Longmaxi formation shales are the major target reservoir for shale gas extraction in Sichuan Basin, southwest China. Swarms of earthquakes accompanying hydraulic fracturing are observed at depths typifying the Longmaxi formation. Mineral composition varies broadly through the stratigraphic section due to different depositional environments. The section is generally tectosilicate‐poor and phyllosilicate‐rich with a minor portion the converse. We measure the frictional and stability properties of shale gouges taken from the full stratigraphic section at conditions typifying the reservoir depth. Velocity‐stepping experiments were performed on representative shale gouges at a confining pressure of 60 MPa, pore fluid pressure of 30 MPa, and temperature of 150°C. Results show the gouges are generally frictionally strong with friction coefficients ranging between 0.50 and 0.75. Two phyllosilicate + TOC (total organic carbon)‐poor gouges exhibited higher frictional strength and velocity weakening, capable of potentially unstable fault slip, while only velocity strengthening was observed for the remaining phyllosilicate + TOC‐rich gouges. These results confirm that the frictional and stability properties are mainly controlled by phyllosilicate + TOC content. Elevating the temperature further weakens the gouges and drives it toward velocity weakening. The presence of observed seismicity in a majority of velocity‐strengthening materials suggests the importance of the velocity‐weakening materials. We suggest a model where seismicity is triggered when pore fluid pressures drive aseismic slip and triggers seismic slip on adjacent faults in the same formation and distant faults in the formations above/below. The effect of pore pressure transients within low‐permeability shale gouges is incorporated. Our results highlight the importance of understanding mechanisms of induced earthquakes and characterizing fault properties prior to hydraulic fracturing.

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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:

The growing hazard of induced seismicity driven by the boom in unconventional resources exploitation is strongly linked to fault activation. We perform laboratory measurements on simulated fault gouges comprising powdered reservoir rocks from major oil and gas production sites in China, to probe the control of mineral composition on fault friction and stability responses during reservoir stimulation. Double direct shear experiments were conducted on gouges with phyllosilicate content ranging from 0 to 30 wt.% and grain sizes <150 μm, at constant normal stresses of 10–40 MPa and conditions of room temperature and water saturation. The velocity step and slide‐hold‐slide sequences were employed to evaluate frictional stability and static healing, respectively. Results indicate that the mineralogy of the gouges exhibit a strong control on the frictional strength, stability, and healing. Phyllosilicate‐rich samples show lower frictional strength μ and higher values of (a − b), promoting stable sliding. For the gouges studied, the frictional strength decreases monotonically with increasing phyllosilicate content, and a transition from velocity weakening to velocity strengthening behavior is evident at 15 wt.% phyllosilicates. Intermediate healing rates are common in gouges with higher content of phyllosilicates, with high healing rates predominantly in phyllosilicate‐poor gouges. As an indispensable component in reservoir rocks, the carbonates are shown to affect both the frictional stability and healing response. These findings can have important implications for understanding the effects of mineralogy on fault behavior and induced seismic potential in geoengineering activities, particularly in reservoirs in China.

The presence of metamorphic epidote on faults has been implicated in the transition from stable to unstable slip and the nucleation of earthquakes. We present structured laboratory observations of mixed epidote and simulated Pohang granodiorite (analogous to the EGS‐enhanced geothermal system site) gouges to evaluate the impact of heterogeneity and contiguity of epidote‐patch structure on frictional instability. Experiments are at a confining pressure of 110 MPa, pore fluid pressures of 42–63 MPa, temperatures 100–250°C and epidote percentages of 0–100 vol.%. The simulated Pohang granodiorite gouge is frictionally strong (friction coefficient ∼0.71) but transits from velocity‐strengthening to velocity‐weakening at temperatures >150°C. This velocity‐weakening effect is amplified in approximate proportion to increasing epidote content. Modes of epidote precipitation likely control the size and contiguity of the epidote‐only patches and this in turn changes the response of 50:50 epidote‐granodiorite mixed gouges for different geometric configurations. However, 50:50 epidote‐granodiorite mixtures that are variously homogeneously mixed, encapsulated and checkerboarded in their structures are insensitive to their geometries – all reflect the high frictional strength and strong velocity‐weakening response of 100:0 pure epidote. This suggests that the epidote present as thin coatings on fractures/faults can enhance velocity‐weakening behavior, independent of individual patch size and can thereby support the potential seismic reactivation of faults. Considering the frictional and stability properties of epidote at conditions typical of shallow depths, the presence of low‐grade metamorphism exerts a potentially important control on fault stability in granitoids with relevance as a marker mineral for susceptibility to injection‐induced seismicity.