Overpressure associated with a convergent plate margin: East Coast Basin, New Zealand

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 low-permeability 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.