Thickness matters: Influence of dolerite sills on the thermal maturity of surrounding rocks in a coal bed methane play in Botswana

Abstract

Abstract Global studies suggest that igneous intrusions usually have thermal aureole thicknesses between 30% and 250% of the intrusion thickness. Here, we discuss data and 1D thermal maturity models from nine coreholes (wells) drilled in Botswana to explore for coal bed gas. The coreholes penetrated multiple dolerite sills with thicknesses ranging from 1.2 m to 73.4 m. The sills intruded the sedimentary rocks in the Jurassic period and significantly affected the rank, properties and thermal maturity of Permian coals. The thermal influence of the sills extends for >5–53 m away from their contacts with surrounding rocks. The relatively thick (>13 m) sills have thermal aureole thicknesses of >31–137% of the sill thickness, in line with global observations. However, the relatively thin ( 1000% of the sill thickness, significantly larger than previously reported from most other locations affected by igneous intrusions. Thin sill intrusions in coals and organic-rich shales may lead to the generation of larger volumes of CH4, other hydrocarbon gases, and CO2 than previously thought, affecting the safety in coal mines and gas resources.