Geological characteristics and exploration direction of oil and gas in the passive continental margin basins in the northern part of West Africa
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摘要: 西非北部盆地勘探程度较低,但资源潜力巨大。基于板块构造理论,通过结合地震与地质方法的综合研究,分析了西非北部主要地质时期的原型盆地,对比了各盆地的结构特征及其沉积充填差异性,解剖已发现油气藏,建立成藏模式,指出下一步勘探方向。研究发现: 西非北部被动陆缘盆地群经历了前寒武纪—二叠纪前裂陷、三叠纪—早侏罗世裂陷和中侏罗世至今的陆缘3个演化阶段,形成了断陷型、断坳型和坳陷型3个被动陆缘盆地亚类。断陷型盆地形成单源-构造型成藏模式,勘探目标以裂谷层系的构造成藏组合为主; 断坳型盆地可形成双源-双组合型成藏模式,大陆架上的缓坡带以裂谷层系构造圈闭为主,在台地边缘应寻找礁体和内幕岩溶,陡坡带上部的主要勘探目标应为规模较大的滑动-滑塌形成的斜坡扇,下部应关注成熟烃源岩层系内扇体所形成的上白垩统和下白垩统砂岩构造-岩性圈闭; 坳陷型盆地以坳陷层系烃源岩为主,主要勘探对象为陡坡下部的浊积砂岩及台地缓坡三角洲砂体,台地边缘礁滩体、台地碳酸盐岩风化壳岩溶和断裂带岩溶也是值得重视的勘探方向。研究可为勘探程度低、资料获取难的被动陆缘深水油气勘探选区评价提供借鉴。Abstract: The basins in the northern part of West Africa were lowly explored, but with huge resource potential. Based on the plate tectonics theory and comprehensive study of seismic and geology methods, the researchers analyzed the prototype basins of the main geological periods and compared the structural characteristics of the basins and their differences in sedimentary filling. The discovered oil and gas reservoirs were dissected to establish the mode of hydrocarbon accumulation and point out future exploration direction. The passive continental margin basin group in the northern part of West Africa has undergone three evolutionary stages, that is Precambrian-Permian pre-rift, Triassic-Early Jurassic rift, and passive continental margin basin from Middle Jurassic to the present, and has formed three subclasses of passive continental margin basins, that is fault basins, fault depression basins, and depression basins. The fault basins formed a “single source-tectonic” accumulation model, and the exploration target is mainly structural reservoir combinations of rift strata. The fault depression basins could form “dual source-dual combination” accumulation model. Structural traps of rift strata are mainly on the gentle slope belt of the continental shelf, and reefs and inner karsts are on the edge of the platform. The exploration target in the upper part of the steep slope zone is slope fans formed by large scale slipping-sliding, and the lower part should focus on sandstone structural-lithologic traps of Upper Cretaceous and Lower Cretaceous formed by fans in mature source rock strata. The depression basins are dominated by source rocks of the depression strata, and the main exploration areas are turbidite sandstones in the lower part of steep slope and delta sandstones in the gentle slope of platform. Meanwhile, the reef-beach bodies on the platform margin, the weathering crust karsts of carbonate rock and the karsts in fault zone are also important exploration direction. The research results could provide references for deep-water petroleum exploration selected areas in passive continental margin with lowly exploration degree and poorly data obtaining.
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