Petroleum prospects are virtually restricted to basins formed after the Early Cretaceous Rangitata orogeny. Basin sedimentary rocks are mainly Cenozoic, but some thick marine Cretaceous sequences fill the Northland and East Coast basins of the North Island; thinner, mainly terrestrial Cretaceous deposits are present in some areas in the northwest and southeast of the South Island. Subsiding epicontinental basins are offshore relatively close to land on the west coast, near 40°S, and the southeast coast, from 44° southward. Along the east coast of the North Island the late Cenozoic fold belt, which extends offshore about 100 km, comprises an extremely thick and continuous marine sequence of Aptian to Pleistocene age. Major areas of submarine rises and plateaus ar und New Zealand are foundered continental blocks with only thin or no sediment cover. Between these, several younger underdeformed sedimentary basins are below water more than 2,000 m deep and are filled with sedimentary deposits several kilometers thick.

Throughout New Zealand sedimentary rocks are commonly of a sand-shale facies with only minor carbonate rocks, mainly in the Oligocene, locally also in the Paleocene-Eocene, Miocene, and Plio-Pleistocene. Along the west side of both islands and east and south of the South Island, the characteristic assemblage is of the shale-sandstone-coal type. Potential reservoirs generally are in sandstones near the base of the sedimentary sequence (Late Cretaceous to early Tertiary), in an environment that was transitional between shallow-marine and nearshore deltaic to estuarine-brackish and nonmarine (sandstones in coal measures). Locally, reservoirs may be in limestones. Many unconformities, pinchouts, onlaps, and lateral facies changes throughout the Cenozoic sequence may have created favorable conditions for extensive stratigraphic traps, but exploration has been concentrated on structural traps.

Production has been obtained only in the Taranaki basin, both on- and offshore, with proved recoverable reserves of about 6 trillion cu ft of gas and 200 million bbl of condensate. Except for a minute percentage produced from Pliocene sandstones, all of the production is contained in the Eocene Kapuni Formation. Good shows in wells, and also surface seepages, are known from the east coast of the North Island and the west coast of the South Island. In Northland one recent well had strong gas shows, but over 9,000-ft thick allochthonous olistostrome deposits make this a particularly difficult area to explore. In general, the prospects are good for further discoveries, mainly offshore, and also on some land areas. The total area of prospective basins on land covers nearly 50,000 sq mi, w ereas offshore the area, to an arbitrary depth limit of about 1,000 m, is roughly 100,000 sq mi. Only 10 wells have been drilled offshore, of which three established the large Maui gas field and one noncommercial well which tested oil at a rate of 600 bbl/d was abandoned.

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