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Potentially petroliferous sediments of early Tertiary age are preserved in several subbasins and graben-synclines along the western parts of both islands of New Zealand. Collectively these constitute the now extensively disrupted, linear, and platformlike Cretaceous to Tertiary "West basin" or "geosyncline," which is separated from the more specifically volcanic "East geosyncline" by the geanticlinal backbone of the New Zealand continental block. The geanticlinal zone which is "geosuturelike" constitutes a major transcurrent fault system within the circum-Pacific tectonic belt.

Following a major break from the Jurassic to Cretaceous graywacke-breccia flysch-type deposition, Upper Cretaceous to Tertiary platformlike sedimentation in the newly forming West basin was regionally extensive and about 10,000-15,000 ft (3,050-4,600 m) thick. The basal unconformity is angular and sharp, whereas internal unconformities tend to be local and marginal. Facies changes are also important locally, but these are superimposed on regional sequences that are traceable through the full longitudinal extent of the platform.

Upper Cretaceous-lower Tertiary sediments are predominantly freshwater and coal bearing. Later sediments are dominantly marine, and include considerable thicknesses of mudstone and limestone.

Oil seeps along the West basin are associated almost entirely with the lower Tertiary coal measures. In the Taranaki basin a small oil field in Pliocene sandstone at New Plymouth has produced a total of 200,000 bbl of oil and 65 MM cu ft of gas (1.85 MM cu m). The Kapuni condensate-gas field in the Taranaki basin, discovered in 1959, is capable of producing 60 MM cu ft (1.70 MM cu m) of gas-condensate per day. Hydrocarbons are considered to be mainly indigenous to the coal measures, but some may be from overlying marine sediments.

Several geophysical surveys have outlined the broader structure of the southern Taranaki basin. Upper Cretaceous-lower Tertiary coal measures, together with a lower Tertiary limestone, provide the principal reflectors, except where masked by thicker Plio-Pleistocene section in the east (D'Urville trough). Sedimentary thicknesses in the area attain 10,000-15,000 ft (approx. 3,000-4,500 m), but thin considerably over conspicuous structural "highs." A variety of structural and stratigraphic traps is predicted.

Broad comparisons have been made with recently discovered major oil and gas fields of the Gippsland basin, Australia, directly across the Tasman Sea.

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