Frictional properties of feldspar-chlorite altered gouges and implications for fault reactivation in hydrothermal systems

As particularly common minerals in granites, the presence of feldspar and
altered feldspar-chlorite gouges at hydrothermal conditions have important
implications in fault strength and reactivation. We present laboratory
observations of frictional strength and stability of feldspar (K-feldspar
and albite) and altered feldspar-chlorite gouges under conditions
representative of deep geothermal reservoirs to evaluate the impact on
fault stability. Velocity-stepping experiments are performed at a
confining stress of 95 MPa, pore pressures of 35–90 MPa and temperatures
of 120–400°C representative of in situ conditions for such reservoirs. Our
experiment results show that the feldspar gouge is frictionally strong
(μ~0.71) at all experimental temperatures (~120–400℃) but transitions from
velocity-strengthening to velocity-weakening at T>120°C. Increasing
the pore pressure increases the friction coefficient (~0.70-0.87) and the
gouge remains velocity weakening, but this weakening decreases as pore
pressures increase. The presence of alteration-sourced chlorite leads to a
transition from velocity weakening to velocity strengthening in the mixed
gouge at experimental temperatures and pore pressures. As a ubiquitous
mineral in reservoir rocks, feldspar is shown to potentially contribute to
unstable sliding over ranges in temperature and pressure typical in deep
hydrothermal reservoirs. These findings emphasize that feldspar minerals
may increase the potential for injection-induced seismicity on
pre-existing faults if devoid of chlorite alteration.