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URUSKI, CHRISTOPHER I., Institute of Geological & Nuclear Sciences Ltd

Abstract: Petroleum Potential of New Zealand's Deep Water Basins

New Zealand is surrounded by large deep-water basins. Some have a history of exploration, and others are barely known. This paper explores the hydrocarbon potential of thick sedimentary accumulations in six basins. Structural style varies from rift basins through strike-slip dominated basins to major accretionary prisms. Source rocks encountered include coal measures, black marine shales and lacustrine facies. Large sedimentary thicknesses, heat flow studies and basin modelling supported by production and numerous seeps in the shelf and onshore, all suggest that these basins may be prolific hydrocarbon producers in the future. New Zealand's deep-water basins offer many different play types.Trap styles observed include a range from fault block drape to compressional anticlines and stratigraphic traps.

Six deep water basins within the New Zealand Exclusive Economic Zone (EEZ) are known, or suspected with good reason, to contain thick sedimentary accumulations, namely: the New Caledonia Basin, the Raukumara Basin, the Pegasus Basin, the Head of the Bounty Trough, the Great South Basin and the Solander Trough.

A is the New Caledonia Basin, perhaps the most attractive of all as it is large, with an extent within the New Zealand EEZ of approximately 90,000 square kilometres and because it lies adjacent to New Zealand's major hydrocarbon producing basin,Taranaki. The New Caledonia Basin also borders the Northland Basin shortly to be tested (at the time of writing) by Conoco using their new drillship, the Deepwater Frontier. Seismic and gravity studies strongly suggest that more than 10 kilometres of sedimentary rock has accumulated in the deeper parts of this rift basin and that minor inversion occurred in the early Neogene.Water depths across much of the part of this basin in the New Zealand EEZ is between 1 and 2 kilometres.

The Raukumara Basin (B) lies to the north of the East Coast Basin in up to 3 kilometres of water. This basin has an area of at least 40,000 square kilometres and is hard to explain structurally as, despite its 13 kilometre thick accumulation of sedimentary rock, very few faults have been observed. It is thought to be a flexural basin in a back-arc position with respect to the Kermadec/Hikurangi subduction front.

C, the Pegasus Basin is located in deep water to the east of Cook Strait where it occupies an area of at least 25,000 square kilometres. It is situated to seaward of the southernmost extent of the Hikurangi subduction system and north of the continental block of the Chatham Rise. This is the region where the tectonic style of the plate margin changes from dominantly subduction to dominantly strike-slip margin.

The fourth basin D, the head of the Bounty Trough covers an area of at least 200,000 square kilometres and appears to be filled by thick rift sequences overlain by Neogene material derived from erosion of South Island. The Bounty Trough was continuous with the New Caledonia Basin before the present plate boundary displaced the two troughs. Gravity evidence suggests a thick sedimentary fill while a seismic line in the west shows its rifted nature.

The Great South Basin (E) was recognised in the 1960s as a large rift basin covering some 85,000 square kilometres of the Campbell Plateau.About 40,000 kilometres of seismic data have been acquired and six wells drilled in this basin. Of those, two were classed as sub-commercial oil and gas discoveries. A large part of this basin (25,000 square kilometres) is.currently licensed as PEP 38211 to Antrim International Inc. F, the sixth basin, the Solander Trough, is a deep-water continuation of the Solander Basin off the southern coast of South Island. Recent seismic data show at least 5 kilometres of sedimentary fill. The basin may have started as a Cretaceous rift basin similar to those described above, but has been subjected to transpressive tectonics, creating some spectacular structures. 

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