Geochemical evaluation of the carbonaceous shale associated with the Permian Mikambeni Formation of the Tuli Basin for potential gas generation, South Africa

Abstract

The increasing demand for energy and global commitment to reduce CO2 emissions to mitigate climate change has spurred countries to pursue unconventional hydrocarbon generation for power production. The investigation of the carbonaceous shale from the Permian Mikambeni Formation of the Tuli Basin in South Africa aims to assess the potential for gas generation. The Rock-Eval 6 pyrolysis of the studied samples shows total organic carbon (TOC) content ranging from 10.10 to 37.61 wt%, indicating an excellent source rock quality since it exceeds the threshold limit of 5 wt%. The produced gas that emanated from the thermal breakup of remaining hydrocarbon (S2) ranged from 14.30 to 65.55 mg HC/g rock while the hydrogen index (HI) ranged from 56.55 to 198.22 mg/g TOC. The plot of the HI against T max indicates the predominance of type-III kerogen, prone to gas generation. The thermal maturity parameters, T max (430–466°C), and vitrinite reflectance, VRo (0.6–1.2), showed a matured organic material. The production index value (0.06) of the samples indicates a moderately mature source generating a biogenic gas. The carbon isotopic composition, δ 13C (−20.0 to −23.4‰), indicates a predominant influx of organic matter derived from a vascular higher plant. The studied samples showed relatively depleted values of oxygen isotopic composition δ 16O (14.1–18.8‰), suggesting a brackish water depositional environment. Additionally, the Ba concentration (303.15–331.27 ppm) exceeds the 200 ppm threshold for the marine environment, indicating a non-marine environment characterised by low marine paleo-productivity and high detrital influx. Based on the evaluation, the studied coaly-shale is found with a matured and type-III kerogen that is prone to gas generation in a fluvial, paleo-depositional environment.