental attributes, suchas evaporite or clay mineralogy, thickness, clasticcomposition, color, or absolute area.

Seeking to explain the common and widespreadoccurrence of these three lacustrine lithofacies associa-tions, we sought to uncover the fundamental controlson the preserved geological record of lakes. One’sintuition might lead one to suppose that wetter cli-mates have lakes that are larger, deeper, and in theancient thicker deposits; however, observations ofboth modern and ancient lakes do not corroborate thisintuition. Modern lake size, depth, and character orancient lake-strata thickness, extent, and attributes donot correlate with measured or inferred climatichumidity (precipitation/evaporation = P/E) (Carrolland Bohacs, 1995, 1997, 1999). No correlation existsbetween P/E and any measure of lake size (e.g., depth,surface area, volume) for large modern lakes, eventhose of tectonic origin (Figure 5a). Also, predictions

Table 1. Representative Attributes of Three Major Lacustrine Facies Associations .

Lacustrine Facies Stratal Stacking Sedimentary Organic Association Patterns Structures Lithologies Matter

Fluvial-Lacustrine Dominantly Physical Transport: Mudstone, marl Freshwater biotaprogradation ripples, dunes, Sandstone Land-plant,

Indistinctly expressed flat bed Coquina charophytic and parasequences Root casts Coal, coaly shale aquatic algal OM*

Burrows Low to moderate (infaunal & TOC*epifaunal) Terrigenous & algal

biomarkersFluctuating Mixed progradation Physical & Biogenic: Marl, mudstone Salinity tolerant

Profundal & aggradation flat bed, current, Siltstone, sandstone biotaDistinctly expressed wave, & wind Carbonate grainstone, Aquatic algal OM*

parasequences ripples; wackestone, micrite Minimal land stromatolites, Kerogenite plantpisolites, oncolites Moderate to high

Mudcracks TOCBurrows (epifaunal) Algal biomarkers

Evaporitive Dominantly Physical, Biogenic, Mudstone, kerogenite Low-diversity, aggradation & Chemical: Evaporite halophytic biota

Distinctly to climbing current Siltstone, sandstone Algal-bacterial OM*indistinctly ripples, flat bed, Grainstone, Low to high TOCexpressed stromatolites, boundstone, flat- Hypersaline parasequences displacive fabrics, pebble cgl* biomarkers

cumulate textures

*OM = organic matter, TOC = total organic carbon, cgl = conglomerate.


based on climatic humidity alone fail to explain thewide variety of modern lakes within a single climaticzone, where they can range from saline to freshwa-ter, eutrophic to organically barren, and purely sili-ciclastic to carbonate forming (Herdendorf, 1984)(Figure 5b). Modern Utah Lake contains fresh water,but drains into the adjacent, hypersaline Great SaltLake, only 70 km away within the same semi-aridarea (Stansbury, 1852). Also note that the drainagebasins of the world’s largest rivers contain only few,

relatively small lakes (e.g. , Amazon, Congo,Orinoco, Mississippi).

A similar discordance between climate and lakesize is also apparent in ancient systems. For example,detailed mapping of the Green River Formation ofWyoming reveals that strata of the freshest water lakephases (e.g., the Luman Tongue) are actually thethinnest and least areally extensive (Sullivan, 1980;Roehler, 1992) (Figure 6). In contrast, more saline lakemembers such as the lower LaClede Bed and upper

Figure 2—Examples of fluvial-lacustrine facies association, typical lithologies, and stratal packages (depositional sequences). Note dominance of clastic lithologies and physical sedimentary structures and similar stratal stacking patterns despite different ages, paleolatitudes, and overall thickness (note differentscales). Subenvironments: 1 = fluvial/floodplain, 2 = lake plain and supralittoral, 3 = littoral, 4 = sublittoral, 5 = lake-floor “fan”/turbidite channel, and 6 = profundal. Subenvironments delineated based on indices ofbottom energy, subaerial exposure (pedogenesis), and trace fossils; preservation of vertebrate fossils; andorganic and inorganic geochemistry. Data derived from Eocene Luman Tongue, Green River Formation,Hiawatha outcrop (Sec 17-T12N-R100W), Washakie basin, Wyoming (Horsfield et al., 1994), Triassic WaterfallFormation, I-66 outcrop (Thoroughfare Gap, Virginia), Culpeper basin, Virginia (Hentz, 1981; Gore, 1988), Cretaceous Lucula Formation, 73-69 core (Malongo North field), offshore Cabinda, Angola (Bracken, 1994),Westphalian lacustrine shales, Traill Ø outcrop (ca. 72°N, 24°W), East Greenland (Stemmerik et al., 1990).

8 Bohacs et al.

Tipton member are the most widespread; furthermore,the most evaporitic Wilkins Peak member is also thethickest stratigraphically, although it had the shallow-est depth lakes (Smoot, 1983). All these observationsstrongly indicate that changes among lacustrine faciesassociations do not result solely from climatically dri-ven changes. Additionally, studies of organicallyenriched lake deposits show little if any correspon-dence with inferred paleoclimate or climatic zone (e.g.,Smith, 1990; Carroll, 1998).

LAKE-BASIN TYPES

To reconcile these seemingly paradoxical observa-tions, we focused on the fundamental controls on lakestrata. We interpret that it is the relative balance ofrates of potential accommodation change (mostly tec-tonic) with sediment+water supply (mostly climatic)that controls lake occurrence, distribution, and charac-ter. Our model addresses lake types and hydrocarbonpotential in the context of these two fundamental con-trols, incorporating meso- and macroscale stratal stack-ing, sediment-supply variations, and subsidencehistory. We propose that climate and tectonics, throughtheir strong effects on these controls, exert coequalinfluence on the occurrence, distribution, and characterof preserved lake strata at both meso- and macroscales(one to tens of meters and hundreds of meters) (see alsoManspeizer, 1985), mainly through their influence onthe time-integrated history of lake hydrology.

Based on numerous empirical observations ofancient systems from Cambrian to Holocene, we there-fore propose that the three most common lacustrinefacies associations correspond to distinctive lake-basintypes: overfilled lake basins, balanced-fill lake basins,and underfilled lake basins (Figure 7). Althoughnamed for interpreted genetic factors (to provide pre-dictive, as well as descriptive, utility), each type ischaracterized by readily observable features, such aslithofacies association and stratigraphic packaging,and possesses predictable hydrocarbon generationcharacteristics (Carroll and Bohacs, 1995, 1999).

A short summary of the major characteristics ofeach lake-basin type follows (Table 2); subsequent sec-tions of this paper expand on stratal attributes, hydro-carbon source potential, and associated hydrocarboncharacter. These sections highlight the most com-monly occurring attributes of each lake-basin typeuseful for recognition and mapping in explorationdata, and are not intended to be exhaustive. Note thatmany of the same factors that control source qualityalso influence the distribution of lacustrine reservoirand seal lithofacies.

Overfilled lake basins (Figure 8) occur when therate of supply of sediment+water consistently exceedspotential accommodation (usually when P/E is rela-tively high or rates of tectonic subsidence are rela-tively low). The resulting lake hydrology is openeither permanently or dominantly over the time spanof accumulation of depositional sequences. Climati-cally driven lake-level fluctuations are minimalbecause water inflows are in equilibrium with out-

flows. These lakes are very closely related to perennialriver systems; their deposits are commonly interbed-ded with fluvial deposits and coals, which is the fluvial-lacustrine facies association. Parasequence develop-ment is driven predominantly by shoreline prograda-tion and delta-channel avulsion.

Balanced-fill lake basins (Figure 9) occur when therates of sediment+water supply and potential accom-modation are roughly in balance over the time span ofsequence development. Water inflows are sufficient toperiodically fill available accommodation, but are notalways matched by outflow. As a result, climaticallydriven lake level fluctuations are common. Lakehydrology is closed during deposition of basinallyrestricted lowstand strata and open during highstanddeposition of obliquely prograding strata. Deposi-tional sequences record a combination of progradationof clastic sediments and aggradation of mostly chemi-cal sediments due to desiccation, which is the fluctuat-ing profundal facies association. These depositstypically have the highest organic enrichments of anynoncoaly source rocks due to an optimal combinationof primary production, mean water depth, chemicalstratification, and rates of burial.

Underfilled lake basins (Figure 10) occur whenrates of accommodation consistently outstrip avail-able water and sediment supply, resulting in a persis-tently closed basin hydrology with deposits ofephemeral lakes or brine pools and playas inter-spersed with those of relatively “perennial” lakes.Individual lakes are geologically short-lived, henceparasequences and sequences are commonly verythin (on the scale of decimeters). Parasequence stack-ing mainly records vertical aggradation of the prod-ucts of desiccation cycles, which is the evaporativefacies association. These deposits are composed ofhighly contrasting lithologies, commonly associatedwith evaporites.

