Impact of diagenesis on reservoir quality in a sedimentary geothermal play: A case study in the Cooper Basin, South Australia

© 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists. Geothermal resources hosted within sedimentary basins with high natural permeability have been targeted for the production of energy in Australia. The Hutton Sandstone (Cooper-Eromanga Basin) - a prolific oil and gas producer known to have good reservoir quality and high reservoir volume - was recently tested for its geothermal potential in the Cooper Region. However, recent exploratory drilling did not produce the anticipated flow rates, raising the question of the impact of diagenesis on the reservoir quality of this sedimentary formation. The combined characterization of the petrology, diagenesis and petrophysical properties of the Hutton Sandstone at Celsius-1 and other surrounding wells indicates variable reservoir properties in the Cooper Region. This integrated study demonstrates that low formation permeability occurs at geothermal target depth and explains the negligible flow rates obtained at Celsius-1. These low permeabilities are the results of the preservation of widespread detrital clayey matrix and the extensive occurrence of authigenic kaolinite, illite and silica cements at the top and base of the Hutton Sandstone. This aspect is confirmed by NMR T2 transversal relaxation time becoming shorter at similar depths. Petrography analysis also reveals that sandstones are affected by diagenetic processes of the eogenetic and mesogenetic phases. However, the Hutton Sandstone at Celsius-1 is presently at pressure-temperature conditions that are below the mesogenetic conditions, which suggests a late episode of uplift and cooling from maximum palaeotemperatures.