Crustal Fault Zones as Underexploited Geothermal Resources: Contribution of Numerical Modelling and Comparison with Natural Systems

Abstract

Crustal Fault Zones (CFZ) are an interesting geological target for high-temperature geothermal resources in naturally fractured and deep basement zones. Field and laboratory studies have already shown the ability of these systems to favor fluid flow down to brittle-ductile-transition. However, several key questions about exploration still exist, in particular the role of structural dip, permeability, and the effect of mechanical stress and more broadly the fundamental role of tectonic regimes on fluid flow in naturally fractured basement domains. Considering 2D and 3D numerical modelling, with TH and THM couplings, two trends can be identified and integrated for the exploration of these targets (i) vertical faults concentrate the highest temperature anomalies at the shallowest depths (ii) strike-slip systems favor the largest temperature anomalies. Geological and geophysical data suggest that, the Pontgibaud fault zone (French Massif Central) is a CFZ that host an active hydrothermal system at a depth of a few kilometers. We conducted an integrated study to assess its high temperature geothermal potential. Field measurements are used to control the 3D geometry of the geological structures. 2D (thin-section) and 3D (X-ray microtomography) observations point to a well-defined spatial propagation of fractures and voids, exhibiting the same fracture architecture on different scales (2.5 μm to 2 mm). Moreover, measurements of porosity and permeability confirm that the highly fractured and altered samples are characterized by high permeability values, with one sample characterized by a permeability as high as 10-12 m2. Finally, a large-scale 3D numerical model of the Pontgibaud CFZ, based on THM coupling and the comparison with field data (temperature, heat flux, and electrical resistivity), allowed to explore the spatial extent of the 150°C isotherm, which rises up to a depth of 2.3 km. Though based on simplified hypotheses, our model reproduces field data. A multi-disciplinary integrative approach based on coupled 3D modeling proved to be an efficient way to assess the geothermal potential of CFZ and predict temperature distributions. It can be used as a predictive tool to develop high-temperature geothermal operations within basement rocks hosting large-scale fault systems.