Mineralogical and Geochemical Characteristics of the Shale and Coal Associated with the Witbank Coalfield for Potential Unconventional Gas Generation in Mpumalanga South Africa

Exploration of the country’s potential for unconventional gas development as an energy mix can complement existing energy sources. In order to expand the energy security of South Africa, this study characterises the Carbonaceous Shale (CS) and coal samples retrieved from boreholes in the Witbank Formation. The mineralogical, major and trace element compositions were analysed using X-Ray Diffractometer (XRD), X-Ray Fluorescence (XRF) and Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), respectively. The XRD quantitatively reveals the presence of quartz (18–36%), kaolinite (15–45%), pyrite (0–0.6%), muscovite (4–9%), anatase (0.3–0.7%), rutile (0–0.8%), microcline (5–10%), siderite (1.2–23%), plagioclase (0.1–3%) and organic carbon (11–20%) in the studied carbonaceous shale. At the same time, the studied coal samples showed similar composition and amount except for a higher content of organic carbon (26–65%) and a lower content of quartz (12–26%) and kaolinite (13–40%). The amounts of the organic carbon in both CS and coal samples indicate sufficient presence of organic matter to generate hydrocarbon since it exceeds the minimum threshold limit of 5 wt% TOC. The major elements in the carbonaceous shale include SiO2, Al2O3, CaO, Cr2O3, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, TiO2, and LOI (17%). A similar trend of elemental composition and amount were present in the studied coal except LOI value (47%) in coal, indicating a higher content of organic car-bon compared to the carbonaceous shale. The trace element showed relatively high concentration in the CS compared to the coal, implying a higher paleo-productivity of abundant organic matter in the shale than the coal. However, the elemental proxy of V/(V + Ni) for the shale and coal are 0.65 ppm and 0.66 ppm respectively, exceeding threshold limit of 0.54 ppm, indicating an anoxic condition that can be favourable for organic matter prone to unconventional gas generation. Furthermore, the average concentration of Ba element (595 ppm) in the CS is relatively higher than in the studied coal (284 ppm), suggesting dissolution of mineral barite in the black shale by action of sulphate-reducing bacteria. The average concentration of V and Cr fall within the threshold limit, 50–250 ppm, indicated for lacustrine unconventional gas.