Fault zone displacement and thickness data (230 measurements) plus orientation and slip vectors from 55 faults were obtained from outcrops of Permo-Triassic sandstones. The data indicate that, generally, as fault displacement increases, the width of a fault zone also increases. In detail, the relationship between displacement and thickness is stepped with thickness increasing sharply above certain values (thresholds) of displacement. This indicates a discontinuous growth of the fault zone. There are at least two thresholds in the data presented. Previous well- and seismic-based studies in the North Sea have shown a clear positive correlation between the displacement, normalized as a fraction of the reservoir thickness, and the probability of a fault sealing. Displacement, therefore, can be used to give a rough estimate of the probability of a fault acting as a pressure barrier to fluids in sandstone-dominated successions. Porosity and permeability of the reservoir rock are reduced within a fault zone, therefore the wider the zone the more likely the fault will seal hydrocarbons. Rapid jumps in fault zone thickness at displacements of roughly 300 mm and 5000 mm indicate that the relationship between fault displacement and fault seal probability may not be linear, but to a fair approximation (±15%) may be taken as such.