Rock Properties of North-Eastern Tasmania: Implications for EGS Development

Abstract

The potential of the Lower Paleozoic basement rocks of north-east Tasmania as a target for Enhanced Geothermal System (EGS) development was assessed by a combination of detailed petrophysical and geochemical measurements and one dimensional modeling. The basement rocks in this area consist of an Ordovician to Devonian turbiditic sequence called the Mathinna Group that has been intruded by Devonian granites. The potential of the Mathinna Group rocks to act as thermal insulators was assessed in this study together with the contributions of enhanced heat production from the granitic bodies.

Laboratory measurements of petrophysical, geochemical and thermal rock properties were used to create one-dimensional thermal models of the upper crust. Density, porosity, magnetic susceptibility, sonic velocity and electrical resistivity were measured using standard equipment and laboratory techniques. New inexpensive techniques were developed and refined to measure thermal conductivity, heat capacity and to determine the elemental abundances of K, U and Th using a hand-held gamma ray spectrometer.

Mathinna Group sandstones have the highest thermal conductivity values (~4.4 W/mK) and shales the lowest (~2.7 W/mK). Granites have intermediate thermal conductivities (~3.5 W/mK). The Mathinna Group rocks all have very low heat production but four granite bodies sampled all have high heat production with values in excess of >7 uW/m3 and a maximum value of 22.7 uW/m3 for a sample from the Royal George pluton.

Modeling suggest that fine-grained Mathinna Group sedimentary packages have the potential to act as basement insulators to increase temperature gradients in the presence of high heat producing granites and hence represents potential targets for EGS. The presence of a widespread low thermal conductivity cover sequence over the Lower Paleozoic basement units further enhances the potential for EGS exploration in eastern Tasmania.