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.
Highlights
Long-term exploitation of geothermal resources is closely linked to reservoir rock petrophysical properties and regional geological settings [1]
Lateral bending and stretching mentioned in (2) and depicted in Figure 2b,c are related to SDVFZ activity, since all kinematic indicators are consistent with progressive southeast oriented dextral shearing. This feature is ubiquitous within the Noble Hills (NH) as described by [33]. Given these new findings dealing with internal Crystalline Bedrock Slice (CBS) structural organization, it is expected that areas along the rear southern limit of the CBS have recorded less transcurrent shear compared to areas situated along its northern front
No signs of a discrete, large-scale deformation structure appear on the map presented in Figure 2, where yellow dashed lines show instead a continuous flow accommodated by brittle deformation processes
Summary
Long-term exploitation of geothermal resources is closely linked to reservoir rock petrophysical properties and regional geological settings [1]. Due to the low matrix porosity and permeability of granitic rocks, underground granitic units are considered as a reservoir only when fractures are present These fractures provide the essential reservoir permeability and porosity for fluid flow [2,3] and are of primary importance regarding geothermal exploitation [4,5,6,7,8]. As Soultz-sous-Forêts in the Upper Rhine Graben, France [6,13] or the Cooper Basin in Australia [14], give invaluable experience in terms of exploration and exploitation feedback. The Noble Hills (NH) range, located in the southern termination of the Death
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