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ABSTRACT: The East Coast Basin, New Zealand: Petroleum systems and exploration

Francis, Dave , Geological Research Ltd, Lower Hutt, New Zealand

Sixty-two wells have been drilled in the 75,000 sq. km East Coast Basin of New Zealand, of which over half were before 1940, one third were on seismically defined structures, and two were offshore. There have been two discoveries: oil in 1886 and gas in 1998. Forty-seven wells had gas or oil shows.

The basin includes up to 7000 m of active convergent margin Miocene-Pliocene-Pleistocene, 2000 m of passive margin Maastrichtian-Oligocene, and up to 5000 m of older Cretaceous active margin sediments.

There are at least 4 different good to excellent marine thermogenic source units in the passive margin sequence, with thermogenic petroleum generated in parts of the basin as long ago as Early Miocene. Elsewhere some source rocks are immature in outcrop. Widespread oil and gas seeps clearly demonstrate thermal maturity.

Biogenic methane has been recognized, and occurs where fast subsidence allowed rapid accumulation of low-TOC shales during Pliocene and possibly Miocene.

Reservoirs include Pliocene coquina limestones and turbidites, Lower, Middle and Upper Miocene turbidites, shelf facies sandstones and fractured limestones, Paleocene-Eocene greensands, and fractured Paleocene shales. Seal formations are widespread stratigraphically and regionally.

Structuring in the transpressive convergent setting is well developed, with a range of structural styles visible in outcrop and seismic.

It is still early days for the East Coast Basin and the odds are good for further discoveries of both oil and gas.

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