Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola

Abstract

Integrated analysis of high-quality three-dimensional (3D) seismic, seabed geochemistry, and satellite-based surface slick data from the deep-water Kwanza Basin documents the widespread occurrence of past and present fluid flow associated with dewatering processes and hydrocarbon migration. Seismic scale fluid flow phenomena are defined by seep-related seafloor features including pockmarks, mud or asphalt volcanoes, gas hydrate pingoes, as well as shallow subsurface features such as palaeo-pockmarks, direct hydrocarbon indicators (DHIs), pipes and bottom-simulating reflections (BSRs). BSR-derived shallow geothermal gradients show elevated temperatures attributed to fluid advection along inclined stratigraphic carrier beds around salt structures in addition to elevated shallow thermal anomalies above highly conductive salt bodies. Seabed evidences of migrated thermogenic hydrocarbons and surface slicks are used to differentiate thermogenic hydrocarbon migration from fluid flow processes such as dewatering and biogenic gas migration. The analysis constrains the fluid plumbing system defined by the three-dimensional distribution of stratigraphic carriers and seal bypass systems through time. Detailed integration and iterative interpretation have confirmed the presence of mature source rock and effective migration pathways with significant implications for petroleum prospectivity in the post-salt interval. Integration of seismic, seabed geochemistry and satellite data represents a robust method to document and interpret fluid flow phenomena along continental margins, and highlights the importance of integrated fluid flow studies with regard to petroleum exploration, submarine geohazards, marine ecosystems and climate change.