Structure and Tectonic Evolution Controls on Hydrocarbon Accumulation in Fractured Basement: A Case study From Melut Basin in South Sudan and Bongor Basin in Chad

The Melut Basin in south Sudan and the Bongor Basin in Chad are two of Late Cretaceous to Tertiary rift basins distributed along the south and north side of Central African Shear Zone(CASZ). Hydrocarbon-hosted in fractured basement reservoir have been found in both two basins, but the hydrocarbon accumulation models of them are different. The objective of this study is to discuss the effects of tectonic evolution on hydrocarbon accumulation in fractured basement, which involves basement lithology, source rock development, hydrocarbon generation history, cap rock distribution and accumulation model. The analysis of regional tectonic evolution history indicates that, from the Late Precambrian to the Carboniferous, the entire Africa plate experienced a relatively quiet tectonic phase, culminating in the collision of Gondwana and Laurentia during the Carboniferous. The basement adjacent to the rift areas are predominantly Precambrian granitic and metamorphic rocks. The two basins have similar basement lithology, which can be proved by core, logging, microscopic identification and characters. The effect of basin evolution on large scale fractures in basement is studied with seismic attributes (Ant-tracking) to indicate the relationship between the change of stress field and fracture distribution. The comparative study on the impact of structure and evolution on hydrocarbon accumulation is carried out according to stratigraphic framework, hydrocarbon generation history, cap rock development, source-related faults and structural framework evolution. The Result shows that the two basins have experienced different tectonic evolution. The Bongor Basin experienced a relatively constant rifting-sagging stage during the Early Cretaceous, and the whole basin suffered a tectonic inversion in Late Cretaceous as a result of the collision of African plate and Eurasian plate. Whereas, the Melut basin experienced three phases of rifting-sagging cycle, which is stronger in the Early Cretaceous and Paleogene and weaker in the Late Cretaceous. The tectonic inversion only occurred in local area. The research on hydrocarbon generation history has shown the characteristics that hydrocarbon accumulated in the Early Cretaceous in Bongor Basin and in the Late Cretaceous in Melut Basin. This different evolution history lead to different accumulation conditions of basement reservoirs in terms of cap rocks, hydrocarbon charge and preservation, and also accumulation model.