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Drift Influence on Deep-Marine Slope Channel Facies and Geometries: Implications for Reservoir Quality Distribution – Block 2, Offshore Tanzania


Bottom current interaction with subaqueous sediment gravity flows influences both the position of sedimentary systems, e.g., channels and lobes, and the facies within them. The coastal basins offshore East Africa are an excellent location to study drift influence on these systems and to improve our understanding of facies distribution and reservoir quality. This study integrates 3D seismic, well-log and core data to investigate drift influence on deep-marine slope channel systems of the Block 2 area, offshore Tanzania. Seismic data indicates complex interplay of subaqueous sediment gravity flows with bottom current deposits and tectonic related relief during the Upper Cretaceous and Neogene. The architecture of several channel complexes shows an interaction with associated drift deposits and the development of strongly asymmetrical levees. Combining these large scale observations with lithological information, we present a model of submarine slope channel development under the influence of deep marine bottom currents. Facies associations include sandstone-dominated turbidite deposits and debris flow deposits that are interpreted as channel fill. Interbedded with these sands are laminated to bioturbated mudstones and bottom current reworked facies, which include coarsening upward trends, periodically occurring sharp-based fine muddy sand- to siltstone beds with starved ripples and abundant bioturbation that are interpreted as toes of the drift deposits. Our observations have a number of implications for the reservoir quality and performance: (i) Flow-stripping of the main channel-fill can improve the reservoir quality by reducing the detrital mud content; (ii) depending on the angle of the channel towards the bottom currents and the frequency of the subaqueous sediment gravity flows, mud and silt ratio may increase within the channel complex; (iii) mud drapes associated with the toes of drift deposits may present fluid flow baffles and barriers within the slope channel complexes; (iv) the likelihood of stratigraphic trapping along the margins of these drift-influenced channels. The data presented indicates that channel-drift interaction has major implications for the understanding of plays along the East African margin and other deep-marine plays globally.