Abstract

Potentially petroliferous sedimentary rocks of early Tertiary age are preserved in several subbasins and graben-synclines along the western parts of both islands of New Zealand. Collectively they constitute the now extensively disrupted, lineal, and platformlike Cretaceous to Tertiary West basin or geosyncline, which is separated from the more specifically volcanic East geosyncline by the geoanticlinal backbone of the New Zealand continental block. The geoanticlinal zone, which is geosuturelike, constitutes a major transcurrent fault system within the circum-Pacific tectonic belt. After a major break from the Jurassic to Cretaceous greywacke-breccia flysch-type deposition, Late Cretaceous to Tertiary platformlike sedimentation in the newly forming West basin was regionally extensive and thick (c. 10,000-15,000 ft or 3,050-4,600 m). The basal unconformity is angular and sharp, whereas internal unconformities generally are local and marginal. Facies changes are also important locally, but are super-imposed on regional sequences that are traceable across the full longitudinal extent of the platform. Upper Cretaceous-lower Tertiary rocks are predominantly freshwater and coal bearing. Younger rocks are dominantly marine, and include considerable thickness of mudstone and limestone. Oil seepages 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 2000,000 bbl of oil and 65 million ft 3 of gas (1.85 million m 3 ). The Kapuni condensate-gas field in the Taranaki basin, discovered in 1959, is capable of producing 60 million ft 3 (1.7 million m 3 ) of gas-condensate a day. Hydrocarbons are considered to be mainly indigenous to the coal measures, but some may be from overlying marine sedimentary rocks. Geophysical surveys have outlined the broader structure of the Southern Taranaki basin. Upper Cretaceous-lower Tertiary coal measures and a lower Tertiary limestone provide the principal reflectors, except where masked by thicker Plio-Pleistocene section in the east (D'Urville trough). The thickness of the sedimentary section in the area reaches 10,000-15,000 ft (about 3,000-4, 500 m), but decreases 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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