Central Sumatra and the East African rift lake sediments : an organic geochemical comparison

Abstract

Abstract Numerous authors have used African rift-valley lakes as modern analogues of ancient lacustrine source rock systems. However, there have been few investigations of the source rock geochemistry of these African lakes and fewer direct comparisons between their sediments and known source rocks. This paper discusses the geochemical characteristics of east African rift sediments and compares them with the Brown Shale (Pematang Group, Paleogene) of central Sumatra. Common to both basin systems are a tectonic origin, low latitudinal position and a tropical setting. There is also evidence that the central Sumatra system contained lakes of varying water depth similar to the modern African rift-valley lakes. East African lakes sediments exhibit highly variable source rock characteristics. Sediments, excluding peats, exhibit total organic carbon contents ranging up to 15 wt % and pyrolytic yields up to 90 mg HC/g rock. The organic matter includes diatom-derived autochthonous material diluted, especially in proximal sediments, by hydrogen-poor organic matter. The best hydrocarbon sources are not associated with the deepest lakes studied (Lakes Tanganyika and Kivu) but with one of intermediate water depth (Lake Edward). The Brown Shale samples compare favorably with those of east Africa with respect to organic richness, potential hydrocarbon yield and product character. In the deep basins a hydrogen-poor component is present but less abundant than in the African sediments, possibly reflecting more efficient trapping of organic matter near lake margins or a lack of input. The Brown Shale lacks diatoms but exhibits evidence of Botryococcus braunii, which is also present in the African lakes. The results of this study indicate that the east African lakes can be used as a valid geochemical analogue for lacustrine source rocks such as the Brown Shale of central Sumatra. It also shows, however, that the systems, are not identical and the there is a direct relationship between hydrocarbon source potential and lake morphology.