OFFSHORE PETROLEUM POTENTIAL IN NEW ZEALAND

Abstract

Extensive shelves and intermediate-depth terraces, rises, and plateaus characterize the New Zealand offshore region. Sedimentary basins with hydrocarbon potential on land, which all have formed after the Early Cretaceous Rangitata Orogeny, are mainly lined up along the west and east of both islands and obviously extend out to sea. Sediment thickness generally increases offshore, while the tectonic disturbances become markedly less severe. Prospects therefore look favourable, and on this basis offshore concessions to a total of nearly 400,000 sq. miles were taken up, covering the area all around New Zealand to a depth of 1,000 m. Ten offshore wells to an average depth of 10,000 ft have been drilled, the second, third and fourth of which establishing a large gas field with an estimated 5 trillion cu. ft of gas, and the seventh being a non-commercial oil discovery (tested flow rate 600 b/d). Marine seismic surveys have been done over about 80,000 line miles. However, the results indicate that in many areas maximum subsidence during the Tertiary occurred relatively close to the present landmass, whereas farther away on submarine rises and plateaus the basement is very shallow or even exposed; also nearer to land some areas have little or no prospects. Consequently, 70,000 sq. miles have been relinquished-with only one well here drilled-and more acreage will probably be relinquished in the near future. But the overall potential still is encouraging; interesting prospects remain particularly along the west coast within the 500 m depth contour and extending from about 35° to 44°, also along the east coast of the North Island, on the eastern half of Chatham Rise, in the south -east of the South Island south of Christchurch and towards the central depression of Campbell Plateau, and in the head of Solander Trough. These areas amount to about 100,000 sq. miles. Concessions totalling 50,000 sq. miles (which for the greater part lie within the above areas) are in their second 5-year term and will expire on 30 September 1975; until then, more drilling is expected to outline existing prospects in greater detail. There is reasonable hope for further discoveries of substantial petroleum accumulations.