Thermal impact of dolerite sills on the shale gas potential of Ecca Group mudstone in a drill core from the Central Main Karoo Basin

M.Sc. (Geology)Abstract: The Main Karoo Basin of South Africa, especially the Whitehill Formation of the Ecca Group is estimated as possessing shale gas reserve potential of between 368bcm to 13734bcm on a wide land area of over 183 000 km2, thus placing South Africa as the 8th largest potential shale gas resource in the world. However, this reserve could be overestimated due to the poorly studied thermal impact of voluminous magmatic intrusions into the basin strata. Jurassic-aged Karoo Large Igneous Province (KLIP) emplaced a myriad network of dykes and sills that takes up 0.55 x 106 km2 of the basin and up to 30% of the total thickness. This could bring additional heat to the thermal evolution of the basin likely to create thermal over-maturation and/or disrupt the preservation of generated hydrocarbon of the Main Karoo Basin source rocks. Here organic-rich mudstones of the Ecca Group with mineralogical, mineral chemistry, organic geochemistry and Ar – Ar age dating proxies were analysed. Illite crystallinity and chlorite chemistry studies show thermal evolution controlled by diagenesis and regional metamorphism as well as local contact metamorphic overprints in the vicinity of dolerite sills. 13Corg and TOC vary in proximity to the dolerite intrusions. Rock-Eval pyrolysis data show that the organic matters are mainly Type III gas-prone kerogens of mature to post mature thermal maturity. The source rocks are mainly over-mature, having poor generation potential which could reflect the thermal overcooking/exhaustion, probably accompanied by gas migration, driven by the thermal impact of dolerite intrusions. The clay mineral ages also show full and partial recrystallized ages in the metamorphic aureoles of dolerite sills. Overall, the finding of this research reveals that the shale gas potential of the basin could be much lesser than originally estimated. Load metamorphism and dolerite intrusions influenced organic matter transformation in the basin. The impact on the thermal maturity and hydrocarbon generation of the basin’s organic matter is greater around multiple to closely spaced thermal aureoles of intrusions. These findings correlate with results from other boreholes studied in the Main Karoo Basin and provide a better understanding of the thermal evolution of the Ecca Group organic rich mudstones and its shale gas potential