--> Modern Analogues of Lacustrine Carbonate Reservoir Facies: Examples from Tropical Lakes of the East African Rift System, Scholz, Christopher, #90100 (2009)

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Modern Analogues of Lacustrine Carbonate Reservoir Facies: Examples from Tropical Lakes of the East African Rift System

Scholz, Christopher1

1Syracuse University, Syracuse, NY.

Many continental rift basins and ancient lacustrine sequences contain important hydrocarbon source rocks, and new exploration on the
South Atlantic margin suggests that lacustrine carbonate facies may host considerable quantities of oil. Most extensional basins are characterized by siliciclastic sedimentation, on account of the high-relief generated on faulted rift margins, as well as associated deep-basin subsidence. The occurrence of siliciclastic reservoir facies in extensional basins is to a first-order predictable, primarily because of the structural controls of drainage systems that enter into the basins. However the spatial occurrence and vertical stacking of lacustrine carbonate facies is far more complex, due to the remarkable geochemical variability exhibited by relatively dilute waters found in most large lakes. Coarse-grained lacustrine carbonate facies are observed in a variety of distinct depositional settings in modern lakes, as well as in ancient successions. Here I present modern examples of mixed-system and carbonate accumulations from several of the Great Lakes of the East African Rift Valley, which represents the most extensive modern network of related and interconnected tectonic lakes. Lakes Tanganyika, Malawi, Albert, and Edward are large freshwater systems extending across several structural basins in the western branch of the East African rift, and are long-lived lakes that have experienced major hydrologic changes over their histories. Smaller lakes of the eastern branch of the rift, such as Lakes Bogoria, Baringo, and Nakuru are at present experiencing low stand conditions, are hydrologically closed, and consequently are alkaline and saline. Carbonate facies observed in these various basins include algal bioherms, stromatolites, or microbialites; ostracode and chara silt beds; ooid beaches and shoals; and bioclastic grainstones composed of gastropods and ostracods. All these lakes have accumulated carbonates at times in the geologic past, but depositional modes have regularly alternated between siliciclastic and carbonate sedimentation, commonly on orbital time scales.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil