Seismic imaging of dolerite sills and volcanic vents in the Central Karoo, South Africa: implications for shale gas potential

Abstract

The Karoo Basin of South Africa covers an area of 700 000 km2 and has been identified as a possible shale gas reserve. Any evaluation of the shale gas potential of the basin must consider the widespread dolerite dykes and sills. These intrusions were emplaced into the Karoo Supergroup and are well dated at around 183 Ma. Their intrusion triggered the explosive releases of gas in the basin, marked on surface by breccia pipes and hydrothermal vents. This outpouring of gas has been proposed as a significant contributor to global climate change. Research into the three-dimensional interconnected structure of these dolerite sills and dykes and their interaction with the hydrocarbon rich layers in the lower part of the Karoo Supergroup has been limited to localized observations of outcrop, magnetic data, legacy seismic data (from the 1970s) and well core. Here we present an interpreted 65 km long higher-resolution 2D seismic reflection profile across the Karoo Basin, approximately 100 km southeast of Trompsburg. These data were collected in the 1990s and at the time deeper structures along the line interpreted. In this study we focus on the top 0.6 to 2 seconds TWT of the data. The seismic line images the interconnected and cross cutting nature of the dolerite dykes and sills along the profile. We also report possible evidence of a gas escape structure (approximately 2.5 km in diameter at surface) emerging near the edge of a dolerite sill in close proximity to the Whitehill Formation, which is the main target for shale gas exploration. This suggests that gas vents in the eastern Karoo Basin close to Lesotho are due to the release of gas from the carbonaceous shales of the Ecca Group. This is similar to breccia pipes mapped on surface in the western part of the Karoo Basin. This seismic section highlights why dolerite sills and dykes must be considered when evaluating the shale gas potential of the Karoo Basin. We propose that better characterization of the Karoo Basin subsurface by seismic and magnetic studies is necessary prior to any efforts to calculate shale gas reserves.