Proceedings Australian Geothermal Energy Conferences 2013 Brisbane, Australia, 14-15 November 2013 1 Diagenesis Impact on the Reservoir Quality of the Hutton Sandstone, Cooper Basin, South Australia Antoine Dillinger, Cameron Huddlestone-Holmes, Ludovic P. Ricard, Lionel Esteban and Horst Zwingmann CSIRO Earth Science and Resource Engineering, 26 Dick Perry Avenue, Kensington WA 6151 antoine.dillinger@csiro.au Keywords: Diagenesis, porosity, permeability, reservoir quality, geothermal, Hutton Sandstone, Cooper Basin, Eromanga Basin ABSTRACT The Hutton Sandstone (Cooper-Eromanga Basin, SA) recently became a target for geothermal heat extraction due to its potential for high natural permeability, decent temperatures, and high reservoir volume. However, recent exploratory drilling conducted by Origin Energy Ltd and Geodynamics Ltd at the Celsius-1 well didn’t produce the anticipated flow rates, raising the question of the impact of the diagenesis on the reservoir quality of this sedimentary formation. Previous studies focused on oil and gas fields located on the ridges of the Nappamerri Trough highlighted the good reservoir quality of the upper Hutton Sandstone. Nevertheless, an extensive literature review reveals that the middle part of the Nappamerri Trough – where Celsius-1 is situated – remains poorly investigated. This study aims to better characterize the petrophysical and petrological properties as well as the diagenesis processes occurring within the Hutton Sandstone. It is based on drill cuttings and cores retrieved from Celsius-1 and other existing nearby stratigraphic and petroleum exploration wells (Della-2, Merrimelia-19, Strzelecki-17 and Packsaddle-1). Porosity and pore size distribution of the Celsius-1 drill cuttings were first measured by using a laboratory low field Nuclear Magnetic Resonance (NMR) under water-saturated condition. Permeability was then inferred from NMR transversal relaxation time T2 via the empirical equation of Timur-Coates. The petrology of the Hutton Sandstone was studied via three different analytical tools: i) Basic automated QEMSCAN for phase assemblage mapping and quantitative analysis of the mineralogy; ii) Scanning Electron Microscope for visualizing the pore geometry and secondary mineral growths; iii) Optical Microscopy for high precision characterization of the mineralogy, the micro-structures, the pore connectivity and the diagenetic cements. The integration of these results gives a robust and complementary evaluation of the diagenetic history in a deep burial environment, its effects on the flow properties of the Hutton Sandstone, and its regional extent. 1. INTRODUCTION Geothermal resources hosted within sedimentary basins, also known as Hot Sedimentary Aquifers (HSA), have been successfully explored and developed for geothermal energy extraction worldwide (Lund et al., 1998, 2011). The best known examples in Australia are in Birdsville, the Latrobe Valley hot-springs and several projects in Perth (Pujol, 2011). These areas have been targeted because of their potential high natural permeability, the presence of water, suitable temperatures and the extent of the resources. Similarly, the Hutton Sandstone – a deep HSA resource – has been tested via deep exploratory drilling at the Celsius 1 well in the Cooper-Eromanga Basin (South Australia). This resource was targeted as it was thought to have high natural matrix permeability similar to the prolific adjacent oil and gas fields, and was not expected to require significant permeability enhancement. Moreover, the region around Celsius 1 is known to be underlain by radiogenic granites, providing heat to the overlying formations including the Hutton Sandstone. Celsius 1 was drilled to intersect the full thickness of the Hutton Sandstone section and evaluate the flow capacity of the formation to investigate the exploitation potential of this known heat resource. However, well tests did not produce the anticipated flow rates at the resource temperatures required to generate electricity, raising the question of the impact of the diagenesis on the reservoir quality of the sedimentary formation. The objectives of the study proposed herein are (i) to give a better insight of the Hutton Sandstone by compiling, classifying and analysing the well bore data provided by existing adjacent stratigraphic or petroleum exploration wells; (ii) to provide information on the diagenetic processes and history affecting the flow properties of the target formation by the analysis of drill cuttings retrieved from Celsius 1 and other offset wells, including mineralogy mapping by Qemscan and imaging by Scanning Electron Microscope and optical microscopy; (iii) to acquire new data on the evolution of petrophysical properties, especially the permeability, of the Hutton Sandstone by the use of a Nuclear Magnetic Resonance on the drill cuttings along with Logging While Drilling (LWD) logs. The integration of all these results provides a strong geological characterization of the Hutton Sandstone. The outcomes give a robust evaluation of the diagenetic history in a deep burial environment, its effects on the flow properties of the Hutton Sandstone, and implications for the geothermal potential of siliciclastic formations. 2. GEOLOGICAL BACKGROUND The Cooper Basin consists of a broad downwarp with two main depocentres – the Poolowanna Trough and the Cooper region – separated by the north-east-trending Birdsville Track Ridge, a complex of related domes and ridges. Three major troughs in the Cooper region (Patchawarra, Nappamerri and Tenappera) are separated by structurally high ridges (GMI and Murteree Ridges)