Northern Lake Tanganyika— An Example of Organic Sedimentation in an Anoxic Rift Lake
A. Y. Hue, J. Le Fournier, M. Vandenbroucke, G. Bessereau, 1990. "Northern Lake Tanganyika— An Example of Organic Sedimentation in an Anoxic Rift Lake", Lacustrine Basin Exploration: Case Studies and Modern Analogs, Barry J. Katz
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Lake Tanganyika is a well-known example of a large anoxic lake that acts as a sink for organic matter. More than 400 grab samples and 25 cores were collected to study distribution of organic facies in the Bujumbura and Rumonge subbasins of the northern lake area. The overall organic richness of the lake sediments (up to 12% TOC) is favored by anoxic conditions, which prevail below about 100 m. This situation is the combined result of (1) organic productivity, (2) lake geometry (deep and narrow), which is controlled by rift architecture and hinders water circulation, and (3) equable warm tropical climate, promoting stable water stratification.
The distribution of organic facies in bottom sediments shows considerable and rather complex lateral variability. Sedimentological and organic geochemical evidence (Rock-Eval, kerogen <513C, hydrocarbon composition) suggests that organic variability is controlled mainly by depositional processes—gravity- flow deposits, containing organic matter that reflects (in terms of concentration and properties) source area environment, interfinger with autochthonous organic-rich pelagic/hemipelagic muds. The importance of redepositional processes at the basin scale probably is a response to the lake bottom's rugged topography, which is controlled by rift tectonism. Such a situation, with its implications in terms of organic heterogeneity, must be kept in mind when considering source rocks deposited in similar continental rift settings.
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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