西非海岸南、北两段主要含油气盆地油气成藏特征对比

Comparison of the hydrocarbon accumulation patterns of petroliferous basins between the north and south parts of the West African coast

西非海岸主要含油气盆地是典型的大陆裂谷和被动陆缘盆地的叠合盆地,具有相同的区域构造演化背景,但是,由于南、北两段板块演化的过程不同,导致了盆地形成发育时期、构造演化特征、沉积充填特征的较大差异,形成了南、北两段油气地质条件、成藏特征、油气富集程度的极大差异。西非北段盆地的形成主要与北大西洋的裂开和非洲与北美板块的分离有关,同时也受北非构造演化的控制,盆地演化早期分异强烈,没有形成统一的沉积中心,晚期进入整体坳陷阶段,受阿特拉斯造山运动的影响,地形高差大,沉积厚度向海上快速增厚。而中段和南段盆地的形成主要与南大西洋的形成和非洲与南美洲板块的分离有关,盆地演化早期形成了统一的盆地,具有相似的沉积序列和地层发育特征,其沉积中心位于中部的加蓬和下刚果盆地,第三纪进入盆地的分异期,沿西非海岸发育一系列三角洲、扇三角洲沉积体系,从北向南规模逐步减小,有尼日尔三角洲和刚果扇盆地等。通过重点探讨西非南、北两段主要含油气盆地形成演化特征及对油气成藏的控制作用,对比分析南、北两段主要含油气盆地油气成藏特征,为中石化海外在西非战略选区提供依据。

The major petroliferous basins along West African coastline are typical superimposed basins of conti- nental rift basins and passive continental margin basins, therefore sharing the same regional tectonic evolution background. However, due to the difference of plate evolution process between the south and north parts of the West African coast, the basins vary in their forming times, tectonic evolution characteristics, and deposition fea- tures, causing the basins in the south part significantly differs from that in the north in geological conditions, ac- cumulation patterns and hydrocarbon abundance. The formation of basins in the north part is mainly connected with the breakup of the North Atlantic and the separation of the African from the North American plate. The structural evolution of North African also contributes to the formation of these basins. In the early stage of basin evolution, tectonic differentiation was strong and no unified depocenter formed. While in the late stage, the basins downwarped as a whole and were influenced by the Atlas Orogeny. As a result, they vary greatly in height diffe- rences and their sedimentary thickness quickly thickens seaward. The formation of basins in the south and middle parts is chiefly related to the formation of South Atlantic and the separation of the African plate from the South American plates. In the early stage of basin evolution, a unified basin with similar sedimentary sequence and strata characters formed and its depocenter was located in central Gabon and lower Gongo basin. In the Tertiary, the unified basin began to differentiate and formed a series of delta and fan-deha depositional systems along the West African coast,such as the Niger delta basin and the Congo fan basin. However,the size or scale of these systems decreases progressively from north to south. This article is focused on the formation and evolution of the major petroliferous basins in the area and their controlling effects over the hydrocarbon accumulation. The com- parative study can be used as a guide to screening of strategic petroleum provinces in West Africa.