Precambrian basement reservoirs: Case study from the northern Bongor Basin, the Republic of Chad

Abstract

Bongor Basin in the south of Chad is a Cretaceous–Paleogene rift basin developed on Precambrian crystalline basement and is part of the central African rift system (CARS). High oil flows have been tested from five basement buried hills, indicating that the Precambrian basement is a good reservoir for hydrocarbon accumulation. Core analyses from 10 wells indicate that the basement rocks are composed of plutonic rocks (including granite, syenite, monzonite, quartz monzonite, and diorite) and orthometamorphites (including migmatitic granite, migmatitic gneiss, and granulite). Structural mapping of the top basement shows that faults in basement have a principal strike orientation of west–northwest-east–southeast and a secondary orientation of northeast–southwest. A combination of mud losses, well testing results, cores, wire-line logs, Formation MicroImager, sonic scanner logs, and three-dimensional seismic data were used to identify and detect the basement reservoirs. Two main sets of high-angle fractures, striking west–northwest-east–southeast and west–southwest-east–northeast, were identified. Three facies are divided based on the degree of weathering, intensity of fracturing, and sonic anisotropy. The basement can be divided vertically into four zones: weathered and leached, fractured, semifilled fractured, and tight zones. The weathered and leached and fractured zones are the main basement reservoirs in the study area. The effective porosities range from 3% to 10%, in places up to 25%. The porosity and permeability of basement reservoirs depend on the structural location and hydrothermal and authigenic precipitates developed during and after the basin formation. The discovery of basement reservoirs in the Bongor Basin will aid the hydrocarbon exploration of a new play in the CARS.