This article is aimed at explaining why sandstone reservoirs in the Haltenbanken area of the Mid-Norwegian shelf frequently exhibit great variations in pore pressure development over short distances. The identification of an overpressure gradient in Upper Jurassic and Cretaceous shale forms the basis for a fluid-flow modelling of the water budget through the reservoirs. The reservoirs, which must be regarded as pressure cells, receive water from the adjacent shale in the deep parts of the basin. This water is transported slowly upwards through the cells and flushed out in the updip parts of the reservoirs. Following the principle of conservation-of-mass, these two fluid-movements must be equal. 1D-modelling of this fluidflow by the Darcy formula for horizontal flow shows that the overpressure in the pressure cells is mainly a function of the depth to the apex of the individual cells. The model may be used as a means of determining the pore pressure in reservoirs with known depth to their apices, the depth to the apices of reservoirs with known pressure, and identifying the permeability gradient in adjacent shale.