Two discrete pressure cells with a 128 bar pressure difference within a 100 m thick Tarbert reservoir interval are separated by a fault with a throw of 50 m. Given that high permeability reservoir rocks are juxtaposed across the fault, the observed across-fault pressure difference cannot be maintained for reasonable fault rock permeabilities over geologically significant periods (> 10,000 years). A high resolution flow model of a 60 km2 area straddling the pressure compartments is used to investigate the main parameters controlling pressure compartmentalisation. Single-phase hydrodynamic flow modelling demonstrates that the observed pressure distribution requires low across-fault transmissibilities and relatively high hydrodynamic flow rates (10 m3/day). The most significant contributor to the high flow rates is gas generation and migration into the high pressure cell. Low fault transmissibilities are attributed either to shale smearing or, less likely, to extensive quartz cementation of the fault rock. Our study shows that pressure compartmentalisation in the North Sea can be controlled by relatively small displacement faults and highlights the importance of high resolution 3-D geological models in understanding overpressure distribution.