The potential for low-grade metamorphism to facilitate fault instability in a geothermal reservoir

Native and fluid circulation-induced metamorphic products may affect the stability of faults in geothermal reservoirs–particularly epidote (Ep) and chlorite (Cl). Our laboratory experiments conducted at hydrothermal conditions show that these two minerals, when precipitated on natural faults, may promote fault instability or failure under conditions typifying a geothermal reservoir. Shear experiments on Ep-rich fault gouges indicate potentially unstable frictional behavior—more pronounced at elevated temperatures and pore fluid pressures—and indicate increased potential for the nucleation of slip instability relative to that of the host granodiorite gouge. Experimental results indicate in particular that Ep can increase the potential for induced seismicity on existing faults. Increased proportions of Cl in gouges stabilize the faults but apparently lower the frictional strength. Our results reveal that the metamorphic minerals exert a subtle but potentially significant control on fault strength and stability in geothermal reservoirs.