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Deciphering an Unconventional African Reservoir: Geological Characterization of Granitic Basement from Bongor Basin, Chad

Abstract

Unconventional reservoirs of Precambrian basement in Africa are rare and poorly understood due to a number of reasons including complex geological setting, limited technical knowledge and availability of data. Development of such reservoir requires a good understanding of fracture network and facies with secondary porosity. The study presents a workflow based on a case study from Bongor basin, Chad to effectively characterize granitic basement reservoirs with borehole images and open hole logs. Based on the integrated approach from geological and petrophysical analysis from the study wells, three main distinct facies and one sub-facies were interpreted. These are Unweathered, Leached, Fractured and Intensely fractured granites respectively. Within the leached granite, there exist fault breccias and major faults with two dominant strike orientations; NE-SW and NW-SE. This resulted to the development of secondary porosity within these reservoir zones. The observed granite basement facies, particularly the fractured and leached granites developed good secondary porosity due to differential leaching and fracturing. Two major classes of fractures striking in two different directions were identified across the study wells; WNW-ESE and WSW-ENE. Majority of these fractures are conductive and high angles occurring within the reservoir zones. This probably indicates that they are open (unless clay-filled). This is further investigated by the resistivity readings which reduced considerably indicating a possible influx of drilling fluid thereby generating the low resistivity response on the borehole Images. Fracture aperture and porosity were computed using the scaled borehole image logs. The maximum fracture porosity observed is 0.3% with a corresponding fracture aperture of 0.3cm and this sets the stage for understanding the flow potential. Present day insitu stresses interpreted from the borehole breakout analysis suggest a slight rotation in stress-fields between the study wells. This can be attributed to the reactivation of the fault lines (due to the continuous migration of the Benue trough) around the triple junction system that extends into southern Chad. This is an innovative work which offers an alternative method of fracture characterization and potentially productive facies and fractures are identified based on their characteristics and in-situ stresses in the absence of dynamic data.