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
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Seismic Imaging of Dolerite Sills in the Karoo Basin, with Implications for Shale Gas Potential Stephanie Scheiber-Enslin, Susan Webb, and Musa Manzi
Abstract
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. Keywords
Karoo dolerites
2.1
Hydrothermal vents
Introduction
Termination of the Karoo Basin sedimentation was characterized by a massive outpouring of lavas around 182 Ma, forming the Karoo Large Igneous Province (LIP, Duncan et al. 1997; Chap. 7 this book). Along with this outpouring, magma intruded as dykes and sills at multiple levels within the basin. These intrusions change from dominant dykes and saucer-shaped sills within the upper and mid-stratigraphic levels (Stormberg and Beaufort Groups), to extensive subhorizontal sills and sheets within the lower stratigraphic levels (Ecca and Dwyka Groups and basement; Du Toit S. Scheiber-Enslin (&) Council for Geoscience, Pretoria, South Africa e-mail: [email protected] S. Scheiber-Enslin S. Webb M. Manzi School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
2D seismic
Shale gas
1920; Chevallier and Woodford 1999; van Zijl 2006). The upper stratigraphic levels in the east are also disrupted by fluid vents interpreted as hydrothermal (Svensen et al. 2006), while breccia pipes are associated with the deeper shales that outcrop in the northwest (Svensen et al. 2007). There is a change in the distribution of dolerites throughout the basin. The largest concentration of dolerites is in the northwestern and eastern parts of the basin (with a combined dolerite thickness of >150 m in each well), with intrusions at all stratigraphic levels (Fig. 2.1, Cawthorn 2012; Scheiber-Enslin et al. 2014). There is a significant drop in dolerite volume in the south-central part of the basin (with a combined dolerite thickness of < 150 m in each well), with intrus
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