The Red Sea — Gulf of Aden: re-assessment of hydrocarbon potential

Abstract

The results and conclusions of the UNDP/World Bank Red Sea — Gulf of Aden Regional Hydrocarbons Study Project (June 1989 – January 1992) are summarized. These derive from detailed examination of pooled public sector data provided to the project by the participating bordering states in the region; these included samples and logs from about 56 wells drilled in the two basins (out of a total of 71) and about 30 000 line km of reflection seismic data (selected from 110 000 km available). Well logs were digitized and correlated, and basinwide biostratigraphic, lithostratigraphic and palaeoenvironmental studies, source rock organic richness, quality and maturity studies and sedimentological analyses studies were performed by contracted specialist staff in the UK and France. These were integrated with basinwide seismic structural and isopach mapping for selected regional reflectors/intervals by national task forces in Cairo under project supervision; academically acquired crustal seismic and other geophysical data and oil industry velocity and other borehole data were also integrated for the Red Sea. Modelling utilizing recently acquired geophysical data indicates crustal asymmetry for the Red Sea rift with oceanic crust of pull-apart basin type flooring some portions of offshore Egypt and Sudan on the west and stretched continental crust underlying the eastern Arabian flank. These interpretations furthermore suggest that sinistral strike-slip processes controlled the initial break up of the Arabian plate, shaping the African flank as a sharp plate boundary; similar processes appear to have initially shaped the Somalia flank of the Gulf of Aden rift. Only later did true seafloor spreading propagate; in the eastern Gulf of Aden 10–12 Ma, about <5 Ma in the western Gulf of Aden, Afar and the southern/central Red Sea. Biostratigraphic work shows that deep subsidence and ocean water inundation commenced in the late Middle Oligocene in the Gulf of Aden, in the Late Oligocene in the southern Red Sea and in the Early Miocene in the Central Red Sea/Gulf of Suez. A regional lithostratigraphic nomenclature scheme for the Miocene-Recent syn- and post-rift Red Sea successions keyed to detailed biostratigraphical distribution charts and microfaunal assemblage characteristics was erected, tied to the well documented and understood petroliferous Gulf of Suez basin scheme; this allows facies variations within the basin to be compared and allows consistent interpretation and correlations across the basin to which local informal nomenclature can be related. Pre-rift (pre-Oligocene) marine sediments are present in most of the Gulf of Aden (Jurassic-Eocene) but only incompletely on the basin margins in the Red Sea (Jurassic in the south, Upper Cretaceous — Early Palaeogene in the north). Significant pre-rift oil/gas prone Upper Jurassic, Cretaceous and Palaeogene source rocks occur in Gulf of Aden sub-basins initiated by Late Jurassic — Early Cretaceous tensional tectonics, with maturity levels ranging from early-late to post-mature, whereas early synrift source rocks are generally immature. In the Red Sea good quality pre-rift source rocks outcrop in the north and south but are probably overmature for oil in the basin except in the extreme south (if present). Good syn-, pre- and post-salt and post-rift source levels occur throughout the basin but in the central/southern sectors only the post-salt remains within the oil window because of high geothermal gradients. Both basins are little explored. Attractive pre-rift Jurassic-Eocene exploration plays in rotated fault and horst blocks in predominantly carbonate but subordinate sandstone reservoirs are present in the Gulf of Aden (confirmed by an important undeveloped Eocene light oil discovery in offshore Yemen) with subordinate synrift potential where maturity is established. Relative continental shelf narrowness is economically limiting, but wider sectors occur. In the Red Sea the principal exploration plays are in Miocene synrift, pre- and post-salt successions. Rotated fault blocks occur in the pre-salt, but maturity for oil in the deeper levels is restricted to the northern sector. These levels in the central and southern sectors are overmature except in the southernmost portion but excellent potential exists in the synrift post-salt and early post-rift sequences with a number of attractive play types. Reservoirs for synrift plays are mainly sandstones and potential is confirmed by several undeveloped gas and condensate discoveries and numerous oil seepages throughout the basin; relatively wide continental shelves occur in the south and along much of the eastern flank.