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Can Sea Surface Oil Slicks Confirm the Presence of an Active Hydrocarbon System in the Mozambique Channel


The Rovuma Basin in northern Mozambique is a a well-established world-class gas province with over 100 TCF of gas discovered so far but the rest of the margin remains unexplored. 242 optical satellite images offshore central Mozambique have been examined for evidence of oil floating on the surface of the sea. These sea surface slicks, when appearing in persistent clusters can be indicative of a working subsurface hydrocarbon system. In total 120 sea surface slicks were identified in the interest area, some comprising of clusters re-occuring over time. The majority follow the continental shelf edge along the East coast of Mozambique. Sedimentary facies from the Mozambique Channel generally dip gently upwards towards the shelf edge, so it is thought that a mature oil source in the centre of the basin is responsible, with hydrocarbons migrating upwards and Westwards towards the mozambique coast. Several potential source, reservoir and seal intervals have been identified with Cretaceous and Tertiary play types including onlaps and drapes over basement highs, stratigraphic and structural traps of deep water slope channel and basin floor fan complexes, lowstand plays (both wedge and pro-delta fan), syn-rift graben hanging wall and footwall plays, and strike slip structural plays. Enhanced clastic reservoir quality is expected from turbidite systems interacting with strong drift currents which are known to winnow turbidite channels leaving behind reservoirs of exceptional quality such as the Lower Grudja formation, a recognized significant reservoir target, with sandstones exhibiting porosities up to 34% and permeabilities up to 5,000 mD. Recent understanding of the influence of active rifting and mantle plume activity causing elevated heat flow in the northern Rovuma Basin, has explained the anomalously high geothermal gradient in that area which has resulted in significant gas generation. The offshore Angoche area is far from rifting and mantle plume activity such that geothermal gradients are modelled to be lower so the main potential source rocks are predicted to be generating oil, as supported by the oil seeps. The distribution and occurrence of the slicks provides strong evidence that there is or has been a mature oil source in the Mozambique Channel. The distribution does not allow us to isolate the age of the hydrocarbon source rock; but three possible source rock intervals have been proposed, from the Late Triassic (Karoo) and Mid Jurassic.