Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

Abstract Occurrence of earthquakes related with geothermal reservoirs highlights the possibility that subsurface fluid injection may reactivate critically stressed faults and trigger seismicity. We report on laboratory experiments conducted at T = 100-250 °C, σc = 110 MPa and Pf = 42-63 MPa and show that two prevalent minerals, epidote and chlorite, impact frictional properties and fault stability under conditions relevant to typical geothermal reservoirs. Numerous geothermal reservoirs worldwide exhibit metamorphic epidotization and chloritization. Shear experiments on simulated epidote-rich fault gouges indicate potentially unstable frictional behavior - more pronounced at elevated temperatures and pore pressures. Increased proportions of chlorite in fault gouges stabilize the faults, which indicates that gouge composition exerts significant control on fault stability. Our results imply a high potential for induced seismicity on faults containing epidote found in many geothermal reservoirs.