Pegasus Basin, Eastern New Zealand: Its Prospects for Oil and Gas, and a Potential Mirror of the East Coast Basin
The East Coast Basin of New Zealand’s North Island has
long tantalized oil explorers with an abundance of oil and gas seeps and
potential trapping structures. All three wells drilled offshore have
encountered promising gas shows. The basin lies entirely on the deforming
Australian Plate within a Neogene subduction margin; its southeastern margin
lies directly against the undrilled Pegasus Basin, which lies on the subducting
Pacific Plate. Prior to the onset of Neogene subduction at c. 23 Ma both basins
were part of the same remnant of the Triassic-Cretaceous Gondwana subduction
margin. Whereas the East Coast Basin has since accumulated great thicknesses of Neogene sediment and been deformed by a post-23 Ma tectonic overprint, the
Pegasus Basin has remained relatively unscathed and therefore provides an undeformed mirror image of the pre-Neogene succession of source and reservoir
rocks. In 2010 the New Zealand Government acquired 3000 km of high quality 2D
seismic across the Pegasus Basin in an attempt to understand the likelihood of
finding commercially-viable accumulations of oil and gas in this unexplored
part of New Zealand. These new seismic data show a range of large potential
trapping structures, most associated with seismic anomalies including
bright-spots, flat-spots, amplitude suppression, and velocity push-down
effects. Additionally, an extensive bottom-simulating reflector (BSR) indicates
the widespread presence of a gas hydrate layer. Six play types identified in
Pegasus Basin include thrust anticlines along the margin with the East Coast
Basin, thrust anticlines along the fossil Gondwana subduction margin, blind
thrusts in Neogene turbidites in front of the East Coast margin, stratigraphic
pinch-outs of Neogene turbidites, traps beneath a gas hydrate layer, and
compound stratigraphic/BSR traps. Presently, the distribution of Cretaceous and Paleogene source rocks in the offshore of the East Coast Basin is poorly known
due to the pervasive post-23 Ma tectonic overprint and the thick blanket of Neogene strata that hinders seismic imaging of older strata. Our revised
geological model of eastern New Zealand, using the succession mapped in the
Pegasus Basin, provides new controls on the thickness of Cretaceous and Paleogene rocks that should underlie southern offshore East Coast Basin. This
new knowledge will in turn help to better understand the region’s petroleum
systems, which will hopefully lead to the discovery of large oil and gas
reserves.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California