Due to changes in climate or tectonic subsidence,nonmarine basins commonly evolve from one laketype to another through a variety of time scales in apredictable pattern (cf. Lambiase, 1990). Predictablesuccessions of lake-basin types can arise fromeither a climatic or tectonic cycle, following amostly vertical or horizontal trajectory on the lake-basin-type phase diagram (Figure 7). These changes arecommonly recorded within a single formation. Forexample, the Green River Formation shown in Fig-ure 6 records a primarily tectonic cycle (horizontaltrajectory) in its subdivisions: fluvial (Wasatch For-mation) to overfilled (Luman and Niland tongues)to balanced-fill (Tipton Member) to underfilled(Wilkins Peak Member) to balanced-fill (lowerLaney/LaClede) to overfilled (upper Laney/LaClede)to fluvial (Washakie Formation). Different laketypes can also coexist in adjacent basins (e.g. ,Eocene Green River Formation in Washakie, Uinta,and Piceance Creek basins) (Bradley, 1964); chainsof modern lakes especially illustrate that the rela-tion of sediment+water supply to potential accom-modation change in each individual lake controlslake type and character.


Ancient deposits may not be directly comparable tomodern lakes, for which only a synoptic (“snapshot”)view is available, because the record of a lake systemis mainly controlled by the response of its sedimen-tary systems to changing hydrology over time (e.g.,Kelts, 1988). The shorthand lake-basin-type names are

intended to summarize the relation of potentialaccommodation and supply of sediment+water andthe resulting lake hydrology over the time span ofaccumulation of the common associations of lithofa-cies and stratal stacking patterns. This time spantypically corresponds to the scale of depositional

Figure 3—Examples of fluctuating-profundal lacustrine facies association, typical lithologies, and stratal packages (depositional sequences). Note complex interbedding of clastic and carbonate lithologies in bothphysical and biogenic sedimentary structures and similar stratal stacking patterns despite different ages, paleolatitudes, and overall thickness (note different scales). Subenvironments: 1 = fluvial/floodplain, 2 = lakeplain and supralittoral, 3 = littoral, 4 = sublittoral, 5 = lake-floor “fan”/turbidite channel, and 6 = profundal.Subenvironments delineated based on indices of bottom energy, subaerial exposure (pedogenesis), and tracefossils; preservation of vertebrate fossils; and organic and inorganic geochemistry. Data derived from Eocenelower Laney member, Green River Formation, Trail Dugway outcrop (Sec 18-T14N-R99W), Washakie basin,Wyoming (Horsfield et al., 1994), Jurassic lower Portland Formation, Park River Tunnel core (41° 45.4’N, 72°42’W), Hartford basin, Connecticut (K. M. Bohacs, 1992, unpublished company report based on Park RiverTunnel project cores), Permian Lucaogou Formation, Tianchi aqueduct outcrop (43.7°N 84.3°E), Junggar basin,China (Carroll, 1998), and Permian Unidad Roja Superior, Fuentes de Izas outcrop (42.45°N 0.28°W), Aragón-Béarn basin, Spain (Valero Garcés, 1991).

10 Bohacs et al.

sequence to sequence set (or tongue to member; sev-eral meters to hundreds of meters). The relative sim-plicity of lacustrine facies associations points out theimportance of the geological “filter” in determiningwhat modern processes leave preservable records (seealso discussion in Shanley and McCabe, 1994). All sed-imentary rocks represent a time-averaging of relatedbut diachronous and evolutionary depositional envi-ronments. Lakes, in particular, are extremely dynamic,with high rates of change of environmental parame-ters, and vary widely in their sensitivity to climaticand hydrodynamic changes (Hutchinson, 1957; Cole,1979; Gierlowski-Kordesch and Kelts, 1994).

Within our framework, water depth is a secondaryattribute of lakes. Water depth can be portrayed as athird, independent axis within the lake-basin-typephase diagram (Figure 7), separating the percent ofsediment+water supply that is water from the potentialaccommodation that remains unfilled with sediment.Although terms such as “deep” and “shallow” are rel-ative, it is possible to subdivide each lake-basin typeinto shallow and deep subtypes based mainly oncoarse clastic facies and stratal geometry. Table 3 listssome key examples of each type. In general, com-pared to their shallow subtype, deep lake-basin typestend to have more deposits associated with high-reliefprocesses, thicker stratal packages, and betterexpressed stratal geometries. Basinally restrictedstrata are more distinct and aggradational and basin-margin erosion, downlap, and onlap are better devel-oped; however, basinward shifts of facies tend to beshorter and more subtly expressed. Clearly, “deep”and “shallow” do not control the essential attributesof each lake-basin type, but “deep” and “shallow” canstrongly influence many accidental attributes, such assequence thickness, areal extent, shoreline type, andgeographic distribution (especially reservoir facies)(see Neal et al., 1997).

Our use of the terms “overfilled” and “underfilled” isdirectly analogous to that employed in marine basins(Marzo et al., 1996; Ravnas and Steel, 1998) and evolvedwithin our company at just about the same time (firstpublished in Carroll and Bohacs, 1995). It is, however,distinctly different in application and consequences forlake-basin fill because of the fundamental differencesbetween lake and ocean basins, detailed in the previoussection. Workers on marine rift basins based their termson the interaction of total accommodation and sedimentsupply alone, and not on potential accommodation andsupply of sediment+water. Most important, the exis-tence of a sill controls the very existence of a lake and itsabsolute maximum amount of accommodation. It neces-sitates the definition of a balanced-fill lake state andenables the explanation of the fluctuating-profundallacustrine facies association, reconciling the closeinterbedding of desiccational and progradational shore-line parasequences.

Other workers have rightly pointed out the varietyof controls on lake strata and organic-rich rock accu-mulation (e.g., Zhou, 1981; Powell, 1986; Kelts, 1988;Watson et al., 1987; summary in Katz, 1990). Whatwe found most helpful was the close examination of

the profundal and sublittoral strata associated with theorganic-rich rocks. Their nature and distributionrecord the complex interaction of depositional andpreservational controls integrated over space and time.This integrated physical-chemical-biological approachallows robust recognition and prediction of lake-basintypes and their associated source and hydrocarbonpotential. By concentrating on fundamental controls,one can see how primary interactions affect all aspects ofa lake system and its geological record.

SEQUENCE STRATIGRAPHY OF LAKE-BASIN TYPES

IntroductionLake-basin type strongly affects the physical char-

acter and distribution of strata and the expression ofthe parasequences and sequences they accumulate.Each type has characteristic expression and distribu-tion of hydrocarbon play elements (Figures 8–10).Their expression ranges from similar to shallow-marine sequences in some overfilled lake basins todifferent in underfilled lake basins. The range ofexpression is detailed below.

The sequence-stratigraphic approach, looking atpackages of rocks bounded by physical surfaces toconstruct a chronostratigraphic framework, workswell in lacustrine and alluvial strata. The expression ofdepositional sequences in these settings can varywidely from the “normal” marine case, but these vari-ations are readily understood and predicted within asequence framework. As with all depositionalsequences, their character is controlled by the com-bined influence of base level (lake level and thegroundwater table), sediment supply, and tectonics.The influence of climate is recorded in both water sup-ply (lake level and groundwater table) and the sedi-ment supply. The expression of lacustrine depositionalsequences varies widely because lake systems them-selves are greatly variable. Lakes are much moreresponsive due to the smaller volumes of water andsediment involved, their frequently closed-basinnature, and their generally closer tie of sediment sup-ply to lake level (Gierlowski-Kordesch and Kelts, 1994;Neal et al., 1997, 1998; Carroll and Bohacs, 1999).Although lacustrine systems can vary more rapidlyand widely (Figure 7), this does not affect the utility ofthe sequence-stratigraphic approach because the rocksare still deposited in layers bounded by physical sur-faces that can be used as time lines. Indeed, sequencestratigraphy is especially appropriate for lakesbecause of its focus on integrating observations ofmany scales of sedimentary changes and hiatuses in ahierarchy that spans millimeters to kilometers.

Each lake-basin type tends to have characteristicstratal patterns that arise from typical phase relationsbetween sediment+water supply and lake level(accommodation) summarized as follows. Lake typeshave distinctive facies stacking patterns at thesequence and parasequence scale that reveal the linkof sediment+water flux to changing lake level.


Understanding this link leads to predictability in thesequence stratigraphy of each lake-basin type.

