Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, California, USA). Part 2: The Influence of Fracturing on Granite Alteration Processes and Fluid Circulation at Low to Moderate Regional Strain

Abstract

Fracture connectivity within fractured granitic basement geothermal reservoirs is an important factor controlling their permeability. This study aims to improve the understanding of fluid–rock interaction processes at low to moderate regional strain. The Noble Hills range (Death Valley, CA, USA) was chosen as a naturally exhumed paleo geothermal reservoir. A series of petrographic, petrophysical, and geochemical investigations, combined with a fracture distribution analysis, were carried out on samples collected across fracture zones. Our results indicate that several generations of fluids have percolated through the reservoir. An increase of (1) the alteration degree; (2) the porosity values; and (3) the calcite content was observed when approaching fracture zones. No correlation was identified among the alteration degree, the porosity, or the calcite content. At a local scale, samples showed that the degree of alteration does not necessarily depend on the fracture density or on the amount of the strain. It is concluded that the combined influence of strain and coeval fluid–rock interaction processes drastically influence the petrophysical properties of fracture zones, which in turn impact geothermal production potential.

Fracture Spacing Variability and the Distribution of Fracture Patterns in Granitic Geothermal Reservoir: A Case Study in the Noble Hills Range (Death Valley, CA, USA)

“…Indeed, the SDVFZ fault segments act with a dextral movement (black lines in Figure 2b), highlighting an intense deformation with local evidences of extreme shearing. These structures are contemporaneous with the syn-kinematic dextral strike faults (orange lines in Figure 2b), highlighted in the recent work done by Klee et al [8]. In addition, compressional structures like thrust faults crosscut outcrops 6 and 8 (red lines in Figure 2b).…”

“…Furthermore, fluid-rock interactions investigations through the NH area, by collecting samples according to profiles approaching major fault segments, have been conducted in recent previous works [8,37]. These major segments consist of the SDVFZ segments, the same ones considered in this present study, since we considered the same faults interpretation in both works based on fieldwork mapping.…”

“…Evidences of fluid circulations have been identified by Klee et al (2021a) [37] and Klee et al, 2021b [8] through (1) the alteration processes that occurred in the whole granite body (propylitic and argillic), and (2) the fracture infills. Among the different natures of fractures infills that have been identified in the granite, carbonates are omnipresent [8]. They mainly filled the fracture following the SDVFZ direction (i.e., NW/SE), which is the main fracture orientation in the area.…”

“…The argillic alteration related to hydrothermal fluid circulations through the fracture network affects dominantly the NH granite [8,37]. During this fluid-rock interaction process, plagioclase and biotite are replaced by secondary minerals, such as illite and kaolinite, which crystallize between 120 and 200 • C [8].…”

“…In deep geothermal systems, many studies have been undertaken to better understand the importance of the natural fractures in various contexts [1,2]. In granitic basement rocks, the permeability is mostly increased by the fracture network and faults [3][4][5][6][7], while the porosity is increased by both the alteration (e.g., dissolution of primary minerals) and the proximity to fracture zones ( [8], this issue). The low rock matrix permeability and porosity allow the fluid flow within fracture networks [9][10][11].…”