Frictional properties of simulated shale-coal fault gouges: Implications for induced seismicity in source rocks below Europe's largest gas field

We report 21 frictional sliding experiments performed on simulated fault gouges prepared from shale-coal mixtures. Our aim was to investigate the effects of local coal seam smearing on the frictional properties and induced seismogenic potential of faults cutting the Upper Carboniferous source rocks underlying the Groningen gas reservoir (Netherlands). We used shale/siltstone core recovered from beneath the Groningen reservoir plus Polish bituminous coal of similar age and origin to coals locally present in the Groningen source rocks. We performed friction experiments in velocity stepping, constant velocity, slide-hold-slide (SHS) and slide-unload-slide (SUS) modes, under near in-situ conditions of 100 °C and 40 MPa effective normal stress, employing sliding velocities of 0.1–100 μm/s and a variety of pore fluids. Samples with 0–50 vol% coal showed friction coefficients ~0.45, with minor slip weakening. Samples with ≥50 vol% coal showed marked slip-weakening from peak friction values of ~0.47 to ~0.3, regardless of experimental conditions, presumably reflecting strain localization in weak coal-rich shear bands, possibly accompanied by changes in coal molecular structure. However, re-sliding experiments (SUS) showed that slip-weakening is limited to small initial displacements (2–3 mm), and does not occur during slip reactivation. At (near) steady state, almost all experiments performed at in-situ stress, pore water pressure (15 MPa) and temperature conditions exhibited stable, velocity strengthening behaviour, regardless of coal content. By contrast, under dry and gas-saturated (CH₄, Argon) conditions, or using water at 1 atm, 50:50 (vol%) shale-coal mixtures showed velocity-weakening and even stick-slip. Our results imply that faults in the Groningen Carboniferous shale-siltstone sequence are not prone to induced earthquake nucleation at in-situ conditions, even when coal-bearing or coal-enriched by smearing. However, the mechanisms controlling coal friction remain unclear at the sliding velocities studied, and the evolution of coal friction at seismic slip velocities remains unknown.