Abstract

Recent interest in the main Karoo Basin of South Africa has been sparked by the possibility of extensive shale gas resources. Historical reflection seismic, petroleum exploration wells and regional magnetic data are used to better understand the distribution and geometry of dolerite intrusions within the basin that could have impacted the shale reservoir. The lowest concentration of dolerites are found in a region stretching from the southwest to the southeastern part of the basin around the town of Graaff-Reinet. These intrusions are confined to the Beaufort Group, ~1000 m shallower than the shale reservoir.

In the southeastern Karoo around Queenstown, 5 to 30 km wide saucer-shaped sills extend down to ~800 m, with dips of between 2° and 8°. Further south, dolerite sheets around Somerset-East extend for over 150 km at dips of between 3° and 13°, and are imaged down to ~5 km. These dips appear to increase closer to the Cape Fold Belt in the south, although there is no correlation between the southern edge of these dolerites (i.e., the dolerite line) and the dip of sediments due to folding.

Magnetic data are useful shale gas exploration to detect shallower (<200 m) dykes that can extend to reservoir depth and are not visible on seismic data. Karoo dykes are usually between 1 and 10 m thick and are shown to often be beyond the resolution of the regional aeromagnetic data. Integrated studies using seismic and higher resolution magnetic data are therefore necessary to better understand the complex dolerite network of the Karoo.

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