西非海岸盆地群形成演化及深水油气田发育特征

Basin Evolution and Development Characteristics of Deepwater Oil and Gas Fields in Coastal Basins,West Africa

西非海岸盆地群的形成演化受非洲板块构造演化和大西洋张裂的控制,总体上可以分为前裂谷阶段、裂谷阶段和后裂谷阶段。沿岸15个主要盆地从北往南可细分为5段,分别为北段盐盆、中段盆地、尼日尔三角洲、中南段盐盆、南段盆地。深水油气田主要发育于后裂谷阶段的上白垩统和第三系海退层序中,以油田为主。在尼日尔三角洲和中南段盐盆的下刚果盆地,所发现的油气田个数及可采储量均比较集中,个数占西非深水油气田总个数比例分别为55%和29%,占总可采储量的比例分别为48%和41%。烃源岩主要有裂谷阶段下白垩统的湖相烃源岩,后裂谷阶段上白垩统、第三系的海相烃源岩。绝大多数储层为浊积岩储层,类型以深水水道砂岩为主,其次为席状砂储层和砂质碎屑流储层。圈闭类型主要为构造-地层复合圈闭,其次为构造圈闭,另外还有地层-岩性圈闭,其所占比例较小。盐构造和泥岩塑性变形构造总体上控制了西非深水区大多数复合圈闭和构造圈闭的形成。

West Africa coastal basins are nowadays the hotspots of deepwater hydrocarbon exploration around the world.Controlled by Africa plate evolution and the Atlantic Ocean rifting,the evolution of West Africa coastal basins can be divided into pre-rifting,syn-rifting,and post-rifting stages.And 15 coastal basins can be subdivided into 5 regional sections from north to south,i.e.the salt basins in the north section,the basins in the middle one,the Niger delta section,the salt basins in the middle-south section,and the basins in the south one.Deepwater fields in these basins,most of which are oil fields,develop in post-rifting Tertiary and Upper Cretaceous regressive sequences.The fields that have been discovered are,in amount and commercial reserves,major in the Niger delta section and the Lower Congo Basin（in the middle-south section）,which respectively occupy 55% and 29% of the total field amount and 48% and 41% of total commercial reserves in West Africa.Source rocks include the Lower Cretaceous lacustrine shales（deposited during syn-rifting stage） and the Upper Cretaceous and Tertiary marine shales（during post-rifting stage）.The most significant deepwater reservoirs are turbidite sandstone,and primary reservoir facies are channel reservoirs,followed by sheet sands and sandy debris flow sediments.The types of deepwater traps are stratigraphic-structural combination traps in majority,and the next are structural traps and stratigraphic traps in minority.Most of the combination traps and structural traps are connected to salt structures or mud-diapirs.