Geology and Oil Prospects of Somalia, East Africa

Abstract

Topographically, the southern half of Somalia is comparatively flat, with elevations of less than 350 m above sea level, whereas the northern half of the country rises northward to an elevation of 2,000 m, where it drops off sharply to the Gulf of Aden. All of Somalia was a peneplain at the end of Paleozoic time. Jurassic limestone then was deposited more or less uniformly over the entire country, but post-Jurassic erosion removed a considerable thickness of these rocks, locally exposing basement. A Cretaceous sea then covered all of Somalia. Transgressions and regressions took place during Cretaceous time, and locally the Cretaceous is disconformable with the overlying Eocene. Marine Eocene sedimentary rocks cover the northern half of Somalia, but are absent in southern omalia except for a narrow strip along the coast where marine Eocene is present in the subsurface together with marine Oligocene, Miocene, and Pliocene, the latter present locally in outcrops. Southern Somalia was subjected to epeirogenic uplift at the end of Cretaceous time, and has remained above sea level ever since except in the coastal areas. The rift faulting of middle and late Tertiary time uplifted the northern half of the country with the result that post-Eocene marine sedimentary rocks are known only along a narrow strip of the coast. Regional structural features include the Mandera-Lugh basin in the far southwest, the Bur Acaba uplift north of it, the Tertiary coastal basin, the Somali embayment which occupies most of central Somalia, the Nogal uplift north of this basin, and the middle Tertiary uplift along the northern coast. There is no evidence of major compressive folding in Somalia, but northeast-southwest-trending anticlines of gentle dip have been mapped in Tertiary rocks in the north, and also have been identified in the subsurface in central and southern Somalia by means of refraction-seismic profiling. The structures are believed to have been caused either by rejuvenation of Jurassic fault blocks causing arching of younger sediments, or by leaching of thick underlying evaporites. The previously mentioned structures have been primary drilling objectives and have yielded negative results. Petroleum has been generated in Somalia as indicated by an active oil seep in former British Somaliland, and by the presence of many oil and gas shows and much staining in the many wells drilled in Somalia and Ethiopia. In southern Ethiopia a gas discovery was made in March 1973, and a well drilled offshore Ethiopia in 1969 blew out, although it subsequently was abandoned. The most promising region for oil and gas prospecting in Somalia is believed to be the Mesozoic shelf and reef area around the Somali embayment and around the Nogal uplift. Lithofacies-isopach maps are of much assistance in determining areas of limestone buildup for subsequent geophysical surveys. Secondary prospective oil and gas regions in Somalia are the coastal and offshore marine Tertiary sedimentary rocks which have had gas shows. Stratigraphic traps in clastic sedimentary rocks caused by facies changes or overlaps against the uplift regions of Somalia, and monoclinal porosity pinchouts in carbonate sedimentary rocks are other possibilities. The total sedimentary column in Somalia is in excess of 27,000 ft (8,230 m) with 14,000 ft (4,267 m) of Tertiary strata in southern Somalia 4,600 ft (1,402 m) of Cretaceous in central Somalia, and the Sinclair 1 Obbia penetrated more than 9,000 ft (2,727 m) of marine Jurassic rocks in the Somalia embayment without reaching basement. It is the opinion of the writer that commercial oil and gas deposits are present in Somalia.