Exploration in many nonmarine sequences requires an understanding of what conditions permit the development of lacustrine hydrocarbon source rocks. Although quantitative predictions are not yet possible, qualitative assessments of the probability of source presence may be made.
To establish the presence of a lacustrine water body requires an understanding of the distribution of topographic depressions and paleoclimatic conditions. The lacustrine water body needs to be areally significant and long-lived to permit the development of substantial volumes of organic-rich rocks. This typically means that “commercial” volumes of lacustrine source rocks can only develop in lakes of tectonic origin.
Paleoclimate and paleogeography not only play major roles in controlling distribution of lake bodies but also influence water chemistry. Saline lakes develop when evaporation exceeds precipitation and during geologic episodes of maximum continentality. Fresh-water lakes develop when precipitation exceeds evaporation and along continental margins, even during times of high continentality. Water chemistry controls the nature and level of organic productivity and influences preservation by altering water density and oxygen solubility and determining availability of other chemical oxidizing agents (e.g., sulfates and nitrates).
The most favorable conditions for development of hydrocarbon sources occur in lakes of moderate water depth (50-400 m) at low latitudes and altitudes. Such lakes typically exhibit elevated levels of organic productivity and preservation.
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Lacustrine Basin Exploration: Case Studies and Modern Analogs
Lacustrine environments are a major contributor of petroleum source rocks. Lacustrine source rock prediction is, however, influenced by numerous, complex variables governing lake sedimentation. Current predictive capability can be improved by attempting to map essential climatic variables to limit in space and time the area of lacustrine source rock exploration. Climatic characteristics that govern lake occurrence and the potential for stratification have been investigated with a General Circulation Model of the atmosphere for the present and for the mid-Cretaceous. In this analysis, the distribution of areas with a positive water balance first is used as an indicator of the distribution of areas conducive to lake formation. Second, the distribution of areas that experience large annual climatic variations is used as an indicator of the distribution of lakes that are less likely to be stratified and, hence, less likely to be sites of high organic-carbon preservation. Four factors used to define large climatic variations include (1) seasonal temperature cycle in excess of 40°C; (2) seasonal temperature extreme of less than 4C°; (3) average seasonal differences in precipitation minus evaporation balance in excess of 5 mm/ day; and (4) distribution of mid-latitude winter storms. Evidence is presented to support the capability of climate models that add insight into lacustrine source rock prediction by simulating geographic regions conducive to lake development and to stratification and organic-carbon preservation