Abstract: The Bongor Basin is a MesoCenozoic intracontinental passive rift basin developed under the influence of the Central African shear zone. It is dominated by the Lower Cretaceous sourcereservoircap assemblage. The basin is underexplored overall, and knowledges of its oil/gas resources and exploration prospects are limited. Taking the Ronier Oilfield as an example, through a systematic organic geochemical analysis and combining with regional geological data, we concluded that the Lower Cretaceous source rocks belong to the Lower Cretaceous dark mudstone with TOC content over 3.5%. The characteristics of saturated hydrocarbon gas chromatography, sterane and terpane showed that the source rocks developed in a brackish waterfreshwater and weakly oxidizedweakly reduced lacustrine sedimentary environment, and the organic matter had typical continental lacustrine characteristics in its development and distribution. The organic matter is type Ⅰ, Ⅱ1, Ⅱ2. It has a thick accumulation of hydrocarbon source rock with huge hydrocarbongenerating potential, and is the main source of oil and gas for the Ronier Oilfield. History of the source rock evolution suggests that the central sag differed from the western sag and reached much higher evolutionary maturity in both source rock and oil products. The main oil and gas resources in the Ronier Oilfield on the northern slope came from the central sag, as evidenced in both geological and geochemical aspects. The inverted tectonic zone, along with the trajectory of hydrocarbon evolution and favorable lithofacies zone, controlled the process of oil and gas enrichment.

Key words: crude oil, source rock, petroleum geochemistry, hydrocarbon accumulation, Bongor Basin, Chad