西非北部塞内加尔盆地油气富集规律与勘探方向

Hydrocarbon Accumulation Characteristics and Future Exploration Direction in Senegal Basin, Northwest Africa

西非北部塞内加尔盆地是位于前寒武系—古生界结晶基底之上的巨型裂谷−被动陆缘叠合盆地。笔者基于地震、典型钻井、测井资料及国际油气商业数据库和勘探的最新成果,结合区域地质背景和盆地构造沉积演化特征,分析盆地油气成藏条件、成藏特征和富集规律,并预测未来勘探方向。研究表明,盆地经历前裂谷、同裂谷和被动陆缘3期构造演化阶段,盆地结构呈“北缓南陡”形态,具体表现为“北部窄陆架缓陆坡、南部宽陆架陡陆坡”特征。盆地发育裂陷期湖相和被动陆缘期海相两套烃源岩,裂陷期烃源岩对盆地深水区白垩系成藏具有重大贡献,被动陆缘期烃源岩在上覆地层较厚时才成熟。盆地可划分为3种油气成藏模式。①陆架区下生上储,断层沟通不整合,陆架三角洲前缘浊积砂岩成藏模式。②斜坡坡脚区下生上储、断砂配置输导、近源供烃成藏模式。③深水−超深水区下生上储,垂向运移,构造−地层型浊积砂岩成藏模式。陆架边缘三角洲前缘砂体、斜坡坡脚处的浊积水道和海底扇、深水区浊积水道和浊积扇是塞内加尔盆地重点勘探领域,深水区大型构造−地层圈闭为盆地最重要的勘探目标。

The Senegal basin in Northwest Africa is a giant rift−passive margin superimposed basin floored by the Precambrian−Paleozoic crystalline basement.Based on combinations of seismic,drilling,well logging data and the latest international commercial petroleum databases and exploration results,along with understanding of regional geological history(tectono−sedimentary evolutions),this study was focused on hydrocarbon accumulation conditions and patterns and enrichment rules of hydrocarbons,and predicted the future direction of exploration in the Senegal basin.The study shows that the basin experienced three tectonic evolution stages:pre−rift,syn−rift,and passive margin.The current structure of the basin is characterized by“a gentle northern slope and a steep southern slope”,specifically,the narrow continental shelf and gentle continental slope in the north the wide shelf and steep slope in the south.The basin has lacustrine source rocks deposited during the rifting period and marine source rocks deposited in the passive margin period. The lacustrine source rocks have a more signifi-cant contribution to petroleum accumulations in the Cretaceous reservoirs in deepwater areas. Marine source rocks became mature until the thicker overlying sediments deposited. Three types of hydrocarbon accumulation models were recognized: ① lower−generation and upper−storage in the continental shelf area characterized by delta−front turbidite sandstones charged through unconformity and faults. ② lower−generation and upper−stor-age at the foot of the continental slope dominated by near−source fault−sandstones configuration. ③ lower−gen-eration and upper upper−storage in deepwater and ultra−deepwater areas dominated by structural−stratigraphic turbidite trap through vertical migration. The shelf−margin delta−front sandstones, turbidite channels and sub-marine fan lobes at the slope foot, turbidite fan in the deepwater areas are favorable targets for exploration. Large structural−stratigraphic traps in deepwater areas are the most important exploration target in the Senegal basin.