Sequence Expression In Lake TypesOverfilled lake basins tend to accumulate fluvial-

lacustrine facies associations in depositional sequencesthat can appear very similar in geometry and develop-ment to both Vail/Exxon-type-1 and Vail/Exxon-type-2

sequences in shallow-marine siliciclastic settings,although they are generally thinner (Figure 8)(Bohacs, 1995, 1998). Observations show that floodingsurfaces tend to have relatively minimal lithologiccontrast across them. Depositional sequence bound-aries tend to show either minimal or very distinctiveerosional reworking. Depositional geometries aregenerally well expressed due to the relatively largesediment supply and are readily recognized on seis-

Figure 4—Examples of evaporative lacustrine facies association, typical lithologies, and stratal packages (depositional sequences). Note wide variety of lithofacies in sedimentary structures due to physical transport,desiccation, and crystallization, and similar stratal stacking patterns despite different ages, paleolatitudes, and overall thickness. Subenvironments: 1 = fluvial/floodplain, 2 = lake plain and supralittoral, 3 = littoral, 4 = sublittoral, 5 = lake-floor “fan”/turbidite channel, and 6 = profundal. Subenvironments delineated basedon indices of bottom energy, subaerial exposure (pedogenesis), and trace fossils; preservation of vertebratefossils; and organic and inorganic geochemistry. Data derived from Eocene Wilkins Peak member, GreenRiver Formation, UPRR 41-43 core (NENE Sec 23-T17N-R109W), Bridger basin, Wyoming (Bohacs, 1998), Triassic Balls Bluff siltstone, Culpeper Crushed Stone Quarry (Stevensburg, Virginia), Culpeper basin, Virginia (Gore, 1988, 1989), Permian Jingjingzigou Formation, Tianchi aqueduct outcrop (43.7°N 84.3°E), Junggar basin, China (Carroll, 1998), and Pleistocene–Holocene Lake Bogoria, cores and outcrops (0.15°N36.06°E), (Renaut and Tiercelin, 1994).

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ssM

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s

1 3 5

Profundal

Sub Environ

1 3 5

Ss

mS

ssM

sM

s

1 3 5

Fluvial

Profundal

Sub Environ

Jingjingzigou Fm

1 3 5S

s

mS

s

sMs

Ms

1 3 5

Fluvial

Profundal

Sub Environ

Wilkins Peak Mbr Lake Bogoria

1 3 5

1 3 5

Fluvial

Fluvial

Profundal

Sub Environ

Balls Bluff Siltst.

Ss

mS

ssM

sM

s

10m

5m

?

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C

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G

#

#

#

#

#

L

#

#

5m

10m

Downlap Surface (max. flooding surface)

Flooding Suface

Transgressive Surface (top Lowstand)

Sequence Boundary

12 Bohacs et al.

mic and in well-log cross sections and outcrop. Based onobservations of two distinctive stratal-stacking patternsof the fluvial-lacustrine facies association, we inter-pret two states of overfilled lake basins, based onwhether and how far lake level falls below sill height.In both overfilled lake states, observations indicatethat shoreline progradation dominates parasequencedevelopment, resulting in the stacking of indis-tinctly expressed parasequences up to 10 m thick.Sequence boundaries and systems tracts are whatvary in development between the two lake states. Inpermanently overfilled lake basins, wherein lakelevel remains at or very near sill height, stratal pat-terns are dominated by variations in sediment sup-ply, and erosive sequence boundaries arenonexistent to minimally developed (Neal et al.,1997). This is analogous to a classic type 2 depositionalsequence boundary, wherein no significant down-ward shifts of facies belts or basinally restricted low-stand deposits are developed (Posamentier and Vail,1988). Flooding surfaces are enhanced in this casebecause they form by decreased sediment supply.Depositional geometries in this overfilled lake statereflect changing sediment flux at fixed lake level. Bycontrast, in dominantly overfilled lake basins, lakelevel occasionally falls significantly below sill height,enabling extensive erosion and incised-valley forma-tion during lake-level fall. These lakes form deposi-tional sequences that look similar to classic type 1sequences in shallow-marine siliciclastic settings.Relatively low organic richness and an abundance of

terrestrial organic matter are typical. Reservoirs gener-ally are best developed in highstand clastic shorelinestrata, and occasionally in charophytic algal litho-somes and in lowstand incised valley fills and lake-floor “fans” (basinally restricted turbidite andmass-flow deposits). Seal facies tend to be best andmost extensively developed in distal transgressive andhighstand prodelta strata. Examples of this lake typeare listed in Table 4.

Balanced-fill lake basins generally accumulatefluctuating-profundal lacustrine facies associationsin depositional sequences that can be somewhat sim-ilar to or distinctly different from shallow-marinesiliciclastic sequences (Figure 9) (Bohacs, 1995, 1998).Observations indicate that parasequences record acombination of progradation of clastic sediments andvertical aggradation of chemical sediments due todesiccation cycles. Carbonates are generally abun-dant in these lakes, hence their parasequences anddepositional sequences are more similar to shallow-marine carbonate or mixed carbonate-clastic settings.Flooding surfaces are commonly well expressed indistinct lithologic contrasts. Sequence boundariestend to be marked by large basinward shifts of depo-sitional environments with minimal erosion and inci-sion. Lowstands (basinally restricted strata) in shallowbalanced-fill lake basins accumulate relatively thinaggradational parasequence sets, with common evi-dence of desiccation, that can be either carbonate orclastic dominated, whereas deep balanced-fill lakebasins can accumulate relatively thick aggradational

Figure 5—(a) Comparison of climatic humidity (precipitation/evaporation or P/E) with surface area and meandepth of large modern lakes. No correlation exists between P/E and any measure of lake size for large modernlakes, even those of tectonic origin. (b) Comparison of climatic humidity with lake chemistry and mixis, showing little relation. Both graphs indicate that predictions of lake existence or character based on climatichumidity alone fail to explain the bewildering complexity of modern lakes, which can range from freshwaterto saline, organically barren to eutrophic, and carbonate-precipitating to purely siliciclastic within a single climatic zone. Data for both figures from Herdendorf (1984).

��yyyyy��yyyyyyyyyy

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0.5100000100001000500

Surface Area (sq. km.)

3.5

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2.5

2.0

1.5

1.0

0.51.0

Pre

cipi

tati

on/E

vapo

rati

on

Average Depth (meters)0.001 0.01 0.10

Modern Lake Observations

(No Lakes observed)

(V. Few Lakes observed)

(No Lakes observed)

(V. Few Lakes observed)

(a)


lake-floor turbidite “fans.” Indeed, the great variabil-ity of lowstand expression is a key defining trait of thislake-basin type. Lake hydrology is interpreted to beclosed commonly during this stage. Transgressivesystems tracts tend to be relatively thick and welldeveloped, marked by significant retrogradationalstacking and some basal erosion. Highstands rangefrom relatively thin to moderately thick and from car-bonate to clastic dominated, interpreted as typicallyaccumulated under open hydrologic conditions. Theycan show well-developed aggradation and someprogradation, expressed as sigmoidal to obliqueprogradation on seismic. Strata generally thin basin-ward by widely spaced downlapping. For this lake-basin type, we interpret that sediment supply is moreclosely linked to lake level because often the same

rivers supply both water and sediment to the lake inan intermittently open hydrologic system. When therivers dry up, lake level falls due to decreased waterinput. The erosive power of the river and its sedimentsupply would also decrease with lake-level fall, butincrease during lake-level rise. This contrasts with theexplicit assumptions for shallow-marine depositionalsequences (Posamentier and Vail, 1988), thus givingrise to the contrasting expression of depositionalsequences in this lake type. Organic matter types canbe mixed algal and terrigenous, although commonlydominated by type I algal-bacterial kerogen. Reservoirfacies can include lake-floor “fans,” incised-valley fills,and shoreline clastics or carbonates deposited duringtransgressions and highstands. Seal-prone facies arewidespread and well developed in late transgressive

Figure 5—Continued.

0

10

20

Precipitation/Evaporation

0

10

20

Precipitation/Evaporation

Fresh

Brackish

Saline

Hypersaline

Amictic

Merom

ictic

Dimictic

Polym

ictic

Mixis Type

Water Chemistry

Large Modern Lakes(b)

14 Bohacs et al.

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and early highstand systems tracts as prodeltamudrocks and sublittoral marls or micrites. Examplesof this lake-basin type are listed in Table 5.

Underfilled lake basins accumulate evaporitic lacus-trine facies associations in depositional sequences thatare distinctly different from shallow-marine siliciclasticor carbonate settings (Figure 10) (Bohacs, 1995, 1998).The nature and distribution of depositional environ-ments within this lake-basin type can change drasti-cally from small evaporitic ponds on broad playamud/salt flats at lowstand to broad “perennial” lakewith lake-plain streams at highstands. Observationsindicate that sequence boundaries are subtly expressedin underfilled lake basins, whereas flooding surfacesare marked by distinct lithologic contrasts. Floodingsurfaces are commonly coincident with sequenceboundaries (FS/SB) across large portions of the basinarea. Depositional geometries are generally parallel tosubparallel and dominated by aggradational stacking.

Strata thin basinward by convergence, although somebasinward thickening can occur in strongly evaporiticsettings. Lowstand deposition is commonly restrictedto evaporites or other chemical/biogenic sedimentsformed in remnant pools in the highest subsidencearea of the basin; the bulk of the lowstand record isonly desiccation features modifying underlying high-stand or other strata (mudcracks, soil formation, etc.).Transgressive systems tracts are typically recorded bythin, widespread clastic sheet-flow deposits at the base(some local erosion is possible), reflecting the rejuve-nation of river input. These deposits are overlaindirectly by distal lake strata, commonly organicallyenriched, marking the rapid spread of the lake over alow-relief surface. Highstands are relatively thick, buttypically composed of only one or two parasequences.Channel-fill sandstones are found in some highstandlake-plain strata, reflecting the development and inte-gration of drainage systems during this wet phase of

Table 2. Characteristics of Lake-Basin Types: Strata, Source Facies, and Hydrocarbons.

