Abstract

The Taranaki Basin, located 400 km west of the Hikurangi Trench and 200 km above the subducting Pacific Plate, contains the only known commercial hydrocarbon reserves in New Zealand. A total of 11 commercial oil and gas fields have been discovered, with total estimated recoverable reserves of about 1300 million barrels of oil equivalent. These accumulations occur within reservoir rocks of widely varying age, with the majority of reserves contained in Late Eocene coastal plain and marginal marine sandstones deposited on a mature continental margin, and sealed by transgressive marine mudstones. All major fields involve structural traps formed in the Neogene by tectonism associated with the convergent Australia-Pacific plate boundary.

Source rocks are principally Late Cretaceous and Paleocene-Eocene coals. 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 geodynamic complexity of the Taranaki Basin, it is difficult to define a single 'critical moment' for the onset of oil migration and charging of traps. In general, source rocks attained sufficient maturity for oil expulsion following substantial Miocene or Pliocene burial. Accordingly most or all of the known hydrocarbon accumulations in Taranaki were probably formed in the Late Miocene or more recently.