Extended abstracts from various authors compiled as the Proceedings volume of the 2011 Australian Geothermal Energy Conference, 16-18 November, Sebel Albert Park, Melbourne.

As part of the Onshore Energy and Security Program Geoscience Australia are producing regional geothermal energy assessments. The initial assessment (Huston, 2010) was conducted in the North Queensland region with a further assessment to be completed in the Gawler-Curnamona region, South Australia. The assessments, which incorporate geological, geophysical, geochemical and rock property data, identify geographic regions of high prospectivity for Hot Rock (HR) and Hot Sedimentary Aquifer (HSA) systems. The North Queensland assessment, consisting of a map of HR and HSA potential ranked from high to low was produced using GIS techniques. A heat production layer of polygons ranked from high to low was generated from solid geology maps and geochemical data. Heat production values for the lithologies were calculated using concentrations of radiogenic elements (U, K and Th). A thermal resistance ranking layer was produced by integrating thermal conductivity data with sediment thickness data. A temperature availability ranking layer was also generated based on the predicted temperature at 5 km using AUSTHERM07 database. The ranked layers were weighted based on their prospectivity potential and in conjunction with data uncertainty rankings,combined in the GIS to produce the final HR prospectivity map. To produce the HSA prospectivity map, aquifer thickness and water temperature ranking layers were added to the HR assessment.

A compilation of extended abstracts as a record of proceedings of the 2nd Australian Geothermal Energy Conference, Brisbane, 11-13 November 2009

Assessments of the uranium and geothermal energy prospectivity of east-central South Australia have been undertaken using a GIS-based geological systems approach. For uranium, sandstone-hosted (including both roll-front and palaeochannel varieties), iron oxide copper-gold-uranium, unconformity-related and sediment-hosted copper-uranium mineral systems were considered. For geothermal energy, both hot rock and hot sedimentary aquifer systems were considered.

Significant volumes of Big Lake Suite granodiorite intrude basement in the Cooper Basin region of central Australia. Thick sedimentary sequences in the Cooper and overlying Eromanga Basins provide a thermal blanketing effect resulting in elevated temperatures at depth. 3D geological maps over the region have been produced from geologically constrained 3D inversions of gravity data. These density models delineate regions of low density within the basement that are interpreted to be granitic bodies. A region was extracted from the 3D geological map and used as a test-bed for modelling the temperature, heat flow and geothermal gradients. Temperatures were generated on a discretised version of the model within GeoModeller and were solved by explicit finite difference approximation using a Gauss-Seidel iterative scheme. The thermal properties that matched existing bottom hole temperatures and heat flows measurements were applied to the larger 3D map region. An enhancement of the GeoModeller software is to allow the input thermal properties to be specified as distribution functions. Multiple thermal simulations are carried out from the supplied distributions. Statistical methods are used to yield the probability estimates of the temperature and heat flow, reducing the risk of exploring for heat.

Geothermal energy has received increasing attention over the last decade as a potentially abundant, large scale, cost competitive, base load, safe and low-emission energy source for electricity generation and industrial applications in Australia. Geothermal resources comprise a volume of rock of suitable temperature and permeability, and a heat-transport fluid. High crustal temperatures in Australia are thought to be generated by high heat producing granites being overlain by thermally insulating sediments. Two types of geothermal plays exist in Australia: Hot Rocks, which require reservoir enhancement and possibly the addition of water; and Hot Sedimentary Aquifers in shallow (<3,500 m), permeable, water-saturated sediments. Ground selection by early geothermal explorers in Australia was made based on direct temperature measurements from deep (up to 5 km) petroleum wells. In areas without previous deep drilling, the most robust measurement for predicting temperature at depth for Hot Rock geothermal resources is heat flow, but there are only ~150 publicly-available measurements continent-wide. Targeting for Hot Rock geothermal resources is increasingly being done using other geological datasets acquired for minerals exploration. These datasets include geological maps (lithology, stress, structure), seismic, geochemistry, gravity radiometrics and magnetics. Heat flow measurements are used in petroleum studies and can be of use in exploration for some types of mineral deposits.

