A quantitative evaluation of shale gas content: A new insight from the Karoo Deep Drilling Project, South Africa

The Council for Geoscience of South Africa undertook stratigraphic drilling in the southern Karoo, to assess and characterise potential shale gas sequences in Permian rocks. Prospective shale sequences occur in the Collingham, Whitehill, and Prince Albert formations of the lower Ecca Group. Borehole KDD-01 was drilled to a total depth of 2 979 m within a prospective area for shale gas in the southern part of the main Karoo Basin in the Western Cape. The geology and shale gas potential were evaluated by means of systematic core sampling and various hydrocarbon analyses. Thirty-six borehole samples were collected at depths between 2 439 m and 2 548 m. All samples were characterised and analysed for Rock-Eval pyrolysis and gas content. The results indicate that the most prolific interval is in the lower part of the Whitehill Formation between depths of 2 465 m and 2 495 m. High Total Organic Carbon content (TOC) with an average value of 6.6 wt.% indicates that the samples may have had high gas generation potential. However, due to high thermal maturity (vitrinite reflectance (Ro) of 3.27%), the organic matter is highly converted and has low hydrogen indices. The lower half of the Whitehill Formation shales showed an average free gas content of 2 m3/t. The results indicate that methane is the dominant hydrocarbon gas (mean 30 vol.%), while nitrogen represents the principal non-hydrocarbon gas (mean 72 vol.%) in the Whitehill Formation. This aligns with the Salzwedel-Peckensen subreservoir classification, validating its potential as a commercially exploitable gas resource. This is further supported by the integrated resource estimate, which estimates at least 30 trillion cubic feet (Tcf) of shale potential in the southern main Karoo Basin. Finally, the results also highlight nitrogen as the dominant non-hydrocarbon gas. Nitrogen removal would require additional infrastructure. This will need to be further considered prior to any exploration activities.