Geological Setting

Introduction
South Australia is situated between the ancient Archaean Shield of Western Australia and the mobile orogenic belts of the eastern states. As a result of this unique tectonic setting, South Australia's rock record extends from late Archaean to Holocene. The State is divided into a number of geological provinces, including Precambrian crustal blocks and younger sedimentary basins, which host a wide variety of mineral, petroleum and geothermal resources.

A viable geothermal prospect needs both rocks that will generate heat, and cover rocks to insulate and trap the produced heat over geologic time.  In South Australia, granites buried by sedimentary cover currently form the main geothermal exploration target.

Geothermal energy exploration has typically involved searching under sedimentary cover for granitic bodies likely to have elevated uranium and thorium contents, then drilling these hot basement rocks and/or drilling the basal sedimentary cover (which is also likely to be hot) and testing temperatures.  The hot rocks may then be fracture stimulated so that water can be circulated via deep injector wells into the heat source.  This heated water is recovered from production wells and circulated to a surface heat exchanger to generate electricity.

Geothermal Provinces
The Gawler Craton (late Archaean-early Mesoproterozoic) and Curnamona Province (Palaeo-Mesoproterozoic) are two large complex basement terrains consisting of mafic and felsic igneous intrusions and volcanics, moderate to high grade metamorphics and minor unmetamorphosed sedimentary deposits (Preiss et al, 2002).  Mesoproterozoic granites, felsic volcanics and gneisses in the Gawler Craton and Curnamona Province contain anomalously elevated uranium and thorium concentrations relative to global Proterozoic averages (Neumann et al., 2000) and generate high heat flows.  For example, the Mesoproterozoic Hiltaba Suite hosts the Olympic Dam copper-gold-uranium deposit.

Currently, regions of interpreted high crustal temperature which have been blanketed by sedimentary cover form the main exploration targets for geothermal energy in South Australia.  Neumann et al (2000) defined the South Australian Heat Flow Anomaly (SAHFA) based on sparse regional heat flow data.  The SAHFA occurs through the eastern Gawler Craton, Delamerian Fold Belt and Curnamona Province. Average heat flow is elevated relative to Proterozoic or younger terrains on other continents – mean heat flow within the SAHFA is 92±10 µWm-2 compared to an average of 51-54 µWm-2 in other countries (Neumann et al, 2000). The central and northern part of the SAHFA has been covered by Neoproterozoic, Palaeozoic, Mesozoic and Tertiary sediments.

As well as ‘hot’ granites, Petratherm Ltd have identified that radiogenic iron oxide deposits also occur in the SAHFA (eg Olympic Dam, Prominent Hill) and may have even higher heat flow than the granites, although over a smaller volume of rock. Petratherm are also exploring enhanced natural thermal systems around Paralana Hot Springs and the associated fracture zone.

At Paralana Hot Springs in the central SAHFA, naturally occurring faults and fracture systems have provided migration pathways for hot (>60°C) groundwater to reach the surface.  Petratherm predict high heat production rocks at depth.  The maximum heat flow measured within the SAHFA, 126 µWm-2 (Neumann et al, 2000) occurs at Parabarana Hill, ~30km northeast of the Paralana Hot Springs, on the Paralana Fault Zone.

Younger (Cambro-Ordovician) granites and granitoids occur in the Delamerian Fold Belt, which lies on the east of the SAHFA.  Many of these granites occur at depth and have been covered by the Murray and Otway Basins, however heat production data are limited.

In the State’s northeast, Mid-Carboniferous granite (Big Lake Suite) has been covered by the Cooper, Eromanga and Lake Eyre basins.  While this granite has lower heat production when compared to older Mesoproterozoic Gawler Craton and Curnamona Province granites, it is blanketed by over 3 km of insulating Late Carboniferous-Recent sandstone, siltstone, shale and coal.  This has effectively trapped heat generated from the granite, and temperatures of 250°C occur at 4.5 km depth.

Granitic basement covered by the Warburton, Cooper, Eromanga and Lake Eyre basins has caused a large temperature anomaly in southeastern Queensland and northeastern South Australia.  In this region, Great Artesian Basin groundwater has been heated to 100ºC at depth.  Hydrothermal energy utilising hot groundwater from the Great Artesian Basin has been harnessed on a small scale in both South Australia and Queensland.  In SA, an experimental 20 kW Organic Rankine Cycle generator was installed at Mulka Bore in 1986 and generated electricity from hot (85ºC) groundwater.  At Birdsville, a 150 kilowatt demonstration plant has been operating using 99ºC groundwater from the town bore since 1999.

The most recent volcanic activity in South Australia occurred in the south east ~4–6000 years ago at Mount Gambier and Mount Shank, producing locally elevated heat flows (Neumann et al, 2000).  Over the border in Victoria at Portland, hot groundwater from the Portland Trough (which has elevated geothermal gradients) has been used since 1985 to heat municipal buildings, the town swimming pool and a motel.

Insulating cover
The geological record in South Australia has preserved a unique history of sedimentation from the Neoproterozoic to Ordovician, and from the Early Devonian to Holocene.  In the context of geothermal energy, these basins form insulating cover over heat sources in crystalline basement, or form potential circulation systems or reservoirs for hot water.

The Cooper and Eromanga basins form important insulators for the Big Lake Suite granites in the Nappamerri Trough.  The Eromanga Basin, and the Otway Basin, are also being explored in their own right as sources of geothermally heated water. Other insulating basins include the Cenozoic Murray Basin, Cambrian Arrowie Basin, Tertiary basins north of Adelaide and the Neoproterozoic sediments of the Adelaide Geosyncline.

Regional sediment thickness data for Australian basins can be obtained from OZ SEEBASE©, which was developed by FrOG Tech in Australia and provides a free GIS model of the geological evolution of both Phanerozoic and Proterozoic Basins. Both the Phanerozoic and Proterozoic Basins products are Public Domain and are available for download or as hardcopy.


Further information on all basins can be found on the Petroleum site
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References
Neumann, N, Sandiford, M and Foden, J 2000. Regional geochemistry and continental heat flow, implications for the origin of the South Australian heat flow anomaly. Earth and Planetary Science Letters, 183, 107–120.

Preiss, WV, Alexander, EM, Cowley, WM and Schwarz, MP 2000. Towards defining South Australia's geological provinces and sedimentary basins. MESA Journal, 27, 39–52.