Hydrothermally altered shear zones: A new reservoir play for the expansion of deep geothermal exploration in crystalline settings

Abstract

Our study investigates the genesis of granitic reservoirs formed within a brittle shear zone in central Finland, evaluating their feasibility as deep geothermal targets. We employ a suite of laboratory-based experiments and cutting-edge analytical mineral techniques to determine the petrophysical and thermal properties of targeted granitic rocks, elucidating their formation processes. The most favorable reservoir properties were observed in granites affected by cataclasis and mineral dissolution, leading to a notable secondary porosity of ∼20 %. Reservoir quality is largely controlled by the pore network morphology. Alongside fractures, interconnected moldic, sieve, and interparticle pores contribute to substantial permeability of ∼5 × 10−14 m2 (50mD), even under high confining pressures of 50MPa (∼2km). A new geothermal play is here presented: brittle shear zones that have undergone high-temperature (∼200–300 °C) hydrothermal alteration. These findings will significantly enhance our capacity to identify and exploit prolific permeable zones in deep crystalline settings globally, thereby making a substantial contribution to our ongoing transition to cleaner energy resources.