南大西洋被动陆缘盆地盐岩对油气成藏的影响

Impact of salt on hydrocarbon accumulation in South Atlantic passive margin basins

南大西洋两岸被动陆缘盆地经历了相似的构造演化历史,在早白垩世阿普特期发育了区域性分布的盐岩。在对南大西洋中段两岸6个主要含盐盆地的区域构造、沉积演化及油气成藏综合研究的基础上,通过各盆地盐构造样式的地震剖面解析,将盐盆划分为伸展区、过渡区和挤压区3个盐构造带。巴西大陆边缘盆地发育的盐下烃源岩现今仍处于生油窗内,而西非盐上烃源岩的成熟速率高于盐下烃源岩,主要由于盐岩具有较高的热导率,能抑制盐下烃源岩的生烃,促进盐上烃源岩热成熟。盐流动产生多种样式的盐构造,为盐上油气聚集提供了构造圈闭条件。盐相关圈闭是盐上重要的圈闭类型,富集的油气储量占南大西洋被动陆缘盆地盐上油气总可采储量的84.8%。伸展区发育的盐窗作为输导通道,控制油气主要于盐上层系成藏。同时盐岩作为厚层的区域盖层控制了过渡区到挤压区的盐下油气成藏。

South Atlantic passive margin basins experienced a similar tectonic evolution and regionally distributed salt was deposited during the Aptian. Based on a comprehensive study of regional tectonics,depositional environment and hydrocarbon accumulation,integrated with the study of salt-related structural styles determined by analyzing regional seismic sections,three tectonic belts,which include extensional domain,transitional domain and compressional domain,were recognized in the 6 main salt basins. The pre-salt source rocks in the Brazil salt basins are still in the oil window. In West Africa,the post-salt source rocks experienced a higher maturation rate than the pre-salt source rocks. The salt has a relatively higher thermal conductivity and can restrain the hydrocarbon generation of the pre-salt source rocks but accelerate that of the post-salt source rocks. The salt flow resulted in several kinds of salt-related structural traps for hydrocarbon accumulation in the post-salt sequences. Reserves in the post-salt related structural traps account for 84.8% of the total post-salt reserves. The salt windows developed in the extensional domain served as hydrocarbon migration pathways and controlled hydrocarbon accumulations in the post-salt sequence. The thick salt provided a regional cap rock,which controlled the pre-salt hydrocarbon accumulations in the transitional-compressional domain.