ABSTRACT: Overpressure , seal breaching and stress regimes in East Coast and Taranaki Basins, New Zealand

Darby, David, Rob Funnell, and Vaughan Stagpoole , Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand

New Zealand's basins are strongly affected by Neogene tectonic movements associated with the Pacific-Australian plate boundary. Quantitative modelling demonstrates that the origin and distribution of overpressure is related to the current stress regime and the recent tectonic history of the basins. The frontal arc East Coast Basin is undergoing active compression. Overpressures >90% of lithostatic pressure have been encountered at depths <1km, causing severe drilling problems. Quantification of subsurface stress suggests that the upper limit to overpressure in the basin is controlled by the present-day stress regime. Sedimentation rates are low (100m/Ma) and undercompaction cannot be recognised from drilling and log data. Modelling of the basin suggests that present-day undrained shear of low-permeability mudstones and structurally-controlled aquifer flow controls the high pressures. The pressure regime in the East Coast Basin contrasts strongly with the tectonically complex Taranaki Basin. Overpressures of 20MPa are widespread within the Taranaki basin, which has been variably uplifted and buried in the Neogene in response to the evolving plate boundary zone. 2D basin modelling shows that deposition of 1-2km of prograding Plio-Pliocene sediments has caused overpressuring below Eocene-Oligocene pressure seals. Mudstones are undercompacted and pressure gradients in transition zones trend parallel to vertical stress gradients, typical of an extensional basin undergoing rapid sedimentation. Contemporaneous uplift has affected many regions, and modelling suggests that overpressure has declined in these regions by 25 MPa during Pliocene uplift. Overpressure at 80% of vertical stress reduced the sealing potential of caprocks, causing seal breaching and cross-formational flow during hydrocarbon migration.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia