--> --> An Integrated Geophysical Approach to Mapping and Modelling the Karoo Dolerite Intrusions in the South-Eastern Karoo Basin of South Africa

2018 AAPG International Conference and Exhibition

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An Integrated Geophysical Approach to Mapping and Modelling the Karoo Dolerite Intrusions in the South-Eastern Karoo Basin of South Africa

Abstract

The south-eastern Karoo Basin is considered to be one of the most prospective areas for shale gas exploration in South Africa. The basin has several dolerite intrusions that possibly introduced heat into the basin and resulted in the large scale conversion of oil to gas. To date, the geometry of these dolerite intrusions and variations in electrical resistivity of the purported shale gas hosting Ecca Group is poorly documented despite over 30 years of research in the area. In this study, we investigate the variations in resistivity of the Ecca Group, estimate the average depths to magnetic signatures, and produce gravity profile models that reveal the configuration of the basin and how dolerite intrusions are interconnected at depth. The magnetic map shows some ring-like structures and lineaments which coincides with the mapped dolerite intrusions. The average depth to the top of the shallow and deep magnetic sources is estimated to be about 700 m and 15100 m, respectively. Depth slicing revealed that the dolerite intrusions are pervasive in the area, extending up to 5400 m. The Bouguer anomaly map shows an increase in gravity values from inland to coastal areas. The 2½D gravity profile models revealed the basin architecture and that the dolerite intrusions form a network of interconnected sills, dykes and inclined sheets at depth. The subsurface apparent resistivity of the Ecca Group varies between 2 Ωm and 557000 Ωm. The pseudosections show that the lower Ecca Group rocks are generally characterised by low resistivity compared to the upper Ecca Group rocks. With the exception of the Cookhouse section, all the measured sections have been intruded by dolerites. These dolerite intrusions could have impacted the shale resources and still poses threat to fracking operation.