THE TECTONIC FRAMEWORK AND REGIONAL HYDROCARBON PROSPECTIVITY OF THE GULF OF ADEN

The Gulf of Aden rifting predates that in the Red Sea. Isolated sub-basins developed during the earliest Oligocene following a major emergent (erosional) period during the latest Eocene. Marine sedimentation began in the middle-to-late Oligocene and progressed upwards from marginal marine to bathyal environments with significant turbidite input (primarily on the Yemen side). Stretched and dike-invaded continental crust exists in water depths greater than 1,000 m in some areas. Sea-floor spreading began in the late Miocene and progressed westward into the Afar region. Only limited faulting has affected the post-rift, uppermost Miocene to Recent section, which is primarily dominated by prograding sequences and thermal subsidence. With over 40,600 square kilometres of continental shelf in less than 300 m of water, the Gulf of Aden is a frontier exploration area. Only minimal exploration work has been carried out in the offshore, including twelve wells resulting in one sub-commercial discovery and numerous indications of hydrocarbons. The wells drilled to-date have encountered source, reservoir and seals in both the pre-rift and syn-rift section. Heat flow and thermal modelling has shown that the Gulf of Aden continental margins are not areas of excessive heat flow, and hydrocarbon-generative "kitchens" can be mapped in the pre-rift sequence. The hydrocarbon prospectivity of the Gulf of Aden differs from that in the Red Sea in that the primary hydrocarbon plays are found in the pre-rift sequences. Pre-rift exploration plays exist in horst and rotated fault-blocks within NW-SE trending sub-basins controlled by Upper Jurassic to Lower Cretaceous tensional tectonics. A combination structural-stratigraphic trap has proved hydrocarbons beneath the pre-Oligocene erosional unconformity. Syn-rift exploration for clastic reservoirs sealed by anhydrites exists in early Oligocene transtensional sub-basins. The primary structure traps are rollover anticlines associated with listric growth faults. Source maturity is a significant risk for syn-rift plays.