Rocks older than the Late Jurassic Rangitata orogeny are highly indurated, deformed, and/or metamorphosed, and constitute effective basement. Sedimentary basins prospective for petroleum began to form in middle to Late Cretaceous time, but their main development occurred during the Cenozoic. In the west and southeast, epicontinental basins or marginal geosynclines resulting from basement collapse and extension are filled with sedimentary rocks several kilometers thick--locally up to 5-10 km--which in their basal portion are terrestrial and include coal measures. In the northeast, the marginal basin corresponds to a complex orogenic system characterized by repeated diastrophic cycles and several marine transgressions and regressions, locally accompanied by intermittent ero ion.

Throughout New Zealand, sediments are commonly of a sandstone-shale facies with only minor carbonate rocks. Potential reservoirs generally are in sandstones near the base of the sedimentary sequence (Late Cretaceous to early Tertiary) that is transitional between shallow-marine and near-shore deltaic to estuarine-brackish and nonmarine (sandstones in coal measures). Locally, reservoirs may also occur in limestones. Structural traps, including various growth structures, occur widely in most areas; good stratigraphic-trap conditions are also widespread, resulting from local unconformities, onlap, pinchout, or lateral facies changes. There seems to be a good potential for petroleum generation in all basins (oil and gas seeps are widespread throughout New Zealand), and prospects for accum lations are particularly favorable where structural and stratigraphic conditions combine. The maximum prospective area is about 50,000 sq mi (129,500 km²) on land, and roughly 100,000 sq mi (259,000 km²) offshore, to an arbitrary depth limit of 1,000 m.

Commercial production has been obtained in the Taranaki basin, where the proved recoverable reserves of the Kapuni and Maui fields amount to over 6 Tcf of gas and about 200 million bbl of condensate. Offshore, only 10 wells have been drilled to date, of which 3 established the large Maui gas field. Another well tested oil at a rate of 600 bbl/day, but was abandoned as noncommercial.