Abstract: Lake-Basin Type, Source Potential, and Hydrocarbon Character: An Integrated Sequence-Stratigraphic-Geochemical Framework

BOHACS, KEVIN M.
Exxon Production Research Company, Houston, Texas

We at Exxon have developed the concept of lake-basin type in order to sort out the complexities of lacustrine deposits and provide a framework for predicting source potential and hydrocarbon character. Based on numerous observations of lake strata of Cambrian to Recent age, three lake-basin types are recognized and named to reflect the interpreted relation of potential accommodation and sediment + water supply: overfilled, balanced-fill, and underfilled.

Lacustrine strata record the integrated history of lake hydrology arising from the interaction of potential accommodation and the supply of sediment + water. These two factors together govern the major stratigraphic features of ancient lake deposits and, in conjunction with lake ecology, geologic age, basin shape, and drainage-basin lithology, exert a strong influence on source character, reservoir-rock distribution, and hydrocarbon potential. Parasequences and sequences are very similar to shallow-marine sequences in some overfilled lake basins, but their expression ranges widely and can be very different in underfilled lake basins. Balanced-fill lake systems contain the most prolific lacustrine source rocks and beneficent facies juxtapositions for hydrocarbon accumulations, based on observations of lacustrine strata of many different basins and ages (e.g., East Africa Quaternary, USA Tertiary, China, and Africa Cretaceous).

This lake-basin approach offers significant advantages over models based solely on paleoclimate 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 is closely linked to the supply of water and lake level. The integrated framework can impact exploration and exploitation strategies by providing an approach for predicting hydrocarbon character from stratigraphic information or lake-strata character from geochemical data. In addition, it enables one to appreciate and begin to comprehend small-scale variations and complexities of lake strata.

AAPG Search and Discovery Article #90927@1999-2000 AAPG Distinguished Lectures