Shale Gas in the Karoo Province of South Africa and Lesotho: A USGS Assessment, 2016

The Karoo Province and the two assessment units (AU) defined in the study. USGS; base map, National Park Service.

Using a geology-based method, the U.S. Geological Survey estimated a mean of 44.5 trillion cubic feet of undiscovered, technically recoverable shale gas in the Karoo Province of South Africa and Lesotho.

The province

The Karoo Province is a geologically complex region of about 608,000 square kilometres. It holds as much as 8 kilometres of sedimentary rock laid down from the Carboniferous to the Jurassic. Organic-rich source rocks in three Lower Permian formations, the Prince Albert, Collingham and Whitehill, are thermally mature enough to have generated gas. Eighty oil and gas exploration wells drilled between 1965 and 1977 reported several gas shows but made no conventional gas discovery.

How the assessment was framed

The USGS defined a Lower Permian Composite Total Petroleum System and, within it, two shale-gas assessment units (AUs). Gas shows in several wells give limited evidence that the system works.

To count as a shale-gas source and reservoir, the rocks must:

1. hold more than 2 percent total organic carbon by weight;
2. contain Type I or Type II organic matter;
3. lie within the right thermal maturity window for generating gas;
4. include organic-rich shale more than 15 metres thick;
5. generally lie deeper than 1,500 metres.

On that basis the USGS defined the Whitehill-Collingham Shale Gas AU and the Prince Albert Shale Gas AU, both in Lower Permian lacustrine source rocks.

The main risk

The biggest geological risk is whether recoverable gas was retained in the source rocks at all. The study's model is an intrusion event that began in the Early Jurassic and continued into the Late Jurassic, about 190 to 154 million years ago. It heated the Lower Permian source rocks in places and drove gas out of them, and breccia pipes in the province were most likely the vents through which the gas escaped. Drilling and testing these rocks will show whether the model holds.

Inputs

Well drainage areas, estimated ultimate recoveries and success ratios were taken from shale-gas plays in the United States used as analogs. Both units were given the same well inputs and a probability of 0.6.

Results

The results are fully risked. F95 is the amount with a 95 percent chance of being at least that large; F50 and F5 are defined the same way. The fractiles add up only on the assumption of perfect positive correlation. All liquids in gas accumulations are counted as natural gas liquids (NGL).

Gas, in billion cubic feet (BCFG):

Natural gas liquids, in million barrels (MMBNGL):

Sources

• Michael E. Brownfield, Christopher J. Schenk, Timothy R. Klett, Janet K. Pitman, Marilyn E. Tennyson, Stephanie B. Gaswirth, Phuong A. Le, Heidi M. Leathers-Miller, Tracey J. Mercier and Thomas M. Finn, Assessment of Shale-Gas Resources of the Karoo Province, South Africa and Lesotho, Africa, 2016, USGS Fact Sheet 2016–3038, June 2016. https://doi.org/10.3133/fs20163038
• R. R. Charpentier and T. A. Cook, USGS Methodology for Assessing Continuous Petroleum Resources, USGS Open-File Report 2011–1167. http://pubs.usgs.gov/of/2011/1167/
• The map and both tables are taken from the fact sheet's PDF.