Hydrocarbon Accumulation Conditions and Exploration Breakthrough in Basement of Bongor Basin, Chad

Abstract

Basement exploration was long ignored due to its poor reservoir properties in basins of Central African Rift System (CARS). As one of Meso-Cenozoic rift basins in CARS, Bongor basin experienced intensive structural inversion during the Santonian Squeeze Event, which resulted in its unique structural features and improved reservoir properties of basement favorable for hydrocarbon accumulation. During the initial rifting stage, high-quality lacustrine shale in M and P Fm. were deposited in several deep half-grabens in Bongor basin. The P and M Fm. shale are characterized by good kerogen type, high TOC and huge thickness, which act as the main source rocks in the basin. The basement is mainly composed of pre-Cambrian granite, granite gneiss and granodiorite, which experienced long-period of weathering and was strongly fractured during Santonian Squeeze Event. Buried hills can be classified based on their location, vertical relief and overlying strata. The widespread and thick M Fm. shale covers most of basement uplifts between small half-grabens, acting as excellent caps rock. Hydrocarbon accumulation conditions show that the early buried hills located by the hydrocarbon enriched depressions are the most favorable targets. Based on this understanding, the recommended prospect LE-2 succeeded and became the first basement discovery in Bongor basin. The following four large buried hill discoveries represented by the BC-1 further strengthened this understanding. The breakthrough does not only reveal a good hydrocarbon potential in Bongor basin, but also provides an important reference for basement exploration in other basins of CARS.