东非海岸坦桑尼亚盆地烃源岩特征与油气来源

Source rock characteristics and oil-gas origins in Tanzania basin,East Africa coast

近年来,东非海岸坦桑尼亚盆地深水区陆续有大的气田发现,展示了巨大的勘探前景。但有关坦桑尼亚盆地烃源岩发育层位、沉积环境、生烃潜力及油气来源的研究报道较少。根据研究区钻井地球化学资料,结合沉积及构造演化特征,对坦桑尼亚盆地烃源岩特征和油气来源进行了分析,结果表明:该盆地发育3套烃源岩,其中二叠系湖沼相烃源岩以倾气型为主;三叠系湖相烃源岩有机质丰度高,以倾油型为主;下—中侏罗统泻湖相、局限海相烃源岩有机质丰度高,以倾油型为主。该盆地深水区天然气乙烷碳同位素组成变化大(为-30.8‰^-21.0‰),甲烷氢同位素组成重(大于-120‰),且热演化程度高,为原油裂解气,天然气主要来自下—中侏罗统局限海相腐泥型烃源岩,下三叠统湖相烃源岩也可能有部分贡献;推测下—中侏罗统倾油型烃源岩主要分布在该盆地海域,油气勘探潜力大。

In recent years, the discovery of large gas fields in deep water area shows great exploration prospects in Tanzania basin, East Africa coast. However, few research in this area on source rock strata, depositional environment, hydrocarbon potential and oil-gas origin is reported. Source rock characteristics and oil-gas origins in Tanzania basin are analyzed based on geochemical data, basin tectonic evolution and depo- sitional characteristics. The results indicate that Tanzania basin develops three sets of source rocks in which Permian lake-swamp source rocks are prone to gas, Lower Triassic lacustrine source rocks with high organic matter content are prone to oil and Lower-Middle Jurassic lagoon and restricted marine source rocks with high organic matter content are prone to oil. The δ^13C2 has a large variation from - 30.8‰ to - 21.0‰ and the δD is heavier than - 120‰ according to the isotope and composition, which indicates oil- cracked gas with high thermal evolution. The gas originates from Lower-Middle Jurassic restricted ma- rine sapropelic source rocks, and Lower Triassic lacustrine source may have the secondary contribution. It is deduced that the Lower-Middle Jurassic oil prone source rocks develop in the offshore of this basin with great exploration potential.