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The East Coast Basin of New Zealand lies in the frontal arc of the Hikurangi subduction zone. Overpressure forms a major hazard for exploration in the basin. Fluid pressures of 12.8 MPa at 600 m depth have been encountered, equivalent to 90% of lithostatic pressure, and mud weights of up to 19.5 ppg are required to control formation fluids at subsurface depths of only 1400 m. The distribution and magnitude of overpressure shows no relation to present-day depth. Lithostratigraphy controls the distribution of overpressure and widespread lowpermeability bathyal mudstones form seals to overpressure across the basin. The transition from normal pressure to overpressure is not associated with any single stratigraphic unit. The Neogene evolution of the plate boundary has given rise to rapid Miocene sedimentation (400 m Ma 1 ) in areas of the basin and has caused disequilibrium compaction. Late Neogene compression has subsequently uplifted the overpressured, undercompacted sediments by up to 3000 m at rates of 1000 m Ma 1 . Lateral tectonic compression associated with the plate boundary has also caused undrained shear of thick mudstones, leading to extremely high overpressures in deformed sediments independent of depth.

K-Ar age dates of authigenic illite from sandstones in the UK south Central Graben have a bimodal distribution. In contrast to established hypotheses of thermal triggers, this illite growth is explained by changes in hydrogeological history. Fluid motion during burial can hence be dated. Illite growth at 84-58 Ma on the graben margins was coincident with rapid subsidence of the Graben axis and consequent expulsion of pore fluids onto the margins. This event pre-dated both overpressure, and the secondary migration and accumulation of hydrocarbon in the region. Illite growth was probably caused by increased solute transport rates during pore fluid motion. Illite growth on an axial high (33-30 Ma) occurred during overpressured conditions. This was triggered either by: (1) downward fluid migration as a consequence of overpressure release. These fluids carried carboxylic acids or hydrocarbons from the highly overpressured Kimmeridge Clay Formation into the less pressured Fulmar Sandstone Formation; or (2) a decrease in water-rock ratios as fluid flow declined within the sandstones, as a consequence of overpressure build-up. Published experiments show that illite growth is favoured by low water-rock ratios.

Jurassic and Triassic reservoirs in the Central North Sea are highly overpressured (>40 MPa above hydrostatic pressure). Simulation of the interplay between rapid Tertiary subsidence, seal permeability and fluid flow allows insight into the geological controls on the distribution and magnitude of the overpressure. One-dimensional models demonstrate that, unlike other basins, the overpressure developed in the Graben is not determined by the thickness and permeability of the shale pressure seal. A two-dimensional model simulating lateral flow beneath the pressure seal provides an accurate simulation of the overpressure distribution. Disequilibrium compaction of shale-dominated off-structure regions forms the principal overpressuring mechanism. Lateral flow in the permeable Fuhnar sandstones leads to high overpressure and focused vertical escape on an axial high, where the seal is thinner above a subcropping Fulmar Fm. A layered hydrogeological regime is suggested, with shallow Tertiary pressure cells separated from deep Cretaceous-Triassic pressure cells by normally pressured, permeable Palaeocene sandstones. The pre-Cretaceous rift-associated configuration of the Graben, in combination with 3 km of Tertiary subsidence, controls the distribution of overpressure.