The characteristics of source rock and hydrocarbon charging time of Precambrian granite reservoirs in the Bongor Basin, Chad

The success of hydrocarbon drilling in Precambrian granite reservoirs expands the petroleum exploration boundary in the Bongor Basin which belongs to the Central and western Africa Rift System. This study integrated the analyses of geochemistry, fluid inclusion and 1D basin models to characterize and identify the source rock of the granite reservoirs, reconstruct the burial history and investigate the time of the hydrocarbon charging in the granite reservoirs. The results indicate all the 5 synrift intervals are organic rich deposition and the kerogen is dominated by type Ⅱ1 and type Ⅰ. However, the Lower Cretaceous P, M and K Formations are regarded as potential hydrocarbon source rock intervals with a high richness of organic matters and thermal - evolution degrees. The close relationship in biomarkers between the crude oil in granite reservoirs and the source rocks in the P and M Formations indicates the P and M Formations are the most potential source kitchens for the granite reservoirs. Core samples obtained from 3 representative burial hills were used to conduct the fluid inclusion analysis. Constrained by the uplift history which was reconstructed based on the apatite fission track analysis obtained from the inner reports and the Ro-calibrated thermal history, four 1D basin models were carried out, and the model of the Well Baobab NE-1 indicates that the hydrocarbon generation began at the time of approximately 89 Ma. By projecting the homogenization temperature to the burial history of the corresponding wells, the hydrocarbon charging time was revealed: twice hydrocarbon charging in the granite reservoirs occurred simultaneously during 87 to 82 Ma and 76 to 72 Ma, corresponding to the uplifting episode in the Later Cretaceous.