South Australia is situated between the ancient Archaean Shield of Western Australia and the mobile orogenic belts of the eastern states, with large regions of interpreted high heat flow and elevated geothermal gradients often associated with buried Carboniferous to Mesoproterozoic granitic bodies which form important Engineered Geothermal System (EGS) exploration targets. In addition there are a number of deep sedimentary basins prospective for Hot Sedimentary Aquifer (HSA) resources.
Much of central and northeastern South Australia is anomalously hot at depth. High heat producing Mesoproterozoic granites, felsic volcanics and gneisses of the Gawler Craton and Curnamona Province contain anomalously high uranium and thorium concentrations resulting in local high heat flow. Average heat flow in the area constrained by the Curnamona Craton and Olympic Cu-Au Province is 92±10 μWm–2, substantially elevated compared to equivalent Australian Proterozoic crust at an average heat flow of 83±10 μWm–2, or the average heat flow of all the Continents at 65±1.1 μWm–2. The maximum heat flow measured to date within South Australia is 126 μWm–2 at Parabarana in the Mt Painter Inlier, ~30 km NE of the Paralana Hot Springs.
A regional temperature anomaly over south-western Queensland and north-eastern South Australia coincides with granitic basement covered by stacked sedimentary basins (the Cooper, Eromanga and Lake Eyre basins). In this region groundwater in the Great Artesian Basin has been heated to over 100ºC, and geothermal energy from this hot groundwater has already been harnessed on a small scale in both South Australia and Queensland. In South Australia, an experimental 20kW Organic Rankine Cycle generator was installed at Mulka Bore in 1986 and generated electricity from 85ºC groundwater. At Birdsville, an 80 kW net demonstration plant has been operating since 1999 using 99ºC groundwater from the town water bore. The Great Artesian Basin thus constitutes a vast Hot Sedimentary Aquifer (HSA) resource which is largely untapped at present.
In this same geographic area, the mid-Carboniferous Big Lake Suite granite has been blanketed by over 3.5 km of insulating sedimentary cover of the Lake Eyre, Eromanga and Cooper basins. While this granite has lower heat production than some of the Mesoproterozoic granites, the thick thermal blanket of coal, sandstone and siltstone has very effectively trapped the heat generated by the Big Lake Suite granite which is a substantial EGS resource target. Temperatures of 250ºC have been measured locally at about 4.5 km depth.
In the South East, higher temperature outliers occur in the Otway Basin where the most recent volcanic activity at Mt Gambier and Mt Shank has produced locally elevated heat flows. These volcanoes were active as recently as ~4000 years ago (eruptions were witnessed by Aboriginal people), as well as under the Murray Basin. Warm artesian water has been used for aquaculture to farm barramundi near Robe since 1985. In Portland, Victoria, hot groundwater from the Portland Trough (which has elevated geothermal gradients) was used between 1985 and 2006 to heat buildings.
To identify geothermal exploration targets in South Australia, companies have used publicly available geological and heat flow data as well as reports on petroleum and mineral exploration wells, seismic, gravity and aeromagnetic data held by DSD. Once identified, targets are drilled to measure heat flow, geothermal gradients and other parameters. If a suitably hot EGS resource is confirmed, rocks within the resource area are fracture stimulated to create a network of interconnected fractures to act as the heat reservoir. Understanding the character and orientation of both the in situ stress field and natural fracture systems in the target rocks is critical to optimise development of the fracture network. Geothermal fluid is circulated via injection wells through the fractured hot rock and the heated fluid is recovered from production wells. Hot fluid is then circulated through a surface heat exchanger and used to generate electricity. The cooled geothermal fluid is then reinjected into the geothermal system and the cycle continues.