A regional seismic survey in north Queensland, with acquisition paremeters set for deep crustal imaging, show a potential geothermal target beneath about 2 km of sediments. Beneath the sedimentary structure there appears to be an area of low seismic reflection signal from about 1 s to 4 s. Combined with the relatively low gravity signature over this location, this area of low seismic reflection signal could be interpreted as a large granite body, overlain by sediments. This body lies near an area of high crustal temperature and suggests a potential geothermal energy target.

The Federal Government has recently committed $58.9M in the Energy Initiative, a four year program scheduled to mid 2011, with the aim of identifying potential new energy sources in Australia. The program is targeted towards a specific range of energy commodities that include onshore geothermal energy. Using the latest geophysical imaging and mapping techniques, Geoscience Australia (GA) aims to provide pre-competitive geoscientific information that will help attract companies to explore in new areas by enhancing the chances of discovery and reducing the risks to investors. GA's Onshore Energy Security Program includes the acquisition of new seismic, radiometric, magneto-telluric, gravity, magnetic, geochemical and drillhole data in support of exploration for energy sources including geothermal, petroleum, uranium and thorium. Available maps of crustal temperature (Figure 1) clearly illustrate that the geothermal energy resource in Australia is vast. Electricity is expected to be generated from both hydrothermal (hot groundwater in situ e.g. the Great Artesian Basin) and hot fractured rock plays (e.g. buried hot granites within the Cooper Basin). Significant potential also exists for lower-temperature hydrothermal resources close to population or industry centres which may be useable by direct means. Currently the only geothermal energy being used in Australia is that which emanates from a 120kW plant located at Birdsville (Qld) which draws from the relative shallow hot waters of the Great Artesian Basin. The Geothermal Energy Project in the Onshore Energy and Minerals Division at GA aims to support ongoing geothermal energy exploration across Australia via the provision of enhanced maps of heat distribution together with a comprehensive national geothermal information system. Heat distribution throughout Australia will be mapped in three ways: (1) new heat flow measurements in existing and new drillholes; (2) a granite source/sediment heat trap map to identify hot fractured rock systems and potential geothermal plays (Figure 2); and (3) enhancements to the 5km temperature map of Chopra and Holgate (2005; Figure 1). The geothermal information system will comprise a wide range of information including (but not limited to) thermal conductivity, thermal gradient, density, and heat production data.

This animation illustrates the various stages of development of Hot Rock geothermal resources for electricity generation.

Exploration models for Rot Rock geothermal energy plays in Australia are based primarily on high-heat producing granites (HHPG) in combination with overlying low-conductivity sedimentary rocks providing the insulator necessary to accumulate elevated temperatures at unusually shallow (therefore accessible) depths. Unknowns in this style of geothermal play include the composition and geometry of the HHPG and thermal properties, and the thickness of the overlying sediments. A series of 3D geological models have been constructed to investigate the range of geometries and compositions that may give rise to prospective Hot Rock geothermal energy plays. A 3D geological map of the Cooper Basin region which contains known HHPG beneath thick sedimentary sequences, has been constructed from gravity inversions and constrained by geological data. The inversion models delineate regions of low density within the basement that are inferred to be granitic bodies. Thermal forward modelling was carried out by incorporating measured and estimated thermal properties to the mapped lithologies. An enhancement of the GeoModeller software is to allow the input thermal properties to be specified as distribution functions. Multiple thermal simulations using Monte-Carlo methods would be carried out from the supplied distributions. Statistical methods will be used to yield the probability estimates of the in-situ heat resource, reducing the risk of exploring for heat. The two thermal modelling techniques can be used as a predictive tool in regions where little or no temperature and geological data are available.