西非宽扎盆地烃源岩分布及油气成藏

SOURCE ROCK DISTRIBUTION AND PETROLEUM ACCUMULATION IN KWANGA BASIN WEST AFRICA

宽扎盆地为南大西洋被动大陆边缘盆地,构造演化可划分为裂谷期、过渡期和后裂谷期3个阶段,过渡期发育阿普特阶盐岩。宽扎盆地主要发育3套烃源岩:盐下下白垩统阿普特阶湖相烃源岩、盐间早白垩世阿普特期海相烃源岩和盐上古近纪始新世海相烃源岩。宽扎盆地油气田均围绕烃源灶分布,烃源岩是该盆地油气藏形成与分布的主控因素。宽扎盆地发育4套含油气系统(盐下下白垩统、盐间下白垩统、盐上上白垩统和盐上古近系)以及2种成藏模式(古生新储和自生自储)。宽扎盆地深水21区块Cameia-1井、Cameia-2井以及21区块Azul-1井盐下地层获得重大油气勘探突破,表明该盆地深水区存在盐下下白垩统含油气系统并发育优质湖相烃源岩,提高了宽扎盆地深水区油气勘探潜力。

The Kwanza Basin, as one of the West African passive continental margin basins, has un- dergone a complex history of basin development consisting of rift stage （Late Jurassic to Early Creta- ceous）, transitional stage （Early Cretaceous Aptian） and post-rift stage （Late Cretaceous to Holocene）. A striking feature of the transitional stage is the wide deposition of the Aptian salt. The Early Cretaceous, Late Cretaceous and Paleocene sequences have been identified as major hydrocarbon source rocks. They include the Early Cretaceous Barremian sub-salt lacustrine shale, the Late Cretaceous Aptian intra-salt marine carbonates and the Paleocene supra-salt marine shale. Oil and gas fields occur in circles around the graben which were filled with source rocks, so the source rocks remain the most significant factor for petroleum formation and distribution in the Kwanza Basin. Four major petroleum systems are defined in the Kwanza Basin, i.e. the Lower Cretaceous sub-salt, Lower Cretaceous Aptian intrasalt, upper Cretaceous supra-salt and Paleocene supra-salt. There are two types of hydrocarbon accumulation models. One is the model of self-generation and self-accumulation, and the other the model of early generatation and later-accumulation. The Cameia-1, Cameia-2 and Azul-1 deepwater exploration wells, which are located in the Block 21 and 23 in the Kwanza Basin respectively, are the significant sub-salt oil＇s discovery in the Lower Cretaceous sequences. It proves that there are petroleum systems in the Lower Cretaceous sub-salt strata and source rocks available in the Early Cretaceous Barremian organic-rich lacustrine shale. It has provided a significant new sub-salt oil exploration target in the deep water area of the Kwanza Basin.