Abstract

 
 
 The Main Karoo basin has been identified as a potential source of shale gas (i.e. natural gas that can be extracted via the process of hydraulic stimulation or ‘fracking’). Current resource estimates of 0.4–11x109 m3 (13–390 Tcf) are speculatively based on carbonaceous shale thickness, area, depth, thermal maturity and, most of all, the total organic carbon content of specifically the Ecca Group’s Whitehill Formation with a thickness of more than 30 m. These estimates were made without any measurements on the actual available gas content of the shale. Such measurements were recently conducted on samples from two boreholes and are reported here. These measurements indicate that there is little to no desorbed and residual gas, despite high total organic carbon values. In addition, vitrinite reflectance and illite crystallinity of unweathered shale material reveal the Ecca Group to be metamorphosed and overmature. Organic carbon in the shale is largely unbound to hydrogen, and little hydrocarbon generation potential remains. These findings led to the conclusion that the lowest of the existing resource estimates, namely 0.4x109 m3 (13 Tcf), may be the most realistic. However, such low estimates still represent a large resource with developmental potential for the South African petroleum industry. To be economically viable, the resource would be required to be confined to a small, well-delineated ‘sweet spot’ area in the vast southern area of the basin. It is acknowledged that the drill cores we investigated fall outside of currently identified sweet spots and these areas should be targets for further scientific drilling projects.
 
 
 
 
 Significance: 
 
 
 
 This is the first report of direct measurements of the actual gas contents of southern Karoo basin shales.
 The findings reveal carbon content of shales to be dominated by overmature organic matter.
 The results demonstrate a much reduced potential shale gas resource presented by the Whitehill Formation.
 
 
 
Highlights
The potential shale gas resource of the Karoo Supergroup (Figure 1), and the ~30-m thick Whitehill Formation of the Ecca Group, remains highly speculative.[1,2,3,4,5,6,7] An original ~18x109 m3 or 485 trillion cubic feet resource estimate8 – which would make the Karoo basin the fourth largest resource in the world – is certainly grossly inflated
Such vein-like deposits were not encountered in our boreholes, it was noted that organic matter in the black shale at 1011.25 m in BH 47 appears as solid bitumen networks of inertinite
We show that hydrocarbon generation and preservation is negatively affected by an elevated thermal gradient and dolerite intrusions
Summary
The potential shale gas resource of the Karoo Supergroup (Figure 1), and the ~30-m thick Whitehill Formation of the Ecca Group, remains highly speculative.[1,2,3,4,5,6,7] An original ~18x109 m3 or 485 trillion cubic feet resource estimate8 – which would make the Karoo basin the fourth largest resource in the world – is certainly grossly inflated. The United States Energy Information Administration downgraded this estimate to place the Karoo basin as the sixth largest global resource at 11x109 m3 (390 Tcf), of which the Whitehill Formation contributed ~6x109 m3 (211 Tcf).[3] Conservative estimates are much smaller. A speculated technically recoverable shale gas resource of 0.37x109 m3 (13 Tcf) for the Whitehill Formation and 0.54–0.65x109 m3 (19–23 Tcf) recoverable free gas represent the lower end of estimates.[2,6]
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