中非裂谷系前寒武系基岩油气成藏组合

中非裂谷系是在前寒武系结晶基岩基础上发育起来的中新生代裂谷盆地群。通过露头和钻井资料研究发现，基岩主要由前寒武系岩浆岩、正变质岩和少量副变质岩组成。根据暗色矿物含量可以将基岩划分为长英质和铁镁质岩石两大类，前者更有利于风化和裂缝的形成，储层物性较好。寒武纪-侏罗纪长期风化作用形成了广泛分布且厚度较大的球形风化壳(段)，早白垩世中非剪切带的走滑拉张应力场与前寒武系刚性基岩的耦合作用在基底产生了大量高角度断层、共轭缝、微裂缝和节理等，风化淋滤、表生和热液作用控制了溶蚀、胶结和次生孔隙发育。风化壳和裂缝储层发育具有“似层状”的特征，垂向上基岩潜山储层序列可进一步划分为风化淋滤带、裂缝发育带、半充填裂缝带和致密带四个区带。中非裂谷系盆地在早白垩世、晚白垩世和古近纪三期裂谷期发育了三期区域性厚层泥页岩，古近系暗色泥岩由于埋藏浅，处于未成熟阶段，不能作为有效烃源岩，上白垩统泥岩以滨浅湖相为主，有机质含量总体不高，以Ⅲ型干酪根为主，下白垩统深湖相泥岩是区域上的优质烃源岩，同时为基岩提供了良好的油源和顶盖层。根据盖层的时代，可以划分出下白垩统、上白垩统和古近系3类成藏组合。乍得Bongor盆地基岩勘探获得极大成功，发现了一批高产稀油油田，打开了中非地区一个新的油气勘探领域。

The Precambrian basement play in the Central African Rift System

The Central African Rift System (CARS) is a group of MesozoicCenozoic rift basins developed on the Precambrian crystalline basement. Our investigation on the outcrops and drilling data in the CARS indicated that the lithology of the basement rocks are mainly igneous rocks and orthometamorphites with small amount of parametamorphites. Based on the content of dark minerals, the basement rocks can be divided into two types: felsic and mafic rocks. The former is prone to be easily weathered and fractured than the latter, and as a result, the felsic rocks have better poroperm. Field investigation showed that a thick wide spread spherical weathered crust formed during the CambrianJurassic; during the Early Cretaceous, the coupling of the strike slip of the Central African Shear Zone and Precambrian brittle basement resulted in a group of high angle faults, en échelon fractures, microfractures and joints within the basement, while weathering, leaching, epithermal and hydrothermal activities controlled dissolution, cementation and development of the secondary pores. Our data further showed that the weathered crust and fractured reservoirs have layer like features, where the basement reservoir was vertically zoned, displaying the weathered and leached, fractured, semi filled fractured and tight zones. Moreover, three sets of mudstones formed in the Early Cretaceous, Late Cretaceous and Paleogene, respectively, potentially acting as a top seal for the buried Precambrian hills. The Paleogene mud shales developed immaturely due to shallow burial depth, therefore they could not serve as active source rocks. The Upper Cretaceous shales, deposited in the shallow lake facies, were composed primarily of kerogen type Ⅲ organic matter, although the overall content of organic matters was not high; whereas the Lower Cretaceous shales, deposited in the deep lacustrine facies, were exceptional regional source rocks, providing an excellent top seal for the basement reservoirs. According to the ages of the cap rocks, there are three hydrocarbon plays in the CARS, i.e, the Lower Cretaceous, Upper Cretaceous and Paleogene plays. The exploration of basement reservoir in Chad has led to the discovery of a number of commercial light oil fields in the crystalline basement reservoirs on the northern slope of the Bongor Basin, and opened a new frontier for the hydrocarbon exploration in the CARS.