关键词:
中非裂谷系, 裂谷盆地, Bongor盆地, 成藏组合, 结晶基岩, 裂缝性基岩储层

Abstract: 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.

Key words:  Central African Rift System, rift basin, Bongor Basin, play, crystalline basement, fractured basement reservoir