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Evaluation of Coal Bed Methane Play in the Eastern Part of the Central Kalahari Karoo Basin (Botswana)


Nine coreholes were drilled by the Kubu Joint Venture (JV) in the northeastern part of Central Kalahari Karoo basin (Botswana) in 2012-2013 to evaluate the resource potential of the Serowe-Morupule (Ecca Group, Permian) coal bed methane (CBM) play. Coal seams were encountered in all wells at ~240-520 m depth. Analytical data from coal samples and desorbed gas samples were interpreted. Coal macerals are dominated by vitrinite and inertinite and have smaller (<13%) portion of liptinite. Most coals experienced relatively simple burial and uplift and have sub-bituminous rank with vitrinite reflectance (VR) below 0.6%. Some coals were affected by contact metamorphism from dolerite intrusions during the Early Jurassic and have high volatile bituminous to anthracite rank with VR up to 5.5%. Our interpretation of the data suggests that the coal bed gas is mostly primary microbial methane (average C1/(C1-C3) ratio for 93 gas samples is 0.989, average δ13C of C1 for 33 samples is -61.2‰). Total gas content varies from ~0.2 m3/t to ~5.3 m3/t (average 1.1 m3/t for 158 samples). Gas saturation varies from 7% to 53% (average 23% for 41 samples). VR data and 1D burial history models suggest that during the Mid-Late Jurassic the coals were buried to a maximum depth of ~1.5 km prior to uplifting to present-day depths. This burial was apparently not sufficient to generate significant amounts of thermogenic methane. Most of the thermogenic gas generated from localized heating of coals adjacent to dolerites appear to have been expelled from the coals. Microbial activity was apparently limited by poor accessibility of meteoric water (carrier of microbes and nutrients) to coal seams. These factors led to relatively low gas content and gas saturation in coals, making the northeastern part of the Central Kalahari basin non-prospective for CBM development for the Kubu JV.