RESERVOIR FORMING MECHANISMS OF THE LAKE ALBERT BASIN IN THE WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM
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摘要:
乌干达Albert湖盆为新生带裂谷盆地，位于东非裂谷西支北端。近年来研究者在Albert湖盆取得大量发现，但对成藏规律仍认识不清。Albert湖盆成藏条件优越，在平面上有2大油气富集带：北部缓坡带和东部断阶带，二者的成藏模式有较大的差异。在对盆地构造、地层特征、基本石油地质条件详细研究的基础上，系统分析了2个油气富集带的成藏特点和成藏机制。研究表明，北部缓坡带具有优质砂体抽吸、油气长距离运移、小断层调节、动态成藏的特点；充足的油源、连续稳定的优质砂体、中途缺少断层的阻截、上覆厚层稳定分布的区域盖层以及良好的原油物性保证油气长距离运移成藏。东部断阶带“二台阶”断层下降盘为自生自储式成藏，成藏层位单一；“二台阶”断层上升盘砂岩和断层对接，发育多个油藏，幕式充注，充满度自下而上逐渐减小。由于油气运移空间跨度大，边界断层侧封较差，油藏充满度整体偏低。
Abstract: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.
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