Distribution of Organic Facies in Recent Sediments of Northern Part of Lake Tanganyika

Alain Y. Huc, Mireille Vandenbroucke, Genevieve Bessereau, Jean Le Fournier

A better understanding of the relation between the organic facies and the depositional environments is a basic prerequisite to allow predictions of the lateral variations of source rocks and then to achieve realistic quantitative evaluation of the petroleum potential of a sedimentary basin.

Lake Tanganyika is a suitable example to address the problem of organic sedimentology in an environment related to a rifting situation. More than 400 dredged samples have been used to construct detailed maps of the organic facies in the surficial sediments of the northern part of Lake Tanganyika. These maps include Bujumbura and Rumonge basins.

Beyond an apparent complex pattern, the distribution of the organic facies can be explained in terms of differential preservation and sedimentological processes including pelagic sedimentation on the top of structural blocks, winnowing processes which drive the low-density organic matter from the shallow agitated waters (above the thermocline) toward depocenters in the deepest parts of the basin, and gravity transport mechanisms which dispatch sediments together with their specific organic content along sedimentary transit pathways.

In this lake the main biological precursors for the sedimentary organic matter are diatoms. Organic geochemical studies including kerogen analyses and pyrolysis-GC show that the preeminent factor controlling the quality of the organic material, principally its hydrogen richness (in other words, its petroleum potential), is the extent of its degradation which is closely related to the depositional environment (oxic environment above the thermocline versus anoxic environment below the thermocline). In this respect, redepositional processes, which are often overlooked when sedimentary organic matter is concerned, are responsible in Lake Tanganyika for a marked heterogeneity of the organic facies distribution because they are the mode of transport of significant amounts of altered organic matter toward the deep parts of the basin. In these depocenters, sediments with high hydrogen-rich organic content are juxtaposed to sediments with lower hydrogen-poor organic content. For instance, in Rumonge basin one can find sediments with petroleum potential (provided by Rock-Eval pyrolysis) as high as 35 kg of hydrocarbons/ton of rock only a few meters away from sediments with petroleum potential as low as 3 kg of hydrocarbons/ton of rock.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.