On the geothermal potential of crustal fault zones: a case study from the Pontgibaud area (French Massif Central, France)

The present study aims to understand the potential of a new and novel type of geothermal play system for high temperature and electricity production: crustal fault zones (CFZ). According to geological and geophysical data, the Pontgibaud fault zone (French Massif Central) is suspected to host an active hydrothermal system at a depth of a few kilometers. The deep geometry of the fault zone and the permeability distribution are the main unknown parameters that are required to assess the geothermal potential of the Pontgibaud site. Structural and thin-section observations, laboratory permeability and connected porosity measurements and X-ray micro-tomography observations suggest that the hydrothermal system behaves like a double matrix-fracture permeability reservoir. Numerical modeling in which we varied the fault dip and the ratio between the fault zone permeability and host rock, R , was performed. Results indicate that three main convective regimes can be identified (weak convection, single cellular-type convection and bicellular convection). For a sufficiently high fault zone permeability (> 1 × 10 −15 m 2 ), buoyancy-driven flow creates a positive thermal anomaly of several tens of °C at a depth of 2–5 km. For a vertical fault zone, the thermal anomaly is larger for higher R values. Numerical models, then applied to the geologically constrained Pontgibaud fault zone, show that a temperature of 150 °C at a depth of 2500 m can be obtained for a fault zone permeability of 1.6 × 10 −14 m 2 . Based on a multi-disciplinary approach, this work establishes a potential predictive tool for future high-temperature geothermal operations within basement rocks hosting large-scale fault systems.