Longmaxi formation shales are the major target reservoir for shale gas extraction in the Sichuan Basin, southwest China. Swarms of earthquakes accompanying hydraulic fracturing are observed at depths typified by 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 (~5 wt.%) the converse. We measure the frictional and stability properties of shale gouges taken from the full stratigraphic section at hydrothermal conditions. Velocity-stepping experiments were performed on representative shale gouges at σ_c = 60 MPa, P_f = 30 MPa and T =150 ℃. Results show that the gouges are generally frictionally strong with friction coefficients spanning a range of 0.50-0.75. Two phyllosilicate+TOC-poor gouges exhibited higher frictional strength and velocity weakening behavior, capable of potentially unstable fault slips, while only velocity strengthening behavior 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 towards velocity weakening. The presence of observed seismicity in majority velocity strengthening materials suggest the importance of the minority velocity weakening materials. We suggest a model where seismicity is triggered when high pore fluid pressures drive aseismic slip in the near-field and triggers seismic slip on adjacent faults. Our results have important implications in understanding the physics of earthquakes in Sichuan Basin and highlights the importance of identifying the location and characteristics of faults prior to hydraulic fracturing.

Experiment details, recorded data during each shear experiment, and friction data used to reflect the frictional properties of Longmaxi shale gouges in Sichuan Basin, southwest China.