Exploring the predictive power of seismic geomorphology to assess sedimentary characteristics of gravity-flow deposits - examples from offshore East and West Africa reservoirs

This research seeks to test the hypothesis that rheology of submarine gravity-flows influences the planform geometry of associated fan lobe deposits. If this control exists, seismic geomorphology has the potential to predict the sedimentary characteristics of these deposits by analysing relatively simple seismic outputs such as time structure maps and amplitude maps. This predictive power could potentially aid identification of prospective reservoirs through automated processes of seismic interpretation.In this work, Upper Cretaceous and Neogene deepwater reservoirs from East and West Africa with porosity in excess of 20Darcy permeability were analysed. The overall external geometry of the reservoirs was described by integrating seismic amplitude extractions, structure and isochron maps derived from high quality 3D seismic data. The maps were generated from detailed interpretation of the top and base reservoir reflections. Core photos of the reservoirs were integrated into the seismic project and their examination allowed identification of dominant sedimentary facies, enabling a proposal for the classification of these deposits in terms of flow processes.The research indicates confinement of flows exerts an obvious control on the shape of the deposits and resulted in sedimentary bodies with marked elongation at the seismic scale. However, the analysis of the reservoirs at the core scale indicated that these are dominated by massive, poorly sorted, medium- to coarse- grained sandstones, commonly with floating oversize clasts, as well as beds with disrupted laminae and sandy conglomerates. These characteristics may indicate ‘depositional freezing’ from high-density flows with yield strength, which may have also contributed to the limited lateral spreading of the deposits analysed.The research also highlights the importance of the non-unique relationship between sedimentary processes and external geometries of resulting deposits at the seismic scale. This means that deposits with similar geometries may have markedly different sedimentary characteristics.This work highlights the increasing importance of integrating detailed seismic geomorphology studies with accurate examination of core data to validate models of gravity flow processes and test their predictive value.