裂谷与被动陆缘叠合盆地的盐构造与油气成藏——以西非下刚果—刚果扇盆地和宽扎盆地为例

Salt tectonics and its relationship to hydrocarbon accumulation in salt basins with a lower rifted section and an upper continental marginal section:A case study of the Lower Congo-Congo Fan basins and the Kwanza Basin in West Africa

西非海岸的加蓬、下刚果—刚果扇和宽扎等盆地均为典型的裂谷与被动陆缘叠合含盐含油气盆地,经历了裂谷期(早白垩世凡兰吟期—巴列姆期)、过渡期(阿普第期—早阿尔必期)和被动陆缘期(早白垩世阿尔必期—现今)3个演化阶段,沉积了裂谷期陆相地层、过渡期蒸发岩和被动陆缘期海相地层3套地层层序。古近纪以来,受非洲大陆隆升和大西洋被动陆缘持续沉降的影响,以过渡期蒸发岩为滑脱层,形成了变形特征迥异的盐上和盐下构造变形层;前者为盖层滑脱型构造,变形强烈,盐构造样式类型多样,从陆向海具有明显的构造分带特征;后者属于基底卷入型构造,变形微弱,基本保持了裂谷盆地的原始构造面貌。盐构造变形不仅控制盐上层系储层发育和圈闭形成,而且盐岩层本身又是盐下层系良好的区域盖层,尤其是盐岩层急剧增厚的深水—超深水地区,客观上对盐下和盐上油气系统具有明显的分隔作用,即盐下层系生成的油气仅在盐下层系运移聚集;但在盐岩层厚度急剧减薄或缺失的浅水至陆上地区,盐窗发育,加之断裂的垂向沟通,盐下油气可以向盐上层系运移并聚集成藏。

The West African coastal basins,such as the Gabon Coastal Basin,Lower Congo Basin and Kwanza Basin,are typical oil-bearing salt basins with a lower rifted section and an upper continental marginal section. Three evolution stages can be identified in those basins,including a rifting period（ the Early Cretaceous Valanginian to Barremian）,a transitional period（ Aptian to the early Albian） and a passive continental margin period（ the early Cretaceous Albian to present）,thus developing three tectono-sedimentary sequences from base to top： pre-salt,evaporites,and post-salt. On one hand,under the influence of the uplifting African craton and the continuing subsidence of the Atlantic passive margin from Paleogene to present,and taking the evaporite layer as a detachment,a gravity gliding tectonic deformation layer was formed in the post-salt sequence. This layer represents a coverdetached structure with complex deformation,various salt structural styles,and obvious tectonic zonation from east to west. On the other hand,tectonic deformation in the pre-salt tectono-sedimentary sequence was very weak,therefore,basement-related geological structures generally remained unchanged since South America and Africa separated some 165 million years ago. Salt tectonics controlled the formation and distribution of post-salt reservoirs and traps. At the same time,the salt layer works as a good regional seal for the subsalt sequence that can be interpreted as a boundary between post-salt and pre-salt petroleum systems. Therefore,in general terms,oil and gas generated in the subsalt petroleum system can only migrate and accumulate locally,particularly in deep to ultradeep water areas where the salt strata have been thickened. However,in shallower water areas,where salt thickness was dramatically thinned or missing,subsalt oil and gas may migrate into the post-salt sequence through salt windows and / or connecting faults. Existing near-shore post-salt fields in West Africa provide support for this migration and accumulation process.