GEOLOGICAL CHARACTERISTICS OF OIL AND GAS ACCUMULATIONS IN THE SIERRA SIERRA LEONE-LIBERIA BASIN, WEST AFRICA

With reference to the tectonic evolution of the Atlantic, we studied the oil and gas geological characteristics of the Sierra Sierra Leone-Liberia Basin. Results show that the Sierra Leone-Liberia Basin has experienced two stages of tectonic evolution, i.e. the rifting stage and drifting stage. Hydrocarbon is mainly accumulated in the lithologic reservoirs formed during the drifting stage. A hydrocarbon accumulation model is then established for clarification of exploration directions based upon the above observation. During the Early Cretaceous of the rifting stage, tectonic activity was strong in the basin and thus the basin was dominated by continental deposits, while during the Late Cretaceous to present of the drifting stage, tectonic activity was weak and marine sediments dominated. The Cenmanian-Turonian marine source rock is the major source of the lithologically trapped oil and gas resources. Large-scale deep-water sediment bodies, which are critical important to the formation of oil and gas reservoirs and lithologic traps, are widely formed during the drifting stage. The hydrocarbon generated in the matured Cenomanian-Turonian source rock has no way but migrating into the lithologic trap within the source rock sequence itself to form a kind of self-generated and self-accumulated oil and gas reservoirs since tectonic movement was weak and there was lacking of fractures. To look for large-scale deep-water sediment bodies in the source kitchen is one of the important exploration directions in the future in deepwater areas.