ENVIRONMENTAL CONTROLS ON THE COMPOSITION OF ORGANIC MATTER IN EAST AFRICAN LAKE SEDIMENTS AND THEIR IMPLICATIONS FOR THE FORMATION OF LACUSTRINE OIL SOURCE ROCKS

The IDEAL project and related coring programmes have provided abundant samples of organic matter-rich sediment from several of the East African great lakes. Based upon several hundred RockEval, elemental (C, N) and stable isotope (C, N) analyses, this talk will review the composition of sedimentary organic matter (OM) in these lakes. Such a large dataset provides some clear indications as to the principal factors that determine the distribution, preservation and composition of the OM, but also throws up a few puzzles. In rift lakes, the combination of steep marginal slopes, and the apparently ubiquitous occurrence of density interflows and underflows, ensures efficient redistribution of terrestrial and degraded OM to basinal areas. OM assemblages are thus typically of mixed origin, even far offshore, with relatively pure autochthonous OM accumulations being restricted to low-relief flexural margins and isolated topographic highs. For sediments with a RockEval Hydrogen Index (HI) of less than 575, the HI/TOC data define a relatively restricted compositional field that encompasses "typical" rift-lake sediments. The relationship seems to be independent of important environmental variables such as water depth, mixing regime and salinity. Deviations from this field are due to exceptional inputs of terrestrial OM. Above an HI of 575, i.e. in sediments dominated by autochthonous OM, the relationship between HI and TOC becomes apparently random, for reasons which are currently obscure.

In terms of hydrocarbon source-rock quality, it is clear that the development of classic lacustrine Type I kerogens is relatively restricted in rift basins. These seem likely to be most characteristic of large basins with relatively low-relief margins, of which Lake Victoria provides the best example in East Africa today.