Aspects of depositional processes of organic matter in sedimentary basins

Abstract

Quantitative evaluation of source rocks is a prerequisite for assessing realistic petroleum potential in target basins. Such an evaluation includes information about the actual occurrence of organic-rich beds, their stratigraphic placement, specific locations in sedimentary sequences, lateral extension, geometry at the scale of the basin and their organic facies variations (richness and quality). To achieve this evaluation, current methods rely on analytical data alone. For several years an attempt has been made to elaborate a predictive approach based on the integration of the various factors controlling the accumulation and distribution of organic matter in sedimentary basins in order to try to model the process. The main effort has been devoted to the control of the production of biomass according to paleogeography, and paleoclimatology, and to the control of the degradation of organic matter according to the development of oxygen-starved water bodies. Depositional processes which are certainly a key factor to understand the internal variations and the distribution of organic bearing sediments have been largely overlooked: Examples of Modern environments (e.g. Black Sea, Caspian Sea, Lake Bogoria) show that the distribution of the organic matter in anoxic basins is generally organized in a concentric pattern, with a centripetal increase of organic content towards the depocenters where the sediment thickness is the greatest. Such a feature, which can be related to the hydrodynamic properties of the organic particles, is of paramount importance as far as the distribution of petroleum potential is concerned. This “centripetal concept,” with its implications for source rocks evaluation, is well exemplified in the Lower Jurassic of the North Sea and in the Lias of the Paris basin. However, this pattern can be strongly affected in anoxic basins when extensive gravity transport mechanisms cause redeposition of sediments. In such a situation sediments, including their specific organic contents, are redistributed along transit pathways. In Lake Tanganyika, these modes of transport drive a significant amount of altered organic matter towards the deepest parts of the basin resulting in a marked lateral heterogeneity of the organic facies in the bottom sediments. Similar processes might contribute to the complex pattern of the distribution of organic facies seen in the Upper Jurassic of the North Sea. On the other hand, gravity flows can introduce organic-rich sediments into basins where environmental conditions would, otherwise have been unfavorable for organic accumulation. A dramatic example of such a situation is provided by the several hundred metres thick sequence of organic-rich turbidites and debris flows deposited since the Late Miocene at the base of the continental slope off South-West Africa.