Deflating the shale gas potential of South Africa’s Main Karoo Basin

The Main Karoo Basin has been identified as a potential shale gas resource (i.e., natural gas that can be extracted via the process of hydraulic stimulation or “fracking”). Current resource estimates of 0.4-11x109 m3 (13-390 Tcf),1-6 are speculatively based on carbonaceous shale thickness, area, depth, thermal maturity, and most of all, the total organic carbon (TOC) content of specifically the Ecca Group’s Whitehill Formation with a thickness of more than 30 m.7 These estimates were made without any measurements on the actual available gas content of the shale. Such measurements were recently conducted from two boreholes, and reported here indicating the presence of little to no desorbed and residual gas despite high TOC values. In addition, vitrinite reflectance and illite crystallinity of unweathered shale material reveal the Ecca Group to be metamorphosed and over mature. Organic carbon in the shale is largely unbound to hydrogen, and little hydrocarbon generation potential remains. This led to the conclusion that the lowest of the existing resource estimates, namely 0.4x109 m3 (13 Tcf), may be the most realistic. However, such low estimates still represent a large resource with developmental potential for the South African petroleum industry. To be economically viable it would require the resource to be confined to a small well delineated “sweet spot” area in the vast southern area of the basin. It is acknowledged that the drill cores we investigated fall outside of currently identified sweet spots and these should be targets for further scientific drilling projects.