High hydrogen concentrations have been measured in hyperalkaline waters (pH 11–12) resulting of the serpentinization of ultramafic rocks at depth. These fluids migrate in the crust mainly through fault systems. What happens to the fluids when these faults cross impermeable rocks like shales? From a series of injection tests performed in a fault at IRSN Tournemire underground laboratory in Toarcian shales, we studied the permeability variation depending on the fluid pressure. Numerical modeling based on these experimental results suggests that the fluid channeling along a fault zone crossing a nonpermeable formation might easily occur at large scale without fault activation.