Integrated interpretation of potential field and seismic data for shale gas potential in the Karoo Basin, South Africa

Abstract

Recent interest in the Karoo basin of South Africa has been sparked by the possibility of extensive shale gas reserves. Here we present an integrated study of historical reflection and refraction seismic (industry and academic), borehole, MT and regional gravity and magnetic data, with the goal of evaluating the shale gas potential. The reflection seismic data, constrained by 5 deep boreholes and MT data are used to map the contact between the base of the Karoo (Dwyka Group) and the underlying basement. This provides an important constraint on the thickness of the Karoo Supergroup (~5.5 km). By constraining the gravity data with the structure known from the reflection seismic and density data from boreholes, this contact can be extended to regions where the seismic data are sparse and suggests that the basin in the region near 24° longitude is thinner than on either side. The flexure of the basin also suggests that at the time of deposition, the Kaapvaal Craton was stiff, preventing a deep basin from developing further northwards on-craton. Additionally, the magnetic and borehole data highlight pervasive Jurassic dolerite sills and dykes. These sills and dykes can be highly fractured at surface, but at depth are possibly impermeable, providing seals and traps. They are also an important source of heat and need to be accounted for when evaluating the gas potential. The borehole data indicate sills can occur at any depth throughout the basin. The near surface sills are easily mapped with the coarse national magnetic data and single flight line modeling suggests that the edges of deeper sills can be delineated with higher resolution magnetic data. This approach indicates the importance of integrated studies to evaluating the shale gas potential in the Karoo.