16 Bohacs et al.

lake development. We interpret that parasequencedevelopment and stacking in the lake center primarilyreflects vertical aggradation of the products of desicca-tion cycles, with minimal advected clastic input. Sedi-ment supply appears tightly linked to lake level. Lakelevel falls by the net withdrawal of water by evapora-tion and percolation, hence sequence boundaries aremarked by extensive indicators of subaerial exposureand minimal erosion. The high primary production ofthese lakes, low fluvial input, and paucity of terres-trial vegetation in the drainage basin result in produc-tion of rich type I algal organic matter, but ultimatepreservation is relatively low due to frequent desicca-tion. Reservoir facies are best developed in transgres-sive sheetflood clastics, early highstand fluvialchannels, and late highstand shoreline carbonategrainstones. Seal-prone facies are most widespread inupper transgressive and basal-highstand systemstract strata. Examples of this lake-basin type are listedin Table 6.

Another key element to recognize is that some lakebasins become connected to the marine realm at high-stand. This allows lake level to vary independently ofsediment+water supply to the lake basin, resulting in a

variety of stratal patterns that are controlled by thephase relations between sea level and sediment+watersupply. (These relations vary by paleolatitude, therebyproviding some predictive capability) (e.g., Perlmut-ter and Matthews, 1990). Intermittent marine con-nection can be beneficial for the accumulation oforganic-rich rocks (Kelts, 1988; Mello and Maxwell,1990; Higgs, 1991). Examples of marine-connectedlake basins are listed in Table 7.

The practical application of recognizing this diversityof lake-basin types boils down to the necessity of usingthe sequence-stratigraphic approach to understand thelake system and to select appropriate models for pre-dicting the distribution of play elements. A two-phased approach is most helpful: (1) Use the standardsequence-stratigraphic approach to make sense of allthe data to recognize and correlate the hierarchy of sur-faces (flooding surfaces/parasequence boundaries,channel bases, parasequence-set boundaries, downlapsurfaces, and sequence boundaries) and (2) apply theunderstanding derived from constructing the sequenceframework to select appropriate lake-basin type andsequence-stratigraphic models for making predictionsaway from control.

Overfilled

Balanced-Fill

Underfilled

SEDIM

ENT+WATERSUPPY

(linkedto

precip'n/evap'n)

Eolian

Fluv

ial

POTENTIAL ACCOMMODATION

(linked to basin subsidence)

P/E=

1.0

P/E=?

(LowPreservation)

Thin

Source Intervals

Thick

Source Intervals

Figure 7—Lake-basin type phase diagram showing existence and character of nonmarine strata in general andlacustrine strata in particular as a function of both sediment+water supply and potential accommodation.Interaction of these two controls is reflected in the lithology, stratal stacking, biota, and geochemistry of lakedeposits. Potential accommodation is the space available for sediment accumulation below the basin’s outletor spillpoint (a key difference from marine systems) (Carroll and Bohacs, 1995), and it is mainly influenced by basin tectonics, along with sill uplift and erosion, and inherited topography. Sediment+water supply isprimarily a function of climatic humidity, along with seasonality, local relief, and bedrock geology. (Clasticsediment yield is a nonlinear, nonmonotonic function of climatic factors, generally peaking in semi-arid, distinctly seasonal climates) (Einsele, 1992).


CONTROLS ON ORGANIC-RICH ROCK DEPOSITION

Lake-basin type also strongly influences production andaccumulation of organic matter that forms source rocks asdetailed in Table 8. Recognizing the lake type allows one topredict geochemical attributes from geologic data.

Organic enrichment in lacustrine rocks is a functionof the same basic factors as for other environments,and can be expressed as a simple relation (e.g., Bohacs,1990, 1998):

Optimum organic enrichment occurs where pro-duction is maximized and destruction and dilution are

minimized. Any appropriate combination of these fac-tors can produce potential source rocks.

Production refers chiefly to the photosynthetic fixa-tion of CO2 by organisms and can include bothautochthonous organic matter derived from algae andaquatic plants and allochthonous organic mattertransported from land (Figure 11). The primary pro-duction of organic matter in a lake is a function ofmany variables, including solar input, wind, precipi-tation, water chemistry, and temperature (Kelts,1988). Of these, solar input and water chemistry havethe largest effect on overall primary production (Katz,1990).

Solar input controls the energy available for photo-synthetic production. Production decreases withincreasing latitude, area/depth ratio, and turbidity dueto the decrease in duration and intensity of solar energyavailable for photosynthesis. In general, primary

Littoral

T O TDS2Epilim

nion

Thermocline

Hypolim-

nion

Littoral

Sub

++

++ +

+

++

+

+

+

+

Profundal

T = TemperatureO2 = Oxygen Conc.

TDS = Total Dissolved Solids(Not to scale) Low-relief margin/shallow basin

High-relief margin/

deep basin

Sandstone

Siltstone

Mudstone

Coal

Scour

Macrophytes

Molluscs

Ostracods

Conglomerate Carbonates

Evaporites

Tufa

Mudcracks

Ooids

Stromatolites

Phytoplankton

##

kmb

Figure 8—Schematic diagram highlights major features of overfilled lake basins: persistently open hydrology,freshwater lake chemistry, high groundwater table, progradational shoreline architecture, close relation to fluvial systems, and commonly interbedded fluvial deposits and coals. (Not shown are bioclastic and otherfreshwater carbonate lithosomes.) This lake-basin type occurs when the rate of supply of sediment+water consistently exceeds potential accommodation (usually when P/E is relatively high compared to rates of tectonic subsidence). Climatically driven lake-level fluctuations are minimal because water inflows are inequilibrium with outflows. (Left side of the diagram schematically represents shallow overfilled lake basin or low-relief margin; right side of the diagram schematically represents deep overfilled lake basin or high-relief margin.)

Organic Enrichment =Production – Destruction

Dilution

18 Bohacs et al.

production in modern lakes peaks at a mean depth ofabout 18 m (Cole, 1979).

The water chemistry of a lake controls the availabil-ity of nutrients to support primary production oforganic matter. Water chemistry is strongly influencedby basin hydrology and climate (Hardie and Eugster,1970), whose integrated effects may be discernedthrough lake-basin type. The nutrients available to alake are brought in by overland flow from the lakecatchment area or by eolian transport, so production isalso, in part, related to bedrock geology (e.g., Cerling,1994). Most nutrients are recycled during mixing oflakes; permanently stratified lakes, in fact, may requirean external nutrient load to support intense primaryproduction (Katz, 1990). Alkaline waters help supporta much higher level of primary production thanwaters of neutral pH because of the abundance of CO3ions available for incorporation by plants in additionto atmospheric CO2 (Kelts, 1988). For these reasons,

the highest natural primary productivities on earthoccur in tropical alkaline lakes (Likens, 1975).

The autochthonous organic matter of a lake may becomposed of benthic, higher plants, planktic and ben-thic algae, and bacteria (Figure 11). Size and waterdepth control the proportion of each of these contribu-tions. Small, shallow overfilled lakes are commonlydominated by benthic higher plants, and the organicsediments tend to be peat rich (Cole, 1979). In contrast,large, deep balanced-fill lakes can have a relativelylimited area of littoral benthic plant growth, and so thedominant organic matter deposited in the lake tends tobe planktic and benthic algal and bacterial matter.

Allochthonous organic matter originates mostlyfrom higher land plants. Deposition of allochthonousorganic matter is greater in the parts of the lake thatare proximal to deltas and fluvial input. Input may beseasonal or episodic, related to weather/climaticevents in the basin. Allochthonous organic matter may

Figure 9—Schematic diagram highlights major features of balanced-fill lake basins: intermittently open hydrology, fluctuating groundwater table and lake water chemistry, interaction of thermal and chemical stratification, mixed progradational and aggradational shoreline architecture, and varied interbedding of clastic andcarbonate strata. This lake-basin type occurs when the rates of sediment+water supply and accommodation areroughly in balance over the time span of sequence development. Water inflows are sufficient to periodically fillavailable accommodation, but are not always balanced by outflow; hence, climatically driven lake level fluctuations are common. (Left side of the diagram schematically represents shallow balanced-fill lake basin or low-relief margin; right side of the diagram schematically represents deep balanced-fill lake basin or high-relief margin.)

T O TDS2

+

++

+ +

+

+

++

+

++

+

Sandstone

Siltstone

Mudstone

Coal

Scour

Macrophytes

Molluscs

Ostracods


Evaporites

Tufa

Mudcracks

Ooids

Stromatolites

Phytoplankton

##



High-relief margin/

deep basin


also be blown into the lake by wind, although this isgenerally a small contribution to the overall deposi-tion of organic sediments in large lakes. Overfilledlake basins are the most likely to receive input ofallochthonous organic matter due to the proliferationof forests in humid climates and to the existence ofwell-integrated fluvial systems (e.g., Kelts, 1988;Sladen, 1994).

For source potential, then, autochthonous aquaticorganic matter is most important because its lipid-richcell membranes usually form oil-prone kerogens.These membranes are sensitive to lake type becausethey are used to control cellular osmotic pressure inaqueous environments; biomarker distributions there-fore reflect the prevailing water chemistry (e.g., Mey-ers and Ishiwatari, 1993; Peters and Moldowan, 1993).Allochthonous organic matter from land plants is, ingeneral, more likely to create gas-prone kerogens.

Destruction of organic matter is primarily a func-tion of the efficiency of various scavengers, particu-larly bacteria, which, in turn, is largely controlled bythe availability of oxygen. The supply of oxygen tolake waters occurs via exchange with the atmos-phere and as a byproduct of photosynthesis. In well-oxygenated water columns, most of the primaryorganic production is destroyed by microbial respira-tion, converting the organic matter back into CO2(Cole, 1979). Oxygen is depleted by biological andinorganic oxidation, and if the oxygen is not renewed,the waters will become anoxic, favoring preservationof organic matter. Anoxic bottom waters enhanceorganic preservation by limiting the activity of scav-engers and bacterial respiration (e.g., Demaison andMoore, 1980; Kelts, 1988). The extent and duration ofoxygen deficiency in a lake are controlled by the inten-sity and frequency of mixing, as well as by primary

Figure 10—Schematic diagram highlights major features of underfilled lake basins: persistently closedhydrology, characteristic chemical stratification, low groundwater table, high solute content of lake waters,extensive desiccation features, highly contrasting lithologies, common association with evaporite deposits,and dominantly aggradational shoreline architecture. This lake-basin type occurs when rates of accommodationconsistently outstrip available water and sediment supply, commonly resulting in a persistently closed basinwith ephemeral lakes interspersed with playas or brine pools or both. (Left side of the diagram schematicallyrepresents shallow underfilled lake basin or low-relief margin; right side of the diagram schematically represents deep underfilled lake basin or high-relief margin.)

T O TDS2Mixolim

nion

# #

# # #

#

++ + + +

+

+ +

+

++

+

+

+

# # ##

# ###

Chemocline

Monimolim

nion

Sandstone

Siltstone

Mudstone

Coal

Scour

Macrophytes

Molluscs

Ostracods


Evaporites

Tufa

Mudcracks

Ooids

Stromatolites

Phytoplankton

##



High-relief margin/

deep basin

20 Bohacs et al.

production of organic matter and water chemistry. Mixing of lake waters is inhibited when a lake is

thermally or chemically stratified, hence oxygen inthe bottom waters of the lake cannot be renewed.Chemical stratification is common in balanced-fill andunderfilled lake basins. It is especially well developedin deep balanced-fill lake basins and in marine-connected overfilled and balanced-fill lake basins.Intermittent input of denser sea water generates per-sistent ectogenic meromixis resulting in excellentorganic preservation (e.g., Raisz, 1937; Mello and Hes-sel, 1998). Thermal stratification is practically essentialfor organic-rich rock accumulation in deep overfilledlake basins (e.g., Dean, 1981; Demaison and Moore,1980). Very deep lakes, however, can have high ratesof organic destruction due to the longer time requiredfor particles to settle through deep, well-oxygenatedsurface waters (Katz, 1990, 1995).

Some lakes remain stratified because high contentsof dissolved organic matter in bottom waters depletethe oxygen content (see discussion in Katz, 1990).These biologically stratified lakes are associated withhigh levels of organic production and currently occurmost commonly in rift basins in tropical climates.They may start out as thermally stratified oligomicticlakes; however, the buildup of dissolved organic car-bon in the bottom waters enhances the stability ofstratification. Excessive primary production (hyper-trophication) can also apparently overwhelm thedelivery of oxygen to the lake bottom, with goodorganic preservation in relatively shallow lakes (e.g.,Horsfield et al., 1994; Bohacs et al., 1996). Theseprocesses are most important in shallow balanced-filllake basins.

Anaerobic degradation is strongly influenced bythe concentration of sulfate in lake waters (e.g., Pow-ell, 1986; Kelts, 1988). Sulfate content of lake waters is afunction of both catchment area input and lake-basinconcentration. Sulfate-reducing bacteria dominateanaerobic consumption in the presence of excess sul-fate, a condition most common in marine-connectedand some underfilled lake basins. Less efficient fer-menters and methanogens prevail in lakes with lowsulfate concentrations (more likely in overfilled andbalanced-fill lake basins).

Dilution by mineral sediments can limit source rockrichness by decreasing the proportion of organic mat-ter relative to inorganic matrix. Organic matter in pro-fundal sediments is diluted by mainly detritalsediments from rivers at deltas or from wave rework-ing of the shorelines and littoral zone. Deltas representa large addition of sediment as bedload and sus-pended load to a lake. Organic-rich sediments formfarther away from points where rivers enter a lake(Kelts, 1988; Sladen, 1994). In many elongate lakes inrift and wrench settings, drainage is axial to the basin,and rivers and deltas are formed at one or both ends ofthe lake. In these cases, deposition of organic-rich sed-iments is displaced away from the clastic-dominatedends of the lake (e.g., Crossley, 1984; Nichols 1987;Cohen, 1989). If organic concentration were low, thequantity of hydrocarbons (particularly oil) expelledduring generation would be limited by retention onmineral surfaces (Sandvik et al., 1992). In extremecases, organic matter can be completely overwhelmedby clastic sediment input.

Table 8 summarizes the effect of different lake typeson each of these factors. Each lake type has characteristic

Table 3. Examples of Shallow and Deep Lake-Basin Types.

Lake Basin Type Formation Basin, County Age

Overfilled, Shallow Modern Sudd, Sudan Pleistocene–HoloceneLuman Tongue, GRF* Washakie, U.S.A. EoceneQuantou-3 Songliao, China Albian

Overfilled, Deep Modern Lake Baikal Pleistocene–HoloceneKissenda N’Komi, Gabon BerriasianMako to Nagyaföld Pannonian, Hungary U. Miocene–M. Pliocene

Balanced Fill, Shallow Modern Lake Victoria Pleistocene–HoloceneLaney Mbr, GRF* Washakie, U.S.A. EoceneLucaogou Junggar, China Permian

Balanced Fill, Deep Modern Lake Malawi Pleistocene–HoloceneBucomazi Cabinda/Gabon NeocomianCandeias Reconcavo, Brazil Neocomian

Underfilled, Shallow Modern Dabusun Lake Pleistocene–HoloceneWilkins Peak Mbr, GRF* Bridger, U.S.A. EoceneBlanca Lila Pastos Grandes ** PleistoceneLagoa Feia (upper) Campos, Brazil Aptian

Underfilled, Deep Modern Issyk-Kul Lake Pleistocene–HoloceneLisan Dead Sea, Israel Pleistocene–Holocene

* GRF = Green River Formation.** Argentina.


ranges of total organic carbon contents (TOC) andhydrogen indices (HI) and associations of organic mat-ter (noted on the right side of the table); a specificexample from the Green River Formation is shown onFigure 12. The optimal lake for source rock depositionis a compromise among production, destruction, anddilution: it is shallow enough to have high primaryproduction, deep enough to preserve a significantproportion of the resultant organic matter, andstarved of sediment but not of nutrients. These condi-tions appear from the geological record of ancientlakes to coincide with balanced-fill lake basins, inwhich deposits commonly attain more than 20% TOCin oil-prone kerogens. For example, in the Washakiebasin, the balanced-fill lower Laney Member has sig-nificantly richer potential source rocks than the otherintervals of the Green River Formation (LumanTongue, Wilkins Peak Member) (Figure 12).

Overfilled lake basins have good aquatic productionand abundant land-plant input. The main challenges toorganic enrichment are preservation and dilution.Good preservation requires thermal stratificationbecause solute-controlled density stratification isunlikely to develop under persistently open hydrology(unless marine connection occurs). Dilution is commondue to strong fluvial influence. Resultant organicenrichment is therefore most likely above flooding sur-faces, especially around maximum flooding surfaces(mid-sequence downlap surfaces), in profundal strata,and within some intervals of lake-plain strata associ-ated with mire and pond environments (Bohacs, 1998).

Balanced-fill lake basins experience optimal combina-tions of production, preservation, and dilution; produc-tion is boosted by intermittent fluvial input of nutrientssubsequently concentrated by evaporation. Shallow bal-anced-fill lake basins have the largest portion of theirvolume in the photic zone for the longest portion of theirhistory. This lake-basin type records the highest primaryproduction levels (e.g., Horsfield et al., 1994; Sladen,1994; Carroll, 1998). Preservation is enhanced by com-monly developed stable chemical stratification and dilu-tion is minimized because the maximum fluvial inputrates occur on transgressions, which traps most clasticsnear shore. Organic enrichment is most likely aboveflooding surfaces at the parasequence scale and in thelower portion of highstand systems tracts (Horsfield etal., 1994; Bohacs, 1998).

