Seismic Imaging of Dolerite Sills in the Karoo Basin, with Implications for Shale Gas Potential

Seismic data provide a unique opportunity to understand the subsurface structure of dolerite sills that have intruded the Karoo Basin at multiple levels. These sills have intruded close to the Whitehill Formation at several locations within the basin, and may impact shale gas exploration of this horizon. In the southeastern Karoo Basin, around Queenstown, seismic data reveal the presence of 5–30 km wide saucer-shaped sills with vertical thickness of up to ~270 m each, and dips of between 2° and 8° at shallow stratigraphic levels. Farther south, dolerite sheets near Somerset-East are imaged down to a depth of ~5 km and extend for over 150 km, with dips of between 3° and 13°. These dips increase closer to the adjacent Cape Fold Belt in the south. Around Lesotho, at the highest stratigraphic levels, intrusions are dominated by dykes and fluid vents that are evident on seismic data as regions of diffused reflectivity. The lowest concentration of dolerite is found in a region in the south-central part of the basin around the town of Graaff-Reinet. Here intrusions are confined to the Beaufort Group, ~ 1000 m above the shale reservoir. This change in dolerite distribution will influence the location of shale gas exploration.