Petro-physical characterization of the Main Fault at the Mont Terri Laboratory

Opalinus clay has been widely studied in the context of deep geological disposal. Its favourable hydro-mechanical properties made it a suitable material for the storage of nuclear waste. But the presence of tectonic systems within the clay formation arise question over the long-tem safety of a potential repository. Here we present a study that focuses on how induced slip on pre-existing faults might enhace rock transport properties To do that, samples from the Main Fault architecture at Mont Terri were collected including the portion of the fault core and intact rock. Within the fault core, the fault gouge is the wear product of the fault surfaces produced by tectonic slip. It appears as a dark and partially continuous band of 8-12 mm thickness that lacks calcite minerals (compared to the protolith) and contains few isolated quartz fragments. The scaly clay portion is a weak zone characterized by networks of anastomosing polished surfaces that are ubiquitous. The scaly clay portion progressively falls apart resembling small fish-like flakes or phacoids. Different petrophysical properties were determined at the laboratory. Connected porosity and grain density were measured using the triple-weight method and helium pycnometry. Pore size was assessed using Mercury intrusion porosimetry technique. Permeability was measured on intact and experimental gouge samples by transient pressure pulse method using fluid at chemical equilibrium with the rock. Finally, petrophysical properties were then, correlated with SEM and FIB-SEM microstructural observations. Preliminary results suggest that porosity along the fault structures suggest a layered system. Fault core was ~21 % porosity and dominated by small pores of size lower than 14 nm. Surrounding damage zone presented porosity values lower than 15%, and nano-pores of about 16 nm. Permeability far from the fault zone is in the order of 1e-20 and 1e-19 m2, if measured perpendicular and parallel to bedding respectively.