Rocks associated with lakes probably account for more than 20% of current worldwide petroleum production (Kulke, 1995; Britannica Yearbook, 1999), and lacustrine organic-rich rocks are significant sources of the petroleum. Lacustrine sources and reservoirs are important in many areas of current and future exploration opportunities including Africa, South America, southeast Asia, and China (Hedberg, 1968; Powell, 1986; Smith, 1990; Katz, 1990).

We have developed the concept of lake-basin type, which is useful for sorting out the complexities of lacustrine deposition to derive a predictive framework (Carroll and Bohacs, 1995, 1999). Three lake-basin types are recognized from recurring lithofacies associations and stratal stacking patterns at scales of meters to decameters, based on numerous observations of lake strata from Cambrian to Recent age.

Lacustrine strata record the integrated history of lake hydrology arising from the interaction of potential accommodation and the supply of sediment+water. Both factors together govern the major stratigraphic features of ancient lake deposits. In conjunction with lake ecology, geologic age, basin shape, and drainage-basin lithology, they exert a strong influence on source character, reservoir-rock distribution, and petroleum potential.

The named lake-basin types, overfilled, balanced-fill, and underfilled, are based on the interpreted relation of potential accommodation and sediment+water supply (Figure 1). Expression of parasequences and sequences ranges from very similar to shallow-marine sequences in some overfilled lake basins to very different in under filled lake basins (Table 1). Balanced-fill lake systems contain the most prolific lacustrine source rocks and beneficent facies juxtapositions for hydrocarbon accumulation,

Figure 1. Lake-basin- type phase diagram shaving existence and character of non-marine strata in general and lacustrine strata in particular as a function of both sediment+water supply and potential accommodation. Interaction of the two controls is reflected in the lithology, stratal stacking, biota, and geochemistry of lake deposits. Potential accommodation is the space available for sediment accumulation below the basin's outlet or spillpoint (a key difference from marine systems; Carroll and Bohacs, 1995). It is mainly influenced by basin tectonics, along with uplift and erosion of a sill and inherited topography. Sediment+water supply is primarily a function of climatic humidity, along with seasonality, local relief and bedrock geology. (Clastic sediment yield is a non-linear, non-monotonic function of climatic factors, generally peaking in semi-arid, distinctly seasonal climates- Einsele, 1992).

End_Page 15---------------

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

End_Page 16---------------

based on observations of lacustrine strata of many different ages and basins (e.g.: East Africa Quaternary; U.S.A. Tertiary; Africa, Brazil, and China Cretaceous).

Lake-basin type commonly evolves among the three end members, often within a single formation, on a variety of time scales as a result of changes in climate or tectonic subsidence. Different lake types can also coexist in adjacent basins. Lake water depth and overall thickness of lacustrine strata are functions of both total subsidence and sediment+water supply and are not necessarily related to lake-basin type.

Lake-basin type offers significant advantages over previous models that were based on paleoclimate alone, and allows lacustrine source rocks to be genetically linked with reservoir and seal lithofacies through sequence stratigraphy. However, sequence-stratigraphic models specific to each type of lake basin are necessary because, unlike most marine systems, the supply of sediment in lake basins commonly can be closely linked to the supply of water and lake level. The integrated framework provides an approach to predicting hydrocarbon character from stratigraphic information or lake-strata character from geochemical data and suggests strategies for successful exploration and exploitation. The framework also enables one to appreciate and begin to comprehend small-scale variations and complexities of lake strata within a bigger picture.

References

Carroll, A. R., Bohacs, K. M., 1995, A stratigraphic classification of lake and hydrocarbon source potential: balancing climatic and tectonic controls. First International Limno-geological Congress. Geological Institute, University of Copenhagen, Denmark, August 21-25, 1995, p. 18-19.

Carroll, A. R., K. M. Bohacs, 1999, Stratigraphic classification of ancient lakes balancing tectonic and climatic controls; Geology v. 27, p. 99 - 102.

Hedberg, H. D., 1968, Significance of high-wax oils with respect to genesis of petroleum: American Association of Petroleum Geo1ogists Bulletin v. 52, p.736-750.

Katz, B. J., 1990. Lacustrine Basin Exploration--Case Studies and Modern Analogues: American Association of Petroleum Geologists Memoir 50, 340 p.

Kulke, H., 1995, Regionale Erdol- und Erdgasgeologie der Erde: Berlin, Gerbr der Borntrager, 2 volumes.

Powell, T. G., 1986, Petroleum geochemistry and depositional setting of lacustrine rocks: Marine and Petroleum Geology, v3, p. 200-219.

Smith, M., 1990, lacustrine oil shale in the geological record, in B. J. Katz, lacustrine Basin Exploration - Case Studies and Modern Analogues: American Association of Petroleum Geologists Memoir 50, p. 43-60.

End_of_Record - Last_Page 17---------------