Identifying subsurface gas origins within the Witwatersrand Basin, South Africa

Several Mines in the Witwatersrand Basin, South Africa, provided access into deep subsurface (>3km) Precambrian geological sequences, to obtain information about ancient environments and extend the knowledge of subsurface systems. To ensure sample quality is maintained precautions must be taken while sampling these regions.; Within the Basin volumetrically significant quantities of hydrocarbon rich gases are encountered when drilling operations intersect faults and dykes. Gas origins are identified based on compositions and stable (δ 2H and δ13C) isotopic analyses of CH₄ to C₄H₁₀. The distribution of results indicates that two distinct gas types are encountered: (1) within the Witwatersrand Supergroup, is thermogenic gas with a microbial gas component; (2) gases within the Ventersdorp Supergroup, are abiogenic in origin. Many isotopic results from the Ventersdorp Supergroup samples indicate a mixing component with the Witwatersrand Supergroup gases. Mixing is further identified by δ2H and δ18O signatures of the associated groundwaters.