The second logging found that Yerila-1 had a higher mean average temperature gradient than first thought – at least 68C a kilometre compared to the 56C first reported – giving it one of Australia’s highest geothermal gradients ever recorded in the near-surface insulating rock strata covering the target hot granites deeper down.

The temperature trend was also in a much straighter line than originally plotted. This is a critical factor in Petratherm determining whether its target of rocks of a minimum 220C temperature can be found no deeper than 3.5km.

“We had no option but to re-log Yerila-1 on realising that the logging tool responsible for the initial temperature readings stuck at several places between 610m and the final depth of 675m,” Petratherm chief executive officer Peter Reid said.

“As a result, the temperature profile reported two weeks ago was not representative of the true situation in Yerila-1. The re-logged results are highly encouraging, well ahead of the sector’s average temperature gradients to date in Australia over distances of consequence, and reinforce our belief we are sitting above some of the – if not the – hottest rock strata in the country.”

The new results were consistent with Petratherm’s interpretation of the area’s Callabonna geophysical anomaly and reinforced its belief that the well was located above thermally anomalous, radiogenic granites with temperatures ranging between 220-250C at the preferred maximum target depth of 3.5km, Reid said.

“The results are also much more aligned to the modelling outcomes developed by Petratherm and the University of South Australia, ratifying our exploration methodology and giving us confidence in now continuing to drill deeper into the well,” he said.

Confirming such temperatures at those depths would significantly reduce the cost of drilling high-temperature geothermal energy extraction wells – the major capital cost component of a geothermal project.

Subject to a complete technical evaluation of the new results, Petratherm would now deepen Yerila-1 to a depth of 1500m using a diamond drilling rig.

This drilling program was likely to begin in December, subject to rig availability and would aim to establish thermal and rock properties at intermediate depths, according to Petratherm.

If the results are positive, Yerila-1 will be drilled to its ultimate depth of 3.5km, using an oil exploration drilling rig.

Yerila-1, located north-east of Arkaroola in GEL 15, spudded on August 2 with an initial target depth of 700m. It is part of an exploration program by Petratherm to locate hot granites at depth through which water can be circulated, superheated, and returned to surface to fuel steam-driven power generators.