Abstract: The BS oil reservoirs are hosted to the GB oil field, Bongor Basin, southwestern Chad. Combined with regional sedimentary background and core data for the BS1-1 well, the present paper focuses on the mudstone colour, rock association, sedimentary texture and structure, grain size distribution, sedimentary rhythms, sedimentary microfacies and their controls on the hydrocarbon distribution in the BS oil reservoirs. The PI pay set in the BS oil reservoirs is interpreted as the fan delta front distal deposits. The subaqueous distributary channel deposits occur only around the faults. The other deposits include the well developed frontal sheet sands, minor amount of distal bar deposits, and less developed interdistributary channel deposits. The sedimentary microfacies types have exercised a major control on the type, distribution and quality of the reservoirs and hydrocarbon distribution. The subaqueous distributary channel microfacies is delineated to be the favourable facies zone for the hydrocarbon accumulation owing to the excellent source-reservoir-seal associations in the BS oil reservoirs.