South Africa's Karoo basin probably has a thirtieth of the shale gas deposits that some estimates had suggested, deflating expectations of an energy bonanza, a study published on Thursday said.

The findings by the South African Journal of Science have been published shortly before the expected award of the first shale gas exploration licences in the region. The government had said an award could happen by the end of September.

The gas development plans have already drawn opposition from campaigners who say they threaten the environment of the semi-arid Karoo, famed for its rugged scenery and rare wildlife.

Geologists at the University of Johannesburg and three other institutions estimated the gas resource was probably 13-trillion cubic feet (tcf), the bottom of a range of estimates that had put deposits between 13 tcf to 390 tcf, the study said.

In 2015, the US Energy and Information Administration (EIA) estimated the Karoo Basin's "technically recoverable shale gas resource" at 390 tcf, making it the eighth largest in the world and second largest in Africa, behind Algeria.

At 13 tcf, it ranks 34th out of 46 nations in EIA estimates.

However, the authors of the study said that "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," they wrote.

The study said previous estimates were "speculative" and had been made without measurements of gas content.

Environmentalists and farmers have staunchly opposed opening up the Karoo for shale gas development, which would require "fracking", a technique involving pumping water and chemicals at high pressure to crack the rock and release the gas.

The sparsely populated Karoo is known for its expansive landscapes and is home to rare wildlife such as the mountain zebra and riverine rabbit. Its fossil treasures include those of ancient species that gave rise to today's living mammals.

South Africa’s government said in May it might award its first shale gas exploration licences by the end of September, after environmental objections delayed the process.

Royal Dutch Shell, Falcon Oil and Gas and Bundu Gas & Oil are among five firms whose applications were being reviewed by the regulator.