Frictional properties of simulated sandstone–coal fault gouge: Implications for seismicity in the Sydney basin and Groningen gas field

Abstract

Carbonaceous materials including amorphous carbon and graphite are observed in seismically active fault zones however their influence on the frictional properties of faults remains poorly understood. The Sydney basin in SE Australia contains extensive coal measures and networks of high angle faults which are susceptible to ML= 5+ earthquakes. The structure of these faults is widely documented however the frictional properties have yet to be studied. This study investigates the influence of coal on the frictional properties of sandstone hosted faults and in doing so contributes to the assessment of seismic hazard associated with faults in the Sydney basin. In addition, it also adds to our understanding of the faults which penetrate the coal bearing Carboniferous shale/siltstone under burden in the Groningen Gas field. Friction experiments were performed on simulated sandstone-coal fault gouge using samples of Coal Cliff sandstone and Illawarra coal collected from the Sydney basin. A combination of velocity stepping and constant velocity experiments were performed at 100 °C and effective normal stress of 40 MPa which are close to in situ conditions of the Groningen reservoir, using slip velocities from 0.1 – 10 μms-1. Rate and state friction parameters (a-b) were derived, combined with X-ray microscopy, quantitative image analysis, SEM and Raman spectroscopy. Both mixed sandstone-coal samples and the sandstone and coal end members displayed a μpeak of ~0.43. Furthermore, the coal end member and coal rich samples (≥50 wt% coal) display intense post weakening before reaching a steady state of ~0.23, which is attributed to shear induced molecular change of coal. Upon shearing aromatic layers become more extensive as functional groups are omitted resulting in a structure closer to that of graphite. Despite both sandstone and coal end members displaying positive (a-b) values, coal rich sandstone-coal samples (≥50 wt% coal) are velocity weakening (negative a-b) at 1 and 10 μms-1 under water saturated conditions. The results indicate that sandstone hosted faults, which have been enriched with coal through smearing, may be susceptible to seismic slip under certain conditions.