Synthetic thermal modelling, constrained by available geological and geophysical datasets, is used to aid in geothermal target identi9fication and prioritization

The hot rock geothermal model in the Australian context comprises high-heat producing granites overlain by thick accumulations of low-thermal conductivity sediments. The granites have low concentrations of radiogenic elements, and over hundreds of millions of years, these elements decay and produce heat. The passage of this heat to the Earth's surface via upwards conduction is slowed by layers of sediments that have low thermal conductivity, creating 'hot spots' beneath the blankets. This thematic map shows granites attributed by heat production and basin depth. The majority of the granites depicted are of surface outcrop. The presence of high-heat producing granites adjacent to deep sedimentary basins may be used as a first-order indicator of where to further investigate the possibility of hot rock geothermal plays. The main frame of the map shows all granites (attributed by calculated heat production - where available), sedimentary basins (and their order e.g. where one basin is overlapped by another) and geothermal licences and applications. The top right inset map shows only those granites with a calculated radiogenic heat generation of >5 uW -3, with the depth of the sedimentary basins. This map provides a rapid view of areas that may be expected to have the greatest hot rock potential. The second-from-top inset map shows all suitable geochemical analyses from OZCHEM, attributed by calculated radiogenic heat generation. This shows both the distribution of data that goes into attributing the granite polygons, and also analyses of granites (and other rocks) that fall outside the mapped granite polygons that are otherwise excluded from the main map. The third-from-top inset map shows the distribution of drillholes with temperature measurements. The bottom inset map shows an image of the Austherm07 database, which is derived from the drillhole temperature information. The image shows the projected temperature of the crust at a depth of 5 kilometres, interpolated between the drillholes. Overlain on this image is the small number of publicly-available heat flow data.

Geothermal energy has been harnessed in Australia for several decades for both direct use applications and power generation, but only at very small scale installations. Australia's geothermal resources are amagmatic and unconventional by the accepted definitions in other parts of the world centred on active volcanism or plate margin collision. Worldwide, there is a lack of experience in exploring for and developing unconventional resources, and few "deposit" or resource models to aide exploration. The conceptualisation of a range of geological environments amenable to geothermal resource development will underpin the large scale development of geothermal utilisation in Australia. This will include developing exploration models spanning the range of unconventional geothermal resources; from "EGS" or "Hot (Dry) Rock" where permeability stimulation is a pre-requisite, to "Hot Sedimentary Aquifer" where no permeability stimulation is required.

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.

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.

The development of the Australian geothermal industry over the last decade owes much to compilations of drill hole temperature data undertaken in the early 1990s in Canberra. The portrayal of this data on maps of predicted temperature at five kilometres depth, and contained heat resource calculations from this data, have shifted the perception that because Australia does not have significant current magmatic activity there is no geothermal potential. The Australian geothermal industry arguably now leads the world in terms of development of amagmatic geothermal systems for electricity generation.

A 3D map of the Cooper Basin region has been produced over an area of 300 x 450 km to a depth of 20 km (Figure 1). The 3D map was constructed from 3D inversions of gravity data using geological data to constrain the inversions. It delineates regions of low density within the basement of the Cooper/Eromanga Basins that are inferred to be granitic bodies. This interpretation is supported by a spatial correlation between the modelled bodies and known granite occurrences. The 3D map, which also delineates the 3D geometries of the Cooper and Eromanga Basins, therefore incorporates both potential heat sources and thermally insulating cover, key elements in locating a geothermal play. This study was conducted as part of Geoscience Australia's Onshore Energy Security Program, Geothermal Energy Project. This 3D data release constitutes the first version of the Cooper Basin region 3D map. A future data release (version 2 of the 3D map) will extend the area to the north and east to encompass the entire Queensland extension of the Cooper Basin. The version 2 3D map will incorporate more detailed 3D models of the Cooper and Eromanga Basins by delineating the major internal sedimentary sequences within the basins. Thermal properties will then be incorporated into the 3D map to produce a 3D thermal model. The goal is to produce a 3D thermal model of the Cooper Basin region that not only matches existing temperature and heat flow data in the region, but also predicts regions of high heat flow and elevated temperatures in regions where no heat flow or temperature data exists.

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

Like many of the basins along Australia's eastern seaboard, there is currently only a limited understanding of the geothermal energy potential of the New South Wales extent of the Clarence-Moreton Basin. To date, no study has examined the existing geological information available to produce an estimate of subsurface temperatures throughout the region. Forward modelling of a basin structure using its expected thermal properties is the process generally used in geothermal studies to estimate temperatures at depth in the Earth's crust. This process has been validated for one-dimensional models such as a drill hole, where extensive information can be provided for a specific location. The process has also seen increasing use in more complex three-dimensional (3D) models, including in areas of sparse data. The overall uncertainties of 3D models, including the influence of the broad assumptions required to undertake them, are generally only poorly examined by their authors and sometimes completely ignored. New methods are presented in this study which will allow estimates and uncertainties to be addressed in a quantitative and justifiable way. Specifically, this study applies Monte Carlo Analysis to constrain uncertainties through random sampling of statistically congruent populations. Particular focus has been placed on the uncertainty in assigning thermal conductivity values to complex and spatially extensive geological formations using only limited data. These geological formations will typically consist of a range of lithological compositions, resulting in a range of spatially variable thermal conductivity values. As a case study these new methods are then applied to the New South Wales extent of the Clarence-Moreton Basin. The structure of the basin has been built using Intrepid Geophysics' 3D GeoModeller software package using data from existing petroleum drill holes, surface mapping and information derived from the FrOGTech SEEBASE study. A range of possible lithological compositions was determined for each of the major geological layers through application of compositional data analysis, using data from deep wells only (>2000 m). In turn, a range of possible thermal properties was determined from rock samples held by the New South Wales Department of Primary Industries and analysed at the Geoscience Australia laboratories. These populations of values were then randomly sampled to create 120 different forward models which were computed using SHEMAT. The results of these have been interpreted to present the best estimate of the expected subsurface temperatures of the basin, and their uncertainties, given the current state of knowledge. These results suggest that the Clarence-Moreton Basin has a moderate geothermal energy potential within an economic drilling depth. The results also show a significant degree of variability between the different thermal modelling runs, which is likely due to the limited data available for the region.

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