There is almost 40 times less shale gas in the Karoo than previously thought, scientists who helped analyse recent rock core samples from test drillings in the Karoo say.
Prof Michiel de Kock, a senior paleomagnetist and head of geology at the University of Johannesburg, said very little sign of shale gas had been found in the first, directly measured, test samples dug from various depths, suggesting that previous claims about huge deposits were “grossly inflated”.
In the latest issue of the South African Journal of Science published on Thursday, De Kock and fellow researchers said previous estimates suggested the Karoo could be home to up to 485-trillion cubic feet (Tcf) of shale gas, which would mean the country had the fourth-largest deposits in the world.
The US Energy Information Agency later downgraded this estimate to about 390 Tcf, the sixth-largest in the world.
But De Kock said these estimates also appear to be “highly inflated” and it is now more likely that the technically recoverable shale gas reserves are closer to 13 Tcf. While these estimates are quite small in comparison to other parts of the world, De Kock and his colleagues nevertheless suggest that the Karoo deposits may still be of commercial interest to large gas-fracking companies such as Shell.
Hydraulic fracturing (fracking) is a controversial technology pioneered in the US, to break apart underground shale rock using high-pressure water, sand and chemicals.
De Kock said further tests still needed to be done in several “sweet spot” areas targeted for exploration, but his research group believed the Karoo deposits were “overmature”.
While the region may have contained large volumes of shale gas in the past, they were overheated by underground heat sources. Most of the gas had since escaped or been transformed over time into a more solid form, such as bitumen.
Using the analogy of a piece of steak on a braai fire, De Kock said the Karoo deposits appear to have been overcooked.
“What petroleum geologists would look for, ideally, is a nice medium steak. If the organic matter is uncooked, hydrocarbons would not have formed. If not cooked enough, there would be oil generation, but no gas generation. If cooked too much, as appears to be the case in the Karoo, the gas also would be driven off.”
The latest study — sponsored by Shell, on behalf of the Department of Science and Technology and National Research Fund’s Centre of Excellence for Integrated Mineral and Energy Resource Analysis — involved researchers from the University of Johannesburg, the Council for Geoscience, University of Cape Town and University of Portsmouth.
No direct gas measurements were taken from the older borehole near Phillippolis, but test results showed gas levels to be “very low” for the Willowvale and Tankwa Karoo samples.
While gas may have been produced in the ancient past, it seemed to have disappeared or been destroyed by overheating.
“The original resource estimates are thus likely highly inflated…. Very likely the most realistic resource estimates for the Karoo basin are between 13Tcf and 49Tcf, with the lower estimate being the most realistic given the sparsity of data.”
While other scientific groups are preparing to dig more boreholes soon near suspected gas “sweet spots” near Beaufort West and Sutherland, De Kock said he expected these to show similar or even lower gas levels.
Responding to queries on whether test results from just three boreholes could be considered to be representative of low gas levels over the entire Karoo basin, De Kock said he was confident the results “provide a true reflection of the actual gas and organic carbon chemistry in the region”.
He acknowledged that shale gas deposits in some areas may have been overcooked by localised heat sources such as dolerite intrusions, but noted that there was also evidence of significant regional overheating in the geological past.
“I think everyone generally thinks that the cooking gets done by dolerite sill intrusions. Because of their localised heating effect, organic matter in the so-called ‘sweet spots’ is expected to be cooked to a much lesser degree.
“Our results confirm the localised effects of dolerite sills, but very importantly illustrate that there is a much more regional heating event that affected the Karoo Basin.
“This might be due to burial of the Karoo sedimentary rocks, being pushed down and heating up under its own weight, or the heat flow under the Karoo basin may have been anomalously high at some point after deposition of sedimentary rocks.
“The effects of this regional heating is what does not bode well for a shale gas resource in the Karoo.”
Shell, one of at least five fracking companies that have been granted exploration rights in the region, was not available to comment immediately on the research findings.
However, at a conference in Port Elizabeth earlier this month, Shell SA spokesman Nigel Rossouw suggested there was only a 10% chance of finding commercially viable reservoirs in the Karoo and said: “There is a strong likelihood that this process may not proceed beyond exploration.”
Dr Phil Mjwara, director-general of the Department of Science and Technology, told the conference that the PetroSA (Mossgas) plant at Mossel Bay had been established based on estimated gas reserves of just 1Tcf.
“This means that, even if a fraction of the estimated 385Tcf gas reserves considered to be technically recoverable by the EIA may be proven, it could have positive impacts on SA.”
• The research article is available at South African Journal of Science