Characterization of geothermal exploitation window in term of fracture evolution: Results from analogue studies with the Southern Death Valley Fault Zone (CA, USA) and the Upper Rhine Graben shoulders (Germany and France)

The goal of the H2020 European MEET Project (Multidisciplinary and multi-context  demonstration of EGS exploration and Exploitation Techniques and potentials) is to enhance demonstration of geothermal energy production throughout Europe.
Dealing with geothermal demonstration, the reference in France is the Soultz-sous-Forêts (SsF) site located in the Upper Rhine Graben (URG). Geothermal wells penetrate the fractured granitic basement and crosscut permeable fractures corresponding to open fractures with a high flow rate.
Our aim is to understand fault zone evolution and fluid/rock interaction processes in order to estimate the reservoir evolution through time and to define the target exploitation zones. To investigate this, 3D subsurface/surface analogues are needed. Therefore, two analogues were chosen:
1) The Noble Hills range (NH) situated at the southern termination of the Southern Death Valley Fault Zone (SDVFZ, CA, USA). The desert climate conditions allow virtually 100% outcrop exposure. Both SsF and NH share a common transtensional tectonic setting, the main difference being a much higher amount of displacement along the SDVFZ compared to the URG border faults.
2) The URG crystalline shoulders themselves. Exceptional quality outcrops are found in
abandoned mining galleries. Ore mineral exploitation took place within fracture corridors
affecting gneissic basement rocks.
In both analogues, high strain zones linked to substantial shearing deformation are observed. Those zones are most of the time rich in clay minerals (mainly illite), meaning that a fluid has circulated within the system during deformation. Due to a high amount of deformation and the presence of clay minerals, the fracture ends up to be sealed. At the opposite, open fractures showing modest shear deformation present systematically geodic minerals of various types (oxydes, silicates, sulfates, native ore...). At SsF exploited site, productive zones are open fractures presenting mainly geodic quartz [1].
We believe that fluid/rock interaction processes assisted by deformation may play a key role in
hydrothermal activity of a given system. We propose a conceptual model in which, incipient deformation allows the sudden arrival of fluids in newly opened fracture voids. Ongoing continuous deformation leads to the onset of cataclastic deformation and higher reactivity of the host rock with the fluid due to grain size reduction. This dynamic system contribute to the onset of clay mineral (mainly illite) formation which in turn drastically changes fault rock petrophysical properties and fault mechanical behavior. In our model, ideal productive stage for a fracture corresponds to a time period where fractures having suffered modest amounts of deformation remain open allowing fluid circulation and act as a drain.

[1] Vidal, J. & Genter, A. 2018: Overview of naturally permeable fractured reservoirs in the central and southern Upper Rhine Graben: Insights from geothermal wells. Geothermics 74, 57-73.