Australian radiogenic granite and sedimentary basin geothermal hot rock potential map

Edition 1 – July 2007

Download

• Download the map in high resolution (PDF, 9.3MB)
• Download the map in low resolution (PDF, 3.6MB)

Abstract

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 contain radioactive elements which over hundreds of millions of years, decay and produce heat. The passage of this heat to the Earth’s surface via upwards conduction is slowed by layers of sediments which 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 prospects.

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 licenses and applications. The top right inset map shows only those granites with a calculated radiogenic heat generation of >5 µWm^-3, with the depth of the sedimentary basins. This map provides a rapid view of areas which 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 which goes into attributing the granite polygons and analyses of granites (and other rocks) falling outside the mapped granite polygons that are otherwise excluded from the main map. The third-from-top inset map shows the distribution of drillholes which have 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 five kilometres, interpolated between the drillholes. Overlain on this image is the small number of publicly-available heat flow data.

This map is Geoscience Australia GeoCat record 65306. ISBN (print): 978-1-921236-44-0; ISBN (web): 978-1-921236-45-7. Webpage: Geothermal Energy project.

Data Sources

The granite layer has been brought together from numerous components, most of which depict outcrop. These include eastern Australia (New South Wales, Queensland, Victoria and Tasmania), the Northern Territory, the Kimberley region, South Australia and the western Nullarbor region. All of these components are outputs of the Geoscience Australia National Geological Maps Project. The Yilgarn granites shown are of outcrop and are taken from the Geoscience Australia AMIRA/MERIWA P482 project. The granites of the Pilbara region are interpreted from solid geology by the Geological Survey of Western Australia, i.e. the depicted granites include subsurface occurrences. Some other granites are included from the Metallogenic Potential of Australian Proterozoic Granites project (see Report or GIS), mostly in areas where subsurface granites have been interpreted from drilling. The granites around Olympic Dam in South Australia are an example.

The geochemical data is sourced from Geoscience Australia’s OZCHEM database. The basin margin information is from the Geoscience Australia Sedimentary Basins (National Geoscience Dataset), AGSOCAT (AGSO Catalogue) Number: 31243 available for download for free. The depth of sedimentary basin information is from OZSEEBASE by FrOG Tech, 2006, OZ SEEBASE™ Proterozoic Basins Study, Report to Geoscience Australia by FrOG Tech Pty Ltd, available here. The image of temperature at five kilometres depth is derived from the Austherm07 database which is proprietary information owned by Earth Energy Pty Ltd CAN 078 964 735.

Data Processing

In the main map, the granites have been attributed with radiogenic heat production values by intersecting with the geochemical coverage in GIS. The radiogenic heat production of each geochemical sample was calculated (shown in the second-from-top inset image), and then averaged across each polygon intersected. The formula for the calculation is A = 10^-5 ρ(9.52_cU + 2.56_cTh + 3.48_cK) where ρ = density in kg m^-3 and c_U, c_Th and c_K are the concentration of uranium and thorium in parts per million and potassium in weight percent. A density of 2.67 g cm^-3 is assumed as representative for granites.

The method used to calculate temperature at five kilometres depth is outlined in Chopra & Holgate (2005), A GIS analysis of temperature in the Australian crust, Proceedings of the World Geothermal Congress, Anatalya, Turkey.

Heat flow, Q (µWm^-2) computed from the expression Q = λß where ß (µKm^-1) is the geothermal gradient and λ(Wm^-1K^-1) is the thermal conductivity of the rock in which the gradient is measured. On land, temperatures are usually measured in boreholes at depths greater than 100 metres to avoid diurnal and seasonal climatic perturbations. The following reference is a useful summary of heat flow data in Australia: Cull (1982), An Appraisal of Australia Heat-Flow Data, BMR Journal of Australian Geology & Geophysics, 7, 11-21.