The Effect of Dolerite Intrusions on the Hydrocarbon Potential of the Lower Permian Whitehill Formation (Karoo Supergroup) in South Africa and Southern Namibia: A Preliminary Study

Abstract

Recent interest in the shale gas potential in the main Karoo Basin of South Africa has focused attention mainly on the carbonaceous Whitehill Formation (Ecca Group, Karoo Supergroup). To unravel the metamorphic effect of the Lower Jurassic Karoo igneous intrusions on the Lower Permian Whitehill Formation, outcrop samples were taken at various distances (ranging from 1 to >30 m) from the intrusive contacts of dolerite sills in the western part of the main Karoo Basin (Northern Cape, South Africa) and in the Karasburg Basin (southern Namibia). Furthermore, to establish the baseline petrological and geochemical composition of the Whitehill Formation, samples have also been collected at localities unaffected by dolerites in the southern main Karoo Basin. Mineral assemblages from all sample localities plot within the chlorite zone and show very low grade metamorphism. Mineral assemblages and illite crystallinity data indicate that the Whitehill Formation has undergone (a) anchizone metamorphism (250 to 300°C) in the southern main Karoo basin as well as in areas close to the dolerite intrusions, and (b) moderate burial diagenesis (~180°C) in areas away from the immediate effect of dolerites in the Northern Cape and southern Namibia. Regional evaluation of unconventional gas potential of the Karoo basins of southern Africa ought, therefore, to consider the subsurface thickness and spacing of Karoo intrusive bodies within the target carbonaceous formations.