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

Abstract

Exploring new geothermal targets requires the understanding of the factors affecting fluid circulation and heat transfer (Rowland and Sibson 2004; Fairley 2009). Understanding the distribution of permeability in the crust remains an essential component for the general comprehension of a geothermal model in the crustal domain, and therefore for the success of a geothermal prospect (Moeck 2014). Nowadays in France, most geothermal exploration licences are located in sedimentary basin areas or within graben structures. The Paris and Aquitaine basins, characterized by simple sedimentary structures, allow geothermal exploitation from low (30 °C) to medium (110 °C) temperature systems.

On morphology and amplitude of 2D and 3D thermal anomalies induced by buoyancy-driven flow within and around fault zones

“…The second major difference deals with the value of temperature anomalies, which do not exceed 50-60°C in the case of an inclined fault zone, whereas it exceeds 80°C in the vertical case. These results are consistent with the 2D models of Duwiquet et al (2019), who also found higher thermal anomalies for vertical faut zones.…”

“…When fault permeability is higher, cold downwellings may be favored within the fault, as in the numerical simulations of the Soultz-sous-Forêts geothermal system where fault width equals 100 m (Guillou-Frottier et al, 2013). In the case of the Pontgibaud crustal fault zone (French Massif Central), Duwiquet et al (2019) reproduced present-day surface heat flow and temperature gradient for a maximum permeability of 1.6 10 -14 m², a value for which a high temperature (150°C) hydrothermal reservoir could settle at a depth of 2.5 km. It may be worth to consider the possibility of a critical fault permeability above which positive thermal anomalies disappear in favor of negative anomalies, but Lopez and Smith (1995) already showed that the role of host rock permeability has also to be taken into account (see also Della-Vedova, 2008).…”

“…It may be worth to consider the possibility of a critical fault permeability above which positive thermal anomalies disappear in favor of negative anomalies, but Lopez and Smith (1995) already showed that the role of host rock permeability has also to be taken into account (see also Della-Vedova, 2008). In addition, the permeability ratio between fault zone and host rocks may correspond to one critical parameter for building a convective regime diagram, as suggested by Duwiquet et al (2019). Consequently, the complex coupling between host rock and fault zone permeability will be also considered in this study.…”

“…In a recent compilation of measured or inferred permeability values in fault zones, Scibek (2020) indicates, when available, the permeability ratio between the fault zone and the embeddings. Duwiquet et al (2019) suggested a regime diagram for different 2D convective patterns as a function of permeability ratios. However, permeability ratio is probably also depthdependent.…”

“…Tectonically active systems, ash-flow calderas, sedimentary basins and especially crustal fault zones correspond to dynamic systems which are sufficiently porous and permeable to host fluid circulation (e.g. Simms and Garven, 2004;Hutnak et al, 2009;Lopez et al, 2016;Patterson et al, 2018a ;Duwiquet et al, 2019). As soon as rock permeability is large enough, meteoric, metamorphic or magmatic fluids can flow along kilometer-scale pathways, at velocities reaching up to several meters per year (e.g.…”