Influence of dolerites on coal rank, maturity and total gas content in coal bed methane play, Botswana

Abstract

Coal bed methane (CBM) is the gas adsorbed into the solid matrix of the coal. Botswana’s energy has heavily been depending on coals. In order to explore CBM potential of Botswana, Kubu Energy Resources (a joint venture between Sasol and Origin Energy) drilled nine coreholes in three different license areas, which cover a total area of 3000 km2 in 2013. Due to extensive magmatic activity that happened about 180 million years ago, Botswana has abundant dolerite sills. In this thesis, I studied the subsurface geology of the Kubu’s license areas to understand the influence of dolerites on coal beds and CBM potential. I used well-logs to reveal lateral and vertical variations in lithology and created structure and isopach maps to understand variations in stratigraphy and structural setting. Coal rank, maturity and total gas content data were provided from Kubu Energy. I used the 1D basin modelling software Genesis (version 5.7) by Zetaware Inc. to build burial and thermal history models and to understand the thermal influence of dolerites on the surrounding sediments, coals and CBM. Thermal influence of sills depends on their thickness. Dolerite sills thicker than 15 m have thermal influence ratio 1:0.55 above the sill and 1:0.48 below the sill, i.e., they affect sediments at the distance about half of their own thickness. Dolerite sills thinner than 5 m have thermal influence ratio 1:5.2 above the sill and 1:6.1 below the sill, i.e., they affect sediments at the distances 5-6 times larger than their own thickness. Timing of the intrusion is also important. Less deposition time results in relatively higher maturity values in the surrounding sediments. Intrusions deposited almost instantaneously have the largest thermal effect on the surrounding sediments. As expected, coal rank, maturity (vitrinite reflectance) and gas content increase towards the contacts of dolerite sills with surrounding rocks. In this study, non-heat affected samples display lower total gas content values than typical productive CBM plays in the U.S basins. However, heat-affected coals in Botswana have gas content in the range of typical productive CBM plays in the U.S basins.