Underfilled lake basins can have high primary pro-duction. Recent shallow saline lakes can have surficialsediments with TOCs over 6% (Burne and Ferguson,1983; Boon et al., 1983). Clastic dilution tends to be min-imal because most fine-grained clastics are trapped inthe lower lake plain and nearshore environments atlake highstand. Peak organic enrichment occurs justabove the initial transgressive surface (base of trans-gressive systems tract) (Bohacs, 1998). Long-termpreservation can be extremely problematic becausefrequent and prolonged desiccation and exposuredegrades most organic matter. In present-day solarlakes in Sinai, organic matter is degraded to a depthof at least 66 cm (surface TOC = 9.7% decreases to3.0%) (Boon et al., 1983), which is about the thickness

of typical underfilled lake-basin parasequences. Forexample, low preservation of organic matter prevailsin Cambrian lake shales in south Australia (TOC<1.1%), and Jianghan basin in China (TOC <1%) (Pow-ell, 1986; Carroll, 1998). Other underfilled lake basinspreserve significant organic-rich rocks, such as theWilkins Peak Member (Wyoming; TOC <19%, HI<1054 mg HC/g) (Bohacs, 1998) and JingjingzigouFormation (Junggar basin; TOC < 6.6%, HI <794 mgHC/g) (Carroll, 1998).

INFLUENCE OF LAKE-BASIN TYPE ONHYDROCARBON TYPE

Lake-basin type influences hydrocarbon generationthrough both the type of organic matter produced andhow it is preserved during burial. Overfilled lakedeposits are likely to contain, on average, higher quan-tities of advected terrestrial organic matter, and there-fore can be expected to generate relatively more gasthan other lake types. In contrast, kerogens from bal-anced-fill and underfilled lake basins both appear tobe principally oil-prone algal-bacterial material. Sulfurcontent of most lacustrine oils is low due to the lowsulfur content of most lake waters and generally highavailability of reactive iron associated with clays (Tis-sot and Welte, 1984). In some underfilled lake basins,however, the combination of extreme evaporative con-centration and low influx of clastic sediments canresult in the preservation of sulfur-rich kerogens,resulting in oils with up to 12% sulfur (Shi et al., 1982;Fu et al., 1985; Sheng et al., 1987).

Especially useful in tying physical and geochemicalcharacter are specific biomarker assemblages of oilsand rock extracts that are associated with organic mat-ter input (terrestrial, aquatic) and lacustrine deposi-tional–preservational environments (solute speciesand concentrations). These associations can help tointerpret lake-basin type from oils and to predict bio-marker distributions from lake-basin type.

Biomarkers can be thought of as molecular fossilswhose basic carbon skeleton is derived from once-livingorganisms. Biomarkers are found in petroleum androck extracts, and provide information about deposi-tional environment, thermal maturity, migration path-ways, and hydrocarbon alteration (Peters andMoldowan, 1993). Biomarker data can be used to inter-pret the relative contributions of terrigenous andaquatic organic matter and the environmental condi-tions under which source rocks were deposited. These,in turn, can be related to lake-basin type (see below).The major diagnostic biomarker compounds used todistinguish lacustrine-sourced oils are tricyclic ter-panes, β-carotane, gammacerane, and, to a lesserextent, the 4-methyl steranes.

Please note that most of the following biomarkerdistributions are from source rocks tied directly to lake-basin type. Oils, however, can show less distinctionbecause they result from maturing an interval ofsource strata that can include several lake-basin typesand mixing of several source facies types. Another

22 Bohacs et al.

Table 4. Examples of Overfilled Lake Basins.

Formation Basin, Country Age References

Anthrocosia shale North Sudetic basin, Permian Mastalerz, 1994Poland

Hongyanchi Formation Junggar basin, Permian Carroll, 1998China

Chinle Formation, Colorado plateau, Triassic Dubiel, 1994Monitor Butte member U.S.A.

Waterfall Formation Culpeper basin, Jurassic Hentz, 1981; Gore, 1989Virginia, U.S.A.

Mangara shale Doseo basin, Cretaceous Genik, 1993Chad

Kissenda Formation N’Komi basin, Cretaceous Kou, 1994Gabon

Pematang shale Kutei subbasin, Eocene Kelley et al., 1995Sumatra, Indonesia

Luman Tongue & Green River basin, Eocene Horsfield et al., 1994upper LaClede bed, Wyoming, U.S.A.Green River Formation

Terengganu shale Malay Basin, Miocene Madon-Mazlan, 1992; Malaysia Creaney et al., 1994

Upper part of Snake Snake River basin, Holocene Wood, 1994; Wood and River basin fill Idaho, U.S.A. Squires, 1998

Lake Baikal Russia Modern Flower et al., 1995

Table 5. Examples of Balanced-fill Lake Basins.


Lucagou Formation Junggar basin, Permian Carroll, 1998China

Lockatong Formation, Newark basin, Jurassic Olsen et al., 1989lower members New Jersey, U.S.A

Alternance de Sokor Termit graben, Cretaceous Genik, 1993Formation Niger

Qingshankou 1 Formation Songliao basin, Cretaceous Schwans et al., 1997China

Lagoa Feia Formation, Campos basin, Cretaceous Carneio de Castro et al., 1981, upper Jiquiá portion Brazil Bertani and Carozzi, 1985,

Abrahão and Warme, 1990Brown shale, Aman trough, central Oligocene Kelley et al., 1995

middle portion SumatraGreen River Formation, Green River basin, Eocene Surdam et al., 1980;

upper Tipton and Wyoming, U.S.A. Horsfield et al., 1994lower Laney members

Middle part of Snake Snake River basin, Pleistocene Wood, 1994River basin fill Idaho, U.S.A.

Units C and D Ribesalbes basin, Miocene Anadón, 1994Spain

Lakes Malawi and East Africa rift system Pleistocene– Scholz and Rosendahl, Victoria Holocene 1990; Scholz , 1995


complicating factor is that hydrocarbons in reservoirsare subject to postemplacement alteration. The numer-ical values in this section are an attempt to summarizerepresentative ranges of oil properties as originallyemplaced with data culled from published literature.

Much of the biomarker signature is due to the twogeneral categories of primary producers (autotrophs)of organic matter in lakes: prokaryotes (blue-greenalgae and bacteria) and eukaryotes (higher plants,algae). The majority of triterpanes are associated with

prokaryotic sources, whereas steranes are producedby eukaryotic organisms. Thus, the triterpane/ster-ane ratio is a rough measure of the prokaryote/eukaryote contribution to the organic material. Assalinity increases in a lake one expects that more sensi-tive eukaryotic organisms (e.g., sterane-producinggreen algae) would give way to the more tolerant bac-teria or cyanobacteria (tricyclic and hopane produc-ing) with a corresponding increase in the triterpane/sterane ratio. Possible exceptions could occur with an

Table 6. Examples of Underfilled Lake Basins.


Jingjingzigou Formation Junggar basin, Permian Carroll, 1998China

Passaic Formation Newark basin, Triassic Olsen et al., 1989New Jersey, U.S.A.

Balls Bluff Siltstone Culpeper basin, Triassic Gore, 1989Virginia, U.S.A.

East Berlin Formation Hartford basin, Jurassic Gierlowski-Kordesch and Connecticut, U.S.A. Rust, 1994

Lagoa Feia Formation, Campos basin, Cretaceous Mello and Maxwell, 1990Alagoas portion Brazil

Argille de Sokor Termit Graben, Cretaceous Genik, 1993Formation Niger

Shahejie 4 Bohai basin, Oligocene Zhou, 1981; Hu et al., 1989; China Chang, 1991; Remy et al.,

1995Green River Formation, Green River basin, Eocene Smoot, 1983

Wilkins Peak member Wyoming, U.S.A.Lake Bogoria Kenyan rift Pleistocene– Renaut and Tiercelin, 1994

HoloceneDabusun Lake Qaidam basin, Modern Yang et al., 1995

China

Table 7. Examples of Marine-Connected Lake Basins.


