--> --> Seismic Imaging of Dolerite Structures in the Eastern Karoo Basin, South Africa: Implications for the Basin’s Shale Gas Potential

2018 AAPG International Conference and Exhibition

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Seismic Imaging of Dolerite Structures in the Eastern Karoo Basin, South Africa: Implications for the Basin’s Shale Gas Potential

Abstract

The Karoo Basin in South Africa is the focus of current shale gas interest. The basin was extensively intruded by dolerites sills at 180 Ma, particularly in the east and the west. If these sills intruded adjacent to the shale horizon (Whitehill Formation) they will have heated the rocks. Knowledge of the geometry of these sills is therefore important, but is limited to outcrop and sparse historic seismic and well data. Here we present a recovered, re-processed and re-intepreted legacy 2D reflection seismic data acquired in the 1970s in the eastern Karoo Basin (west of Lesotho). The closest well to the profile, WE1/66, is 40 km to the east. The well intersects 19 sills ranging in thickness from 3 to 125 m. We would expect sills at all stratigraphic level to cause “ringing” (multiple reflections below the strong reflector) on the seismic data and therefore make it difficult to interpret. Despite these multiples, several sills have been successfully delineated in the seismic data, as well as the top of the Whitehill Formation and the basement. The seismic data also show imaging of a possible escape event originating within the shale layer. This is a deeper origin than previously expected (i.e., within the Beaufort and Stormberg Group). This suggests that the vent might have formed due to the explosive release of gas from heating of the shale layer by the sill, rather than from an interaction between the sill and moisture in the sediments at higher stratigraphic levels.