Establishing Hot Rock Exploration Models for Australia

Abstract

Hot Rock geothermal exploration methods used in Australia are based on high-heat producing granites (HHPG) as the presumed heat source, while low-conductivity sedimentary rocks provide the insulator necessary to create an accumulation of heat and elevated temperatures. Unknowns in this style of geothermal play include the composition and geometry of the HHPG and thermal properties and thickness of the overlying sediments. A series of 3D geological models have been constructed in to investigate the range of geometries and compositions that may give rise to Hot Rock geothermal systems. A 3D 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 modeling was carried out by incorporating measured and estimated thermal properties to the 3D map lithologies. The Cooper Basin thermal model can be used as a predictive tool in regions where little or no temperature and geological data is available. A series of synthetic 3D maps were constructed using different granite geometries beneath varying thicknesses of cover sediments. The gravity, heat flow and vertical temperature gradients were forward modelled using typical density contrasts, heat production values and thermal conductivities. A geothermal explorer can use the results of the density modelling to identify the geometries and depth of burial of potential granitic bodies and the results of the thermal modeling to predicted the heat flow and temperature gradient produced by the body.