A Geological Assessment of Unconventional Geothermal Targets in Queensland, Australia

Abstract

In the absence of active volcanism in Australia, geothermal exploration generally targets high heat producing (HHP) intrusives at depth for enhanced geothermal systems. However, in Queensland, very few of these intrusives are within viable range of centres of population and industry, so alternative heat sources must be sought. Queensland possesses abundant coal measures, which can act as excellent insulators within unconventional geothermal systems. By targeting coal-bearing stratigraphy, less established heat sources may also be prospective for geothermal energy development. The Queensland Government’s Coastal Geothermal Energy Initiative (CGEI) targeted a variety of heat sources in a shallow drilling, heat flow, and temperature modelling program across northern and eastern Queensland. In addition to high heat producing intrusives, the program targeted low-medium heat producing granitoids and residual tectonic heat insulated, where possible, by coal-bearing sedimentary basins. Ten sites were drilled in the CGEI, and moderate to high heat flow was modelled for seven of the sites. Further modelling produced estimated temperatures greater than 187°C at 5 km depth within the Millungera, Surat, Hillsborough, and Maryborough basins. Excellent results were obtained from the Millungera Basin sites, where there are HHP intrusives at depth insulated by stacked sedimentary basins. However, modelling of the Hodgkinson Province site – the only other site targeting HHP granite – produced high heat flow but very low modelled temperatures at depth, due to the slightly high thermal conductivity of the overlying strata. Thus, the traditional rationale of targeting highly radioactive intrusives may not be the most effective. Modelling indicates that high temperatures are retained at depth within the less traditional heat sources of the Surat, Hillsborough and Maryborough basins, which targeted low to moderate heat producing intrusives and residual heat from Cainozoic tectonism. This is due to the excellent insulation provided by the coal measures present in the basins. These data show that geothermal energy potential may exist in regions previously thought to be unprospective, and that Queensland’s abundant coal may be instrumental for future geothermal energy exploration.