Phase-1 of Yerila–1 was drilled to 693.5m to evaluate the geothermal potential of the Callabonna Gravity Low, which has been interpreted to represent a possible high heat producing granite at 2.5km depth.

At 693.5m, the temperature reached 64C. Petratherm said the raw mean gradient was 56 degrees per kilometre, with the mid-hole gradient as high as 66 degrees per kilometre.

“Temperature results over the first 693.5m drilled support Petratherm’s initial model for locating shallow geothermal resources in South Australia in excess of 200C at around 3.5km depth,” the company said this morning.

Other down-hole geophysical logs of Yerila-1 are currently being processed.

Petratherm said it now planned to use diamond drilling to extend Yerila-01 to a total depth of about 1500m. The second drilling phase will determine the thermal properties of underlying strata and confirm the subsurface model established in the first phase.

The Callabonna geothermal body, defined by the regional gravity that clearly marks its boundary, covers roughly 1200sq.km. Petratherm holds two licenses covering 1000sq.km over the centre of this body.

The company has now spudded an evaluation well, Paralana-1B, at the nearby Paralana geothermal target. Drilling started on the weekend.

Phase-1 of the Paralana geothermal evaluation well, located 40km east-north-east of Arkaroola on the Frome Plains, had reached 134m this morning, Petratherm said.

Like Yerila-1, Paralana-1B will be drilled through the artesian aquifer, which Petratherm estimated to be located at 460m, and into underlying rock sequence. A total depth of around 550m has been projected for the first phase of drilling.

On completion, the hole will be cased to isolate the artesian flow. Projections of the temperature gradient will be made to the target depth of around 3.5km.

If results are encouraging, the hole will be deepened using a diamond rig to about 1500m to gain further temperature, rock physical property and stress measurements.

The Paralana project covers three tenements totalling 1500sq.km immediately east of the outcropping thermally anomalous Mt Painter granites. Interpretation of historical seismic data indicates these high heat-producing granites may extend under favourable insulating cover to the east in an area informally called the Poontana Basin.

Evidence of elevated sub-surface temperatures included the Paralana Hot Springs where surface water temperatures have been measured at 62C. Numerous shallow wells in the area have also record significant thermal anomalies, according to Petratherm.