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