Evolution of the structural fault permeability in argillaceous rocks in a polyphased tectonic context

Abstract

Deep argillaceous formations have petrophysical and hydrodynamic properties favourable to long-term radioactive waste confinement (very low intrinsic permeability, high sorption capacity,…). However, these properties may be modified by the development of discontinuities in the host-rock. The tectonic activity is responsible on the one hand for creating the fractures and on the other hand for reactivating them. Today, the calcite crystallisations in faults give evidence of paleofluid flows during the tectonic deformation. The microstructural study shows that faults were alternately and temporarily impermeable, permeable or “semi-permeable” during the tectonic activity. These “hydraulic states” were controlled by the nature and the architecture of the microstructures and by variations in the petrophysical properties of the rock in the core zone (CZ) and damage zone (DZ) of the faults. Within DZ, the structural fault permeability evolution is associated with (1) microcracking and (2) a probable ductile behaviour of the shales. Within CZ, the structural fault permeability is associated with the development of cavities generated by (1) dilation, (2) shearing and openings in extensional stepover and (3) microcracking in pre-existing calcite fillings. During the tectonic evolution, the development of a new structural porosity both in CZ and DZ gave up the faults permeable. The crystallisation sealing of the total structural porosity gave up the faults impermeable. But, when only the CZ was sealed, the fault was “semi-permeable”. Finally, we show that (1) the fluid transfers occurred principally from the DZ to the CZ, (2) the DZ constituted a “storage zone” in fluids for the CZ, (3) the DZ then remained longer permeable than the CZ and became permeable with weaker stress intensity and (4) the sealed discontinuities constituted zones of weakness (fracture reactivation with or without shearing) in the argillaceous material.