Hydrothermal fluid circulation and alteration processes in an analogue of a fractured granitic reservoir (Noble Hills range, CA, USA)

This work presented here is part of the H2020 European MEET Project (Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials). The overall goal of this project is to enhance demonstration of geothermal energy production throughout Europe. Part of this project is the study of the interaction between the hydrothermal fluid and the rock in granitic fractured reservoirs as the one present of Soultz- sous-Forêts (SsF) located in the Upper Rhine Graben (Alsace, France). Indeed, alteration of the encountered rock changes the petrophysical properties of the reservoir and minerals can precipitate and clog the fractures. Depending on host rock alteration and fracture infills, the reservoir productivity can drastically evolve trough time and space. Additionally, stimulations strategies can become challenging and great care has to be taken to select them. It is observed that fractures in the SsF reservoir are filled by calcite and quartz and more sporadically by barite. The host rock alteration only affected plagioclases and biotite. Plagioclases (oligoclases) are transformed into tosudite and biotite into illite. The problem in the SsF plant consists in a limited borehole dataset. It is for instance under question if the alteration and the fracture infills are comparable in the whole reservoir trough space and time. In order to circumvent this data limitation, it has been chosen to work on an analogue showing virtually 100% outcrop exposure on a geographical extend comparable to the inferred reservoir at SsF.

This analogue is located in the Noble Hills range (NH, Southern Death Valley, CA, USA). Preliminary observations and results are presented here and based on petrological analyzes of encountered rocks. We concentrate on 1) the nature of fracture infills and 2) on the host rock properties depending on alteration processes. The NH granite is highly fractured, with fractures spanning over several orders of magnitudes in length and width. These can be either opened or filled by precipitation of secondary minerals due to fluid circulations and the complex interplay with tectonic. We have been investigating those infills at both outcrop and sample scale. At the outcrop scale different infills are observed:

• At the middle and frontal part of the range fractures are diffusely filled by carbonates across the fracture network.
• At the rear part carbonates and oxides diffusely filled fractures in a localized corridor. Moreover, late generation of oblique fractures showing limited amounts of shearing and possibly linked to the strain incompatibility with the surrounding Garlock system are present. These are filled with geodic barite.
• In the middle of the range a major fault gouge (up to 10 meters width) completely filled with clay minerals runs the entire length of the range.
• Quartz veins are rare.

At the thin section scale, the same association of oxides, carbonates and sometimes quartz veinlets are found. Microscopic analyzes reveal that the carbonates veins are composedby calcite and/or dolomite and that carbonate veins are composed by different generation of filling. It has also been observed that the granite present traces of alteration due to the interaction between granite and fluids, which have circulated through the fracture network. Plagioclases, which are oligoclases, and sometimes biotite are the only minerals affected by the alteration processes. Quartz and K-feldspars remain intact. The alteration of plagioclase causes its transformation into calcite, illite and mostly into kaolinite, which is considered as a geochemical analogue of tosudite. Biotite alteration is less developed but according to SEM-EDS the product of its alteration corresponds to illite. During this process, Mg and Fe are released from biotite and formed oxides which are nearby present.

Based on those preliminary data, it is obvious that the NH allows a clear analogy with alteration and circulation processes observed in SsF. However, these investigations show that the infills are geographically different across the reservoir and also different in term of evolution through time. It is emphasized that geothermal reservoirs are complex and dynamic systems evolving trough time. Fractures evolve with strain but this evolution can be modified depending on the nature of the infill and play the role of barriers or drains. Care has to be taken during reservoir exploration and exploitation strategies as a fault network petrophysical properties might drastically change depending on the fault tectonic history and fluid/rock interaction processes.