A model of the variation in permeability of clay-bearing fault gouge in the brittle crust is presented. Factors affecting the permeability from laboratory experiments are considered and quantified. These are pressure, temperature and permeating fluid. Effective pressure ($overline{sigma}$) and temperature (T) effects on permeability (k) are described by k = k0exp(-0.01Δ$overline{sigma}$)exp(-0.01ΔT) where k0 is a reference permeability at a particular temperature and pressure. At low temperatures (<80¿C), with aqueous pore fluids, the above relation is modified to k = k0exp(-0.01Δ$overline{sigma}$)$left(frac{T}{T_{c}}right)^{1.2}$exp(-0.01ΔT) in order to describe low temperature (less than a critical temperature, Tc, of 80¿C) inferred physicochemical interactions between pore fluids and the fault gouge. The model is used to predict the variation in permeability as a function of depth for different ratios (λ) of pore fluid pressure to confining pressure.