PETROLEUM PROSPECTS OF TASMAN AND GOLDEN BAY AREAS, SOUTHERN COOK STRAIT, NEW ZEALAND

Abstract

Petroliferous sediments of Lower Tertiary age are preserved in a number of sub-basins, basin synclines and graben along the western portions of both islands of New Zealand.

These form part of the extensively disrupted lineal and platform-like Cretaceous-Tertiary "West Basin" or geosyncline which is separated from a comparable "East Geosyncline" by the geanticlinal backbone of the New Zealand island-chain.

Sedimentation in the West Basin was thick (10,000 to 20,000 feet or more) and continuous throughout much of the Cretaceo- Tertiary interval. Unconformities, where present, tend to be local features, but may still represent marked erosional and/or structural breaks. Lithologic and facies changes are not infrequent on a small scale, but these are superimposed on regional sequences that are traceable over the full longitudinal extent of the overall basin.

Cretaceous to Lower Tertiary sequences tend to be freshwater and coal-bearing, but may be "paralic" locally. More marine facies are predicted beneath Cook Strait. Later Tertiary sediments are more predominantly marine, and include big thicknesses of mudstone and limestone.

Oil seepages occur in a number of situations in the West Basin, and are associated with Lower Tertiary coal measures. In the Taranaki sub-basin the small oil field at New Plymouth has produced a total of 200,000 barrels of oil and 65 million cubic feet of gas, while the more recently discovered (1959) Kapuni condensate-gas field is capable of producing 60 million cubic feet of gas per day, of which 40 per cent is CO, accompanied initially by 4,500 million barrels per day of condensate.

The Nelson and Farewell Spit sub-basins of present interest respectively plunge north beneath Tasman and Golden Bays on either side of the Pikikirunl Range "horst". They then coalesce beyond Separation Point and link with the Kapuni sub-basin beneath Cook Strait as part of the modern continental shelf. These graben-like developments are characterised by strong north plunge.

Geophysical surveys leading to the better understanding of basin sub-structure about southern Cook Strait are discussed, followed by consideration of possible petroleum potential. Gravity anomalies in both the Tasman and Golden Bay areas, supported by seismic surveys, have confirmed anticlinal structure. Structural and stratigraphic traps for petroleum are predicted in relation particularly to these developments.