Bude Formation Westphalian basin, U. Carboniferous Higgs, 1991England

Lagoa Feia Formation, Campos basin, Cretaceous Mello and Hessel, 1998Alagoas portion Brazil

Coquiero Seco Formation, Sergipe-Alagoas basin, Cretaceous deAzambuja Fihlo et al., 1997Moro do Chaves member Brazil

Oligocene lake strata Daban Basin, Oligocene Sagri, et al., 1994of Daban Basin Sudan

Coorong Lakes Younghusband Holocene Warren, 1994peninsula,South Australia

Lake Maracaibo Maracaibo basin, Modern Redfield, 1958Venezuela

24 Bohacs et al.

Tab

le 8

. Con

trol

s on

Org

anic

-ric

h R

ock

Dev

elop

men

t in

Lak

es.*

Lak

e T

ype

Pro

du

ctio

nD

estr

uct

ion

Dil

uti

onS

ourc

e P

oten

tial

Ove

rfil

led

+ N

utri

ent i

nput

incr

ease

d–

Incr

ease

d o

xyge

n su

pply

– A

bund

ant c

last

ic d

etri

tus

• M

oder

ate

to p

oor

oil/

gas

– Fr

esh

wat

er in

put d

ilute

s–

to b

otto

m±

Abu

ndan

t ad

vect

ed•

Mix

ed g

as/

oil

– nu

trie

nts

– H

omog

eneo

us w

ater

±

terr

igen

ous

clas

tics

• M

arke

d la

tera

l var

iabi

lity

– O

vera

ll pr

oduc

tion

–

mas

s m

akes

win

d m

ixin

g T

OC

: <1–

7% (m

uds)

– d

ecre

ases

wit

h in

crea

sing

–

mor

e ef

fect

ive

TO

C: <

80%

(coa

ls)

– la

ke v

olum

e–

Col

d u

nder

flow

OM

T:

mix

ed a

lgal

/

– In

crea

sed

turb

ulen

cete

rrig

enou

s (I

/II

)H

I: 5

0–60

0 m

g H

C/

gR

elat

ivel

y th

ick

(< te

ns o

f met

ers)

Bal

ance

d f

ill

+ A

ppre

ciab

le n

utri

ent i

nput

+ C

lose

d b

asin

and

epi

sod

ic

+ V

aryi

ng, b

ut r

elat

ivel

y •

Mod

erat

e to

exc

elle

nt o

il+

Nut

rien

ts c

once

ntra

ted

by

+ d

ryin

g pr

omot

es d

ensi

ty

+ m

inor

cla

stic

det

ritu

s•

Mos

tly

oil,

som

e ga

s?+

epi

sod

ic d

ryin

g+

str

atif

icat

ion

+ M

inor

com

pone

nt o

f •

Lit

tle

late

ral v

aria

tion

+

Lar

ger

perc

ent o

f lak

e vo

lum

e+

Lar

ge a

mou

nt o

f +

ad

vect

ed te

rrig

enou

s T

OC

: 1–

30%

+ in

pho

tic

zone

+ p

rod

ucti

on c

onsu

mes

+ o

rgan

ic m

atte

rO

MT

: m

ostl

y al

gal (

I), s

ome

+ o

xyge

n at

bot

tom

– E

piso

dic

floo

ds

or fl

ashy

terr

igen

ous

(II)

– d

isch

arge

may

del

iver

H

I: 5

00–7

00 m

g H

C/

g–

sign

ific

ant c

last

ic d

ebri

sR

elat

ivel

y th

in (1

–10m

)U

nd

erfi

lled

±V

aria

ble

nutr

ient

inpu

t- E

piso

dic

dry

ing

oxid

izes

–

Sem

i-ar

id c

limat

es y

ield

•

Poo

r to

exc

elle

nt o

il+

Nut

rien

ts c

once

ntra

ted

by

– or

gani

c m

atte

r–

high

est c

last

ic in

put

• M

ostl

y oi

l+

epi

sod

ic d

ryin

g- E

piso

dic

fres

heni

ng

+ M

inim

al in

put o

f •

Min

imal

late

ral v

aria

tion

– E

xtre

me

conc

entr

atio

n of

– in

trod

uces

oxy

gen,

+

terr

igen

ous

orga

nic

TO

C:

<0.

5–20

%–

solu

tes

kills

org

anis

ms

– co

nsum

ers

+ m

atte

rO

MT

: Alg

al (T

ype

I),

– W

ater

ava

ilabl

e fo

r +

Sig

nifi

cant

am

ount

of

HI:

650

–115

0 m

g H

C/

g–

prod

uctio

n on

ly p

art o

f tim

e+

fill

due

to p

reci

pita

ted

R

elat

ivel

y th

in (m

eter

s)+

min

eral

s

* +

Pos

itiv

e fo

r or

gani

c en

rich

men

t, –

Neg

ativ

e fo

r or

gani

c en

rich

men

t, ±

Var

iabl

e in

flue

nce

on o

rgan

ic e

nric

hmen

t, T

OC

= to

tal o

rgan

ic c

arbo

n, O

MT

= o

rgan

ic m

atte

r ty

pe, H

I = h

ydro

gen

ind

ex.


increased input of a limiting nutrient such as nitrogen,or in a shallow balanced-fill to underfilled lake that ishighly stratified, with a fresh surface layer and a salinebottom layer. In the latter case, both freshwater organ-isms and salinity-tolerant organisms could coexist inthe photic zone with little change in the triterpane/sterane ratio. The other major component of the bio-marker fraction comes from reworking of primaryorganic matter (photosynthetically generated) by het-erotrophs, bacteria whose signature are recordedmostly in hopanes.

Stressed environments (high alkalinity/salinity,typical of underfilled and balanced-fill lake basins) arecharacterized by tricyclics (C20–C24; m/z 191), β-carotane (a C40 compound; m/z 125), and gammacer-ane (a C30 triterpane; m/z 191), all with prokaryotesources. Gammacerane and β-carotane are specificallyassociated with nonmarine, highly saline environ-ments (Peters and Moldowan, 1993). Gammacerane isthought to be derived from protozoa (ciliates) or bacte-ria or both. Gammacerane is derived from tetrahy-manol, which is produced by protozoa in an anoxicenvironment; however, if free sterols are available inthe anoxic environment, protozoa will assimilate themand not synthesize the tetrahymanol precursor ofgammacerane (Tissot and Welte, 1984). In addition, ifthe environment is oxic (to possibly dysoxic), then theprotozoa can generate their own sterols to rigidifytheir cell membranes. The implications are that if asource rock has a low triterpane/sterane ratio (i.e.,possibility of free sterols), then gammacerane may notbe abundant even though the environment could beboth saline and anoxic. In this case one needs to lookfor other indicators of salinity (β-carotane) and anoxia(C35/C34 triterpane ratio). As ever, one needs to place

all geochemical data into geological context for mostrobust interpretations.

Most dominantly overfilled lake basins contain amix of terrigenous and aquatic organic matterdeposited in freshwater, suboxic to oxic conditions(Table 8) (Carroll and Bohacs, 1995, 1999). The result-ing hydrocarbons are typically oil plus some gas andgas condensate (e.g., Pannonian, Eromanga basins)(Clayton et al., 1994; Powell, 1986). The oils are paraf-finic with large contents of high-molecular-weight n-alkanes (waxes >20%) mainly generated fromcuticular tissues of vascular plants and membranelipids from some freshwater algae (Tissot and Welte,1984; Goth et al., 1988; Tegelaar et al., 1989). API grav-ities range widely (24–57°API), as do pour points (–5to >20°C) mainly as a function of gas/oil ratio (GOR)(Powell, 1986; Clayton et al., 1994; Telnaes et al., 1992;Kulke, 1995). Sulfur contents are low and NSO con-tents are low. The molecular character is dominated byterrigenous organic matter input and relatively oxicpreservational conditions: pristane/phytane (Pr/Ph)and hopane/sterane ratios are high and C29 desmethylsteranes are relatively abundant (Powell, 1986; Isak-sen, 1991; Carroll, 1998). Some overfilled lake systemscontain a predominance of 4-methyl steranes (e.g.,Brassel et al., 1986). Gammacerane contents are lowand β-carotane is usually not detected. Tricylic indicesrange from 12 to 100 (Carroll, 1998). If an overfilledlake is not in an appropriate climate and deep enoughto maintain thermal stratification and anoxic bottomwaters, the resulting organic matter generally willhave a low hydrogen content and be completelyswamped by terrigenous organic matter. In theextreme case, the organic matter is largely terrigenouswith characteristics similar to coals or coaly shales

Figure 11—Organic production within lakes is chiefly due to photosynthetic fixation of CO2 by organisms andincludes both autochthonous organic matter derived from algae and aquatic plants and allochthonous organicmatter transported from land. The primary production of organic matter in a lake is a function of many variables, including climate, solar input, wind, precipitation, water chemistry, and temperature. Of these, solarinput and water chemistry have the largest effect on overall primary production.

26 Bohacs et al.

(e.g., Eromanga basin). In these cases, typical of per-manently overfilled lake basins, Pr/Ph is even higher(>3), the proportion of isoprenoids to n-alkanes ishigh, and diterpenoids (from higher plants) are com-mon (Powell, 1984).

