Detection of potential methane gas pathways in deep South African gold mines

Abstract

At a number of gold mines in South Africa, methane gas has been encountered when drilling into faults and/or dykes extending to depths beyond 7000 m. Methane gas has been reported to migrate up through structures from within the basin to the mine working environments (∼3000 m depth) and caused explosions. The Booysens Shale is one of the possible source rocks for hydrocarbons and it forms the footwall to the gold-bearing Ventersdorp Contact Reef (VCR, ∼1.5 m thick). The Booysens Shale lies at depths between 3500 and 4500 m below the surface and can be best described as a base of the divergent clastic wedge, hosting the quartzite and conglomerate units that sub-crop against the VCR towards the west of the gold mining areas. Geometric attributes (dip and dip azimuth) and instantaneous attributes (phase, frequency and envelope) computed for the Booysens Shale and VCR horizons provide insight into structures that extend from the Booysens Shale into the overlying mining level (i.e. VCR). These attributes successfully mapped the structures (faults and dykes) that have displaced both the VCR and Booysens Shale horizons. The edge-detection attributes were particularly useful in delineating faults with throws below the traditional seismic resolution criteria (e.g. a quarter of the dominant wavelength) that were not visible in the conventional seismic interpretation. The structural analysis using seismic attributes gives a new visual representation of geological structures that may be probable conduits for hydrocarbons, particularly methane, migrating from the Booysens Shale to the VCR mining levels.