Abstract The evaluation of fluid flow through fractures and faults is of primary importance for the long-term performance assessment of radioactive waste repositories in shale formations and could be used to assess CO 2storage security or the integrity of caprocks and reservoir capacity. This study focuses on the structural evolution within brittle to ductile shear zone in order to understand permeability enhancement and sealing processes affecting a strike-slip fault system within the Toarcian shale formation from the Tournemire Underground Research Laboratory (URL), southern France. A combination of quantitative field measurements and laboratory and in situexperiments was used to estimate the fluid-flow properties of a fractured shale formation. Results indicate that microfractures govern the matrix porosity in the damage zone and play an increasingly dominant role in fluid flow along the boundary between fault core and fault damage zone.