The fault in our clay: Variation in fault cementation in evaporite-bearing mudrocks and implications for sealing

Mudrocks are a crucial resource for subsurface storage of gases or radioactive waste, both as host rock and caprock. They are characterised by  low primary permeability and high creep rates, limiting the lifespan of fractures, both decreasing the chance of leakage. Large faults pose a significant risk to this, as they act as coupled hydro-mechanical weak points, with potential for increased flow up-dip and along-strike. Gouge generation is well established as a process which limits this permeability increase, however cementation of void space in faults is less well studied.

We studied faults hosted in the Mercia Mudrock Group in the Bristol Channel (southwest UK). We find that gouge and cementation work cyclically with more than one mineral phase, indicating the hydro-mechanical development of a fault through its lifespan. In the fault core, veins tend to be redirected parallel to the mechanical discontinuity between damage zone and gouge, indicating the location of preferential flow. In the damage zone, veins are not always planar in the soft mudrocks and are often part of an anastomosing network which is influenced by other non-fault related structures, such as older veins, sand dyking, and mechanical stratigraphy. The elongated, fibrous gypsum and calcite crystals observed emphasise the importance of high fluid pressures to open and maintain these fractures.

We interpret episodic overpressures that build up layers of gypsum veining in the gouge core of a number of faults; however, the limited special extent of these veins suggests patch-style dilation and slip that may not pose a significant leakage risk. Gypsum veining appears to destroy most remaining evidence of a damage zone in these instances.

This is in contrast to brittle calcite, associated with rifting and inversion, which shows more fluid movement up-and-down-dip and appears to preserve the damage zone. This results in structures that can be reactivated, both by more calcite-bearing fluid but also overpressure caused by rehydration of anhydrite to. This reactivation causes brecciation of the calcite that is not seen in the gypsum despite the overprinting of stress events.

In summary, tectonic environments which open large fault patches potentially pose a higher risk to leakage through mudrock-hosted faults, than fluid overpressure events. However, cementation of these faults, while providing mechanical discontinuities for later dilation events, do appear to seal voids generated by fault activity.