Geology and Petroleum Resources of Northwestern Africa

James A. Peterson, H. Douglas Klemme

The main onshore basins of northwestern Africa are (1) basins in the Atlas folded geosynclinal belt adjacent to the Mediterranean Sea, (2) the Tindouf, Bechar, and Reggane basins of western Algeria and southern Morocco, and (3) the Taoudeni basin of Mauritania and Mali. Coastal basins are (1) the Essaouria basin of southwestern Morocco, (2) the Tarfaya basin of Western Sahara, (3) the Senegal basin of Senegal and western Mauritania, (4) the Sierra Leone-Liberia basin, and (5) the Ivory Coast basin.

Much of the Atlas folded geosynclinal belt contains a very thick but highly disturbed section of marine Paleozoic, Mesozoic, and lower Tertiary carbonate and clastic rocks that includes a substantial thickness of probably highly mature or overmature petroleum source rocks. The small, elongate, infolded Tertiary basins appear to have some hydrocarbon potential, but limited drilling there has had little success thus far. The Tindouf, Bechar, and Reggane basins contain a thick section of marine Paleozoic rocks that has some gas potential and minor oil potential. Carbonate mounds and reefs of Devonian and Carboniferous age in these basins may not have been adequately explored. The Taoudeni is a large basin and has a high exploration risk. Most of its sedimentary section is early Paleozoic and late Precambrian in age; problems include questionable source rock quality, inadequate burial depths in much of the basin, and lack of extensive regional seals.

Mesozoic and Cenozoic clastic and carbonate rocks, as much as 10 km (35,000 ft) thick, are present in the Essaouria, Tarfaya, and Senegal offshore basins. A moderate number of test holes in the Essaouria and Tarfaya basins have had no success. The low to moderate hydrocarbon potential of these basins is probably due to inadequate or immature source rocks; sandstone and carbonate potential reservoir rocks are present. The Senegal basin is fairly well tested in the northern and central parts with no success. Moderate potential may exist in the southern part. The Sierra Leone-Liberia basin has a relatively thin sedimentary cover with low hydrocarbon potential, although the shelf edge and deeper water areas may have some gas potential. A few small to medium-sized oil and gas discoveries h ve been made in the Ivory Coast basin, which has moderate potential for undiscovered petroleum resources. These coastal basins appear to be gas-prone and have their highest potential in the deeper water shelf edge.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.