RESERVOIR FORMING MECHANISMS OF THE LAKE ALBERT BASIN IN THE WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM

The Lake Albert Basin in Uganda is a Cenozoic rift basin located at the north end of the western branch of the East Africa Rift System (EARS). Many hydrocarbon discoveries have been made recently. However, the reservoir-formation mechanisms of the basin remain unclear, and thus dry wells appear sometimes. In fact, there occur two major hydrocarbon enrichment zones in the basin with excellent reservoir-forming conditions. They are the northern gentle slope zone and the eastern step-fault zone. The hydrocarbon accumulation models are quite different between the two zones. Based on the research of basin structure, stratigraphic sequence and petroleum geological conditions, the accumulation characteristics and mechanism of the two hydrocarbon enrichment zones are systematically analyzed in this paper. The research shows that the northern gentle slope zone has high-quality sands suction, long-distance hydrocarbon migration, small faults control and dynamic accumulation in addition to ample hydrocarbon supply, continuous high-quality sandbody, no fault interception of migration pathway, stable distribution of thick regional caprocks and good physical properties of crude oil to ensure the long-distance hydrocarbon migration and accumulation. As to the "two-step" fault in the eastern step-fault zone, the down-thrown wall is dominated by the self-generation and self-storing reservoir systems in a definite layer. However, on the up-thrown wall, it is dominated by episodic petroleum recharge into the connected sandstones following the faults on migration pathway. Owing to the huge skip distance of hydrocarbon vertical migration and poor lateral sealing of the boundary fault, the fullness of reservoirs is generally low and decreasing upwards.