Both deep and shallow balanced-fill lake basins canhave significant hydrocarbon generation potential.Organic matter in both is dominated by aquatic organ-isms, with some land-plant material. TOC and HI canbe high (≤27%) (Horsfield et al., 1994). Hydrocarbonsare dominantly oil with low GORs (e.g., 40.4–48.5m3/t, Songliao basin) (Yang, 1985). Oils are typicallyparaffinic and rich in n-alkanes from membrane lipidsof aquatic organisms. Wax contents range from 5 to>25% (e.g., Daqing field, Songliao basin), API gravitiesfrom 18° to 45°API, and pour points from 25° to 59°C(mostly >35°C), with surface-condition viscositiesfrom 6 to 24 cp (Tissot et al., 1978; Yang, 1985; Powell,1986; Schull, 1988; Kulke, 1995). Sulfur contents areuniformly low, although NSO+aromatic contents canrange from 5 to 40% (Tissot and Welte, 1984; Yang etal., 1985). Their molecular character is dominated bycompounds derived from aquatic organisms and theirmicrobial degradation, commonly under suboxic tooxic depositional conditions (e.g., Powell, 1986). Pr/Phratios range from <1 to 2.0, reflecting persistentanoxia and moderate to low terrigenous input. Alsoin contrast with freshwater overfilled lake systems,CPI (odd-carbon number compound preference) islower and n-C30+ compounds are less abundant.Enhanced algal procaryote input is recorded in mod-erate amounts of β-carotane and increased amounts

of tricyclic terpanes. Gammacerane is typically ele-vated due either to overall higher salinities (e.g., Melloet al., 1988) or well-developed chemical stratification(e.g., Schoell et al., 1994; Sinninghe Damsté et al.,1995). C29 steranes are most common, tricyclic indicesrange from 80 to 200, and hopane/sterane ratios rangefrom 0.5 to 15. Dinosteranes, along with other 4-methyl steranes, dominate biomarker distributions ofseveral shallow balanced-fill lake systems, consistentwith preponderant aquatic organic matter input insalinity stratified lakes with anoxic bottom waters(Horsfield et al., 1994; Carroll, 1998).

Underfilled lake basins have variable overallhydrocarbon generation potential, although primaryproduction can be very high, preservation is prob-lematic. For example, the potential of the underfilledWilkins Peak Member (mean HI ~ 730 mg HC/g) ismuch greater than that observed in the JingjingzigouFormation of the Junggar basin (mean HI ~ 351 mgHC/g) (Grabowski and Bohacs, 1996; Carroll, 1998),but their detailed petroleum character is generallysimilar. Hydrocarbons are typically oils with mini-mal associated gas (Tissot and Welte, 1984; Powell,1986). Oils can have significant asphaltic and aro-matic contents and wax contents are relatively low(<5–10%) (Yang et al., 1985; Powell, 1986). API gravi-ties range from 12 to 37°API, pour points from –5 to23°C, and GORs from 10.6 to 149 m3/t (Chen-yongChang, 1991; Kulke, 1995). Sulfur contents are higherthan in other lake-basin types, typically around 1%,but ranging as high as 12% (Shi et al., 1982; Fu et al.,1985; Sheng et al., 1987). The molecular character of

Figure 12—Each lake-basintype has characteristic rangesof total organic carbon andhydrogen indices and associations of organic matter,illustrated here with examples from the EoceneGreen River Formation ofWyoming. The optimal lakefor source rock deposition is a compromise between production, destruction, and dilution; it is shallowenough to have high primaryproduction, deep enough to preserve a significant proportion of the resultantorganic matter, and starved ofsediment, but not of nutrients. These conditionsare most common in balanced-fill lake basins, hereshown that the balanced-filllower Laney Member has significantly richer potentialsource rocks than the otherintervals of the Green RiverFormation.


the oils is derived from low-diversity assemblages oforganisms highly specialized for rapidly changingand often hypersaline conditions. Pr/Ph ratios aretypically <1, probably due to distinctive bacterialcontributions, as well as persistent anoxic deposi-tional conditions. Commonly β-carotane is dominant,and concentrations of gammacerane and tricyclic ter-panes are elevated (Powell, 1986). Hopane/steraneratios are low, consistent with a restricted diversity ofproducers and excellent syndepositional preserva-tional conditions.

Figure 13 demonstrates these variations in organic-matter character. In the Upper Permian of the Junggarbasin the three lake types are represented by theHongyanchi Formation (overfilled), the Lucaogou For-mation (balance-fill), and the Jingjingzigou Formation(underfilled). The triterpane chromatograms (m/z 191)show an increase in the tricyclics (C20–C25) and gam-macerane from the overfilled lake to the underfilled

lake. Also note that the C24 tetracyclic triterpane ismore abundant relative to the adjacent C26 tricyclicsin the overfilled and balanced-fill lakes in compari-son to the underfilled lake. Similarly, the GC (gaschromatogram) traces of the saturate fraction showthe underfilled lake basin extracts to contain elevatedβ-carotane and a Pr/Ph ratio of <1, in contrast to theoverfilled lake basin extracts, which show little or noβ-carotane and Pr/Ph ratios of >3. The best sourcepotential for this series is the balanced-fill (mean HI= 693 mg HC/g) followed by the underfilled (meanHI = 351 mg HC/g). The overfilled Hongyanchi For-mation is essentially a nonsource for liquids. Similartrends between lake types are observed in theWashakie basin of the United States, except that theoverall oil potential of the underfilled lake basin isbetter. (Remember, however, oils may demonstrateless distinction because they result from maturingan interval of source strata that can include several

Figure 13—Examples ofvariations in biomarker distribution among lake-basin types from a single basin. In the Junggarbasin (Upper Permian) the three lake types are represented by theHongyanchi Formation(overfilled), the LucaogouFormation (balanced-fill),and the Jingjingzigou Formation (underfilled).The triterpane chromatograms (m/z 191)show an increase in the tricyclics (C20–C25) and gammacerane from theoverfilled lake to the underfilled lake. Also notethat the C24 tetracyclic triterpane is more abundantrelative to the adjacent C26tricyclics in the overfilledand balanced-fill lakes in comparison to the underfilled lake.

28 Bohacs et al.

lake types and consequent mixing of several sourcefacies types.)

SUMMARY AND CONCLUSIONS

So, where are we in our understanding of lacus-trine systems? Studies over the last century havepretty well boxed the compass of variations of lakebehavior and stratal records, explored the wide rangeof variation, and seen enough examples to begin tosee essential common elements. We stand on theshoulders of people who have done a giant amount ofwork (e.g., Lyell, 1830, 1847; Livingston, 1865; Gilbert,1890; Bradley, 1929, 1964; Picard and High, 1972;Manspeizer and Olsen, 1981). We can start synthesiz-ing all these observations now because we also havethe large-scale 3-D perspective of reflection seismicdata and the framework of sequence stratigraphywith which to integrate all the disparate data on phys-ical, biological, and chemical attributes from bothmodern and ancient examples. We found it essentialto look through the geologic/stratigraphic filter ofpreserved strata to sort out and reconcile the essen-tials of process, response, and record aspects of lakesystems. This approach was necessary because mod-ern process studies, so helpful in marine systems,have yet to yield broadly successful models forancient strata. This makes sense, because lakes are notjust small ocean systems. Their much smaller volumesmake them more dynamic and sensitive to changingboundary conditions, and potentially subject toclosely linked processes of accommodation creationand fill. These many and complex interactions andfeedbacks should not be reduced to consideration ofjust one variable such as climate.

By starting with the rock record we are able tofocus on what processes at what time scales are mostimportant to generating preserved strata. We hereinhave presented a short synthesis of the range of lakesystems and strata that addresses the essential con-trols on the occurrence, distribution, and character oflake strata and hydrocarbon play elements—source,reservoir, and seal. We proposed a lake-basin typeclassification based on observations of lake strata ofmany ages worldwide that focuses on the major con-trols on preserved strata. The interaction of the ratesof sediment+water supply (mostly climatic) withpotential accommodation (mostly tectonic) thatmainly controls lake occurrence, the distribution oftheir strata, and the character of their inorganic andorganic components. We presented these lake-basintypes within a sequence-stratigraphic framework thatfacilitates the integration of all aspects of the deposi-tional system: physical, chemical, and biological. Thisapproach allows one to understand the nature of lakestrata and their hydrocarbon potential from data at allscales, i.e., from basin architecture, through seismic,well log, outcrop/core, to molecules and isotopes. Asever, this framework works best when informed byregional data and an appreciation for local and arealvariations.

ACKNOWLEDGMENTS

Our understanding of lakes has benefited from dis-cussions and cooperative projects with L. Cabrera, Y. Y. Chen, K. S. Glaser, G. J. Grabowski, N. B. Harris,N. C. de Azambujo Filho, L. Magnavito, M. Marzo, K. Miskell-Gerhardt, P. E. Olsen, D. J. Reynolds, C. A.Scholz, and K. O. Stanley. We thank R. P. Steinen (Uni-versity of Connecticut) for introducing KMB to thewonderful world of lake deposits, insightful interac-tions, and for access to the Park River Tunnel Projectcores. We are grateful to Exxon Production ResearchCompany for permission to publish. We also thank ourtwo anonymous reviewers for their very thorough andhelpful comments and E. Gierlowski-Kordesch for hereditorial encouragement and patience. Alan Carrollthanks the donors of the Petroleum Research Fund,American Chemical Society for financial support.

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