Geology and hydrocarbon potential of the East African continental margin: a review

The East African margin has a complex structure due to multiple phases of rifting with different stretching directions. The main phase of rifting leading to Indian Ocean opening lasted from the Late Pliensbachian to the Bajocian (c. 183–170 Ma). This occurred during impingement of the Bouvet hotspot which weakened the lithosphere sufficiently to allow continental break-up. Thick salt and marine shales were deposited during the Toarcian in the Majunga, Ambilobe and Mandawa basins and the onshore Ogaden Basin, marking the onset of the Indian Ocean marine incursion, and good quality oil-prone source rocks were deposited at this time. The recent giant gas discoveries in Tanzania and Mozambique are believed to be sourced from overmature Jurassic or, possibly, deeper Permian age Karoo shales. The margin from the Lamu Basin in the north to the Zambesi Delta in the south is covered by thick Tertiary and Cretaceous sediment derived from the African rift shoulders, and Lower Jurassic source rocks are predicted to be in the gas window along most of the margin. However, the margins in South Africa, south Mozambique, northern Somalia and Madagascar are less deeply buried, and have better oil potential. The large Tsimimo and Bemolanga tar sand deposits and the recent announcement of an oil rim in the Inhasorro Field indicate that there are good oil-prone source rocks in the Karoo rifts and in the Albian Domo shales; and the search for oil continues with companies exploring in areas where Jurassic source rocks may be less deeply buried, and/or potential Albian–Turonian-aged source rocks are sufficiently buried to generate oil.