Oil-gas exploration and significance of the lower assemblage in western Bongor Basin,Chad

Hydrocarbon exploration has been carried out in the lower assemblage (P and F formations) of the western Bongor Basin for many years without breakthrough. Through in-depth studies of the hydrocarbon accumulation patterns in western Bongor Basin, Well D-2 was deployed and drilled by China National Petroleum Corporation in 2024; oil reservoirs were encountered during drilling in the lower assemblage of Bongor Basin, and the well production during well testing exceeded 200 tons per day, thus determining the exploration potential in the study area. To better summarize this discovery and guide further exploration, a systematic study, which is based on analysis of regional geological setting and exploration history, has been conducted on the tectonic evolution, stratigraphy, sedimentation, and source-reservoir-seal assemblages of the western Bongor Basin. On this basis, the hydrocarbon accumulation models and exploration deployment strategies have been clarified. The results show as follows. (1) The lower assemblage of Bongor Basin developed in the fault depression period. Thick lacustrine mudstones and near-source deltaic sand bodies were developed in the deep depression area, forming favorable lithological combinations. (2) The thick lacustrine mudstones of P and M formations, with high total organic carbon content and good organic matter type, have already entered into the oil generation window. They served as not only the excellent source rocks but also regional seals in western Bongor Basin. (3) Due to tectonic inversion, uplift and denudation during late stages, the deltaic sand bodies of P Formation had a certain porosity despite large burial depths, thus being considered as good reservoirs. The underlying buried hills experienced long-term structural fracture, weathering and denudation, and formed composite reservoirs with the sandstones of P Formation. (4) The Bongor Basin underwent multiple stages of tectonic evolution and inversion, leading to extensive fault development, which allowed oil and gas to migrate along the faults and accumulate in both upper and lower assemblages and buried hills. (5) Based on the characteristics of reservoirs in the lower assemblage, a comprehensive three-dimensional exploration deployment strategy was recommended to explore both shallow and deep formations, structural and non-structural traps, which can achieve breakthroughs in exploration of new strata and new fields. (6) The quartz sandstone above the basement is speculated to be a set of older formation than P Formation, and widely developed in western Bongor Basin, indicating good hydrocarbon accumulation conditions. The exploration breakthrough of Well D-2 has confirmed the resource potential in western Bongor Basin and exploited new reservoirs in Bongor Basin. Moreover, the three-dimensional exploration deployment strategy can provide guidance for oversea risk exploration in the future.