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Abstract: An overview of Taranaki Basin petroleum systems, New Zealand

KING, P.R. Institute of Geological and Nuclear Sciences Limited.

The Taranaki Basin is the sole commercially-producing hydrocarbon province in New Zealand, with estimated recoverable reserves totalling about 1300 million barrels of oil equivalent within eleven oil and gas fields. The basin has had a complex evolution since the Late Cretaceous, and now lies in a pan stable, part extensional, and part compressional setting, 400 km west of the Hikurangi Trench and 200 km above the subducting Pacific Plate. All major petroleum fields involve Neogene-aged structural traps linked to the development of the Australia-Pacific plate boundary.

Hydrocarbon reservoir rocks range in age from Paleocene to Pliocene, although the majority of reserves occur in Late Eocene coastal plain and marginal marine sandstones deposited on a semimature passive margin, and sealed by transgressive marine mudstones. Source rocks are principally Late Cretaceous and Paleocene-Eocene coals. Geochemical fingerprinting has matched produced oils to source rocks of specific age, but no well has yet penetrated to kitchen depths of equivalent maturity as the oils. Geohistory modelling of source areas, taking into account transient thermal effects within the basin and incorporating kinetic parameters determined from pyrolysis experiments on Taranaki source rocks, indicates that the onset of hydrocarbon generation generally occurs at 3.5-5.5 km depth, depending on existing thermal regime.

Because of the basin's geodynamic complexity, there is no single `critical moment' for the onset of oil migration and charging of traps. Most or all of the known hydrocarbon accumulations were probably formed following substantial burial of source rocks in the Late Miocene and Pliocene.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah