The complex interplay of different generations of fractures and the varying stress regime within the Bongor basin of Chad has posed serious problems in understanding the impact of these fractures on the producibility from the granitic reservoirs. These reservoirs correspond to Type I fractured reservoirs in which there is little or no primary porosity, and the fractures provide the essential secondary porosity and permeability.
The producibility of granitic reservoirs is strongly related to a few specific lithofacies where secondary porosity develops. The basement granites encountered in the wells have unique facies assemblages with differing characteristics controlling the reservoir properties. Three major facies; Unweathered Granite, Fractured (normal and intensely fractured sub-facies) and Leached Granites were delineated based on their geological and petrophysical characteristics, the Leached Granites being the best reservoir facies accompanied by fault breccia.
Knowledge of the orientation and variation in stresses within a basin is important for fractured reservoir characterization, stimulation and further exploitation. Two main generations (WNW-ESE and WSW-ENE) of fractures striking perpendicular to each other were identified. The WSW-ENE fracture set has the same orientation as the identified maximum horizontal stress which also corresponds to the strike of the regional Central Africa Shear Zone. This fracture set forms an integral part of the fracture network that give strong indications for future production from the basement. Slight rotations in in-situ stresses towards the south-eastern part of the basin may be indicative of possible reactivation of fault lines in response to migration of the Benue trough.
The placement of subsequent wells in the leached and fractured reservoir facies with preferred fracture orientation depends on several determinants, especially in light of the rotating stresses identified in the study; coming from an interplay of tectonic forces from the Central African Shear Zone arising from the West and Central African Rift system. This is particularly important for future production plans such as a stimulation treatment through hydraulic fracturing where specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing vertical fractures to open.
This study not only provides a methodology to characterize such complex reservoirs, but also suggests ways for optimal exploitation.