The Potential for Low‐Grade Metamorphism to Facilitate Fault Instability in a Geothermal Reservoir

Abstract

Epidote (Ep) and chlorite (Cl) are two low-grade metamorphic minerals typically present in geothermal reservoirs • Fault instability is promoted by Ep-rich gouges at hydrothermal conditions but minimized with the addition of Cl • Interplay of Ep/Cl mixtures exerts a detectable control on fault strength and stability for geothermal reservoirs Supporting Information:

Frictional Stability of Metamorphic Epidote in Granitoid Faults Under Hydrothermal Conditions and Implications for Injection‐Induced Seismicity

“…The transition temperature (100–125 ° C) promoting the occurrence of velocity‐weakening behavior of epidote gouges is similar to, or even lower than, that currently reported for granitoid faults (An et al., 2021; Blanpied et al., 1991, 1995; Kolawole et al., 2019; Mitchell et al., 2016). This suggests that the enrichment of epidote in granitoid fault zones may be another important control on granitoid fault stability under hydrothermal conditions.…”

“…Both materials were crushed and then sieved to <75-μm diameter to simulate fault gouge. Mineral compositions of the two rock powders were analyzed using X-ray diffraction (XRD; apparatus type: D/max-rB) at Beidazhihui Micro Analytical Laboratory, Beijing, China with the results shown in the Supporting Information of An et al (2021). The crystal epidote contains only trace inclusions (Figure S2 in Supporting Information S1) with the epidote gouge showing a >99 wt.% purity.…”

“…And thus, the epidote can be regarded as an index mineral associated with the local temperature, permeability and fluid composition (Grapes & Hoskin, 2004; Hu et al., 2017). Previous experimental results show that epidote or epidotite gouge is frictionally strong, shows velocity‐weakening behavior and promotes unstable fault slip at lower temperatures, generally at T (temperature) of ∼100–125 ° C (An et al., 2021; Fagereng & Ikari, 2020). Therefore, if the fluid‐rock interactions under hydrothermal conditions contribute to the growth of epidote on fractures/faults, the epidote may control the response of the fracture or derivative gouge and therefore the frictional stability.…”

Frictional Characteristics of Oceanic Transform Faults: Progressive Deformation and Alteration Controls Seismic Style

“…In contrast, the chlorite-rich STTFZ matrix-rich breccia and fault gouge are velocity-strengthening (Figure 3b), behavior consistent with other studies of foliated, chlorite-rich fault gouges (e.g., Imber et al, 1997;Schleicher et al, 2012;Smith & Faulkner, 2010). The tendency for frictional weakness to be associated with velocity-strengthening behavior (Figures 3a, 3b and S5 in Supporting Information S1) is consistent with previous work on a wide range of natural and analogue fault gouges (e.g., An et al, 2021;Ikari et al, 2016;Shimamoto & Logan, 1981;Tesei et al, 2012).…”

Competing Controls of Effective Stress Variation and Chloritization on Friction and Stability of Faults in Granite: Implications for Seismicity Triggered by Fluid Injection

“…Field investigations frequently document that chlorite is an important alteration mineral in greenschist facies and is a ubiquitous product of hydrothermal alteration (Elders et al., 1979; Schiffman & Fridleifsson, 1991). This is especially true within the fault cores within granites hosting geothermal reservoirs that are believed to play a significant role in modulating fault strength and stability (An et al., 2021; Kwon et al., 2019). Natural fractures in granite, recovered from borehole cores at 4.2‐km depth from the Pohang geothermal reservoir, South Korea, show greenschist alteration along the fracture surface reaching 7 wt.% chlorite (An et al., 2021).…”