UNCONVENTIONAL SHALE GAS DEVELOPMENT IN SOUTH AFRICA: POSSIBLE TECHNICALLY RECOVERABLE VOLUMES IN THE SOUTHERN KAROO BASIN

The Karoo Basin covers 620,000 km² of South Africa and contains 1 to 5 km thick sedimentary rock sequences intruded by dolerites and covered by Lower Jurassic flood basalts. It was explored for hydrocarbon in the 1960s with a number of deep boreholes drilled by SOEKOR-the Southern Oil Exploration Corporation in the southern (deeper) part of the basin. Geochemical, petrophysical and petrographic studies were carried out on the material recovered and concluded that the lower Karoo sequences (Carboniferous-Permian) were possibly prospective for dry gas in the area immediately north of the Cape Fold Belt and south of the main concentration of dolerite intrusions (the Karoo escarpment). In the light of recent renewed interest in the shale gas potential of the Karoo Basin, we re-examine this borehole data and perform new petrographic and geochemical analyses of the archived cores.

Sequences of black shale of variable thickness, Total Organic Carbon (TOC) and geochemical content occur within the Dwyka and Ecca Groups. We mapped a first sequence of black shale, enriched in dolomite and hydrocarbon (TOC= 0.3-8.0 wt%), between the top of the Dwyka diamictites and prominent chert and sandstone beds of the upper section of the Lower Ecca Group. This potential Source Rock1 is 70 to 150 m thick and 2,000 to 4,200 m below surface in the southern Karoo Basin, but in proximity of the fold belt (<70 km) it is duplicated/thrusted and penetrated by abundant quartz/calcite veins and other fractures. The overlying and significantly thicken turbidites of the Upper Ecca Group in the southeastern part of the basin contain abundant black shales that could represent another potential Source Rock2. This second sequence is 1,300 m thick in average, at depths below 1,000 to 1,500 m, and with relatively lower TOC contents (0.4-1.1 wt%) and more sandstone and limestone interbeds. However, the data is very limited across this region of the Eastern Cape and new borehole cores are required for robust resource assessment. But if we consider a prospective area of 60,000 km² these two sequences represent hypothetical gas shale reservoirs of 6,000 km³ and 100,000 km³, respectively, with possible recoverable shale gas of 10-50 tcf (Source Rock 1) and 65-400 tcf (Source Rock 2).