The equations in NAMMU were modified to allow the fault permeability to depend on the effective stress. The effective stress would increase as pressure dissipated, but this nonlinear effect had relatively little impact for our estimates of compressibility and porosity-permeability relationships. One process that might change the deep reservoir pressures on a shorter timescale in the North Sea is glaciation. Significant glacial events occurred 10000 and 100000 years ago, together with associated changes in sea level. The influence of these changes on the deep pressure distribution is uncertain, but if the effective stress change is transmitted to the pore water pressure, then the pressure perturbations should persist today. A very simple model with the pressure being increased beneath the ice and then removed again when the ice retreats will leave an overpressured region ahead of the ice sheet and an underpressured region behind the ice sheet after it retreated.

This Project used a hydrogeological approach to understand an anomalous pressure distribution at the basin scale. Pressure measurements show that the aquifer pressure in the North Sea deviates from hydrostatic conditions, and appears to show distinct pressure compartments bounded by faults. A review of pressure generation mechanisms shows that the pressure anomalies
cannot be explained by the observed temperature and salinity variations. Mechanisms that operate over geological timescales (including disequilibrium compaction, gas generation, dehydration reactions and sediment deposition) can only be invoked to explain the pressure anomalies if they can be shown to persist over timescales
of the order of 10 million years.

The model qualitatively reproduces some of the measured features of the pressure data, and also predicted the observed pressure compartmentalisation arising from a pattern of low permeability faults. This is illustrated in the figure.