Architectural element variations across the offshore basin of Tanzania continental margin: influence of local factors in sedimentary processes and their implications for petroleum prospectivity

Significant gas discoveries have been made in the southern offshore basin of Tanzania continental margin while no gas accumulation has been found in the northern part of the basin. This study was aimed at finding out the reasons behind absence of gas discoveries in the northern offshore Tanzania continental margin despite the presence of potential gas flow indicators. 2D seismic interpretation and well logs analysis allowed assessment of depositional architectural elements and structural features across the basin. These aspects revealed architectural variations that have been used to account for the current non-discovery status in the northern part of the basin. Results of this work have shown that basin topography, influence of tectonics, sediment supply and depositional and post-depositional processes varied significantly over the whole offshore Tanzania during the Cretaceous-Holocene period. For example, the available 2D seismic profiles show that the Quaternary extensional tectonics created a N-S to NE-SW trending fault controlled sub-basin in the northern part of the study area, but similar feature could not be seen in the southern part of the basin. The variations of key factors controlling sedimentary development also caused dissimilarities in deposit types and dominance of sandy and muddy successions. The northern part of the study area is dominated by complex channel-levee systems containing sandstone bodies while the southern part contains hybrid turbidite-contourite (HTC) deposits and their respective thick drift successions that are stepping southward onto the HTCs. Deposit types play important role in accumulation of hydrocarbons. The HTCs for example contain clean sandstones with high net-to-gross ratio, and may be used to explaining why the existing commercial gas discoveries are found in the southern offshore Tanzania and not in the northern part of the basin.