In search of hot buried granites: a 3D map of sub-sediment granitic bodies in the Cooper Basin region of Australia, generated from inversions of gravity data

Abstract

The Cooper Basin region straddles the Queensland/South Australia (SA) border (Figure 1) and is coincident with a prominent geothermal anomaly (Cull & Denham, 1979; Cull & Conley, 1983; Somerville et al., 1994) (Figure 2). The region forms part of a broad area of anomalously high heat flow which is attributed to Proterozoic basement enriched in radiogenic elements (Sass & Lachenbruch, 1979; McLaren et al., 2003). Thick sedimentary sequences in the Cooper and overlying Eromanga Basins provide a thermal blanketing effect resulting in temperatures as high as 230° C at depths <5 km (Holgate, 2005). High heat-producing granites, granodiorite of the Big Lake Suite (BLS), at the base of the Cooper/Eromanga sequences form a significant geothermal play which were targeted by Australia?s first Hot Rock development at Habanero (near Innamincka in SA). The relationship between high heat flow, high temperature gradient and anomalous heat production in the BLS is well established (Middleton, 1979; Gallagher, 1988; Beardsmore, 2004). There is a good probability that analogous geothermal plays exist in association with other granitic bodies lying beneath the Cooper/Eromanga Basins. For the most part the location and characteristics of these bodies are poorly understood and accurately identifying them is an important first step toward future geothermal exploration in this region. A 3D map was produced over an area of 300 x 450 km to a depth of 20 km. The 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 map, which also delineates the 3D geometries of the Cooper and Eromanga Basins, therefore incoporates 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.