New Zealand's deepwater frontier

The islands of New Zealand cover an area of approximately 250,000 km(2), but the New Zealand Exclusive Economic Zone (EEZ) extends to around 4 million km(2) and recent confirmation of New Zealand's Extended Continental Shelf (ECS) has added a further 1.7 million km(2) to the country's marine estate. Within the 5.7 million km(2) of New Zealand's marine territory, approximately 1.2 million km(2) are underlain by sedimentary accumulations which may be thick enough to expel petroleum. New Zealand is almost entirely surrounded by sedimentary basins and all that have been explored for petroleum have deepwater extensions. In addition, several deepwater basins have no onshore or shallow water portions (Fig. 1). The northeastern seaboard of the country was once part of the Eastern Gondwana margin. A series of basins that developed along that margin may be related to the Gondwana trench system. They include the Northland Slope Basin to the northeast of the Northland Peninsula, probably the East Coast Basin of North Island and the Chatham Slope Basin (Figs. 1 and 2). The eastern seaboard of North Island is occupied by the East Coast Basin, a fold and thrust belt above the subducting slab of the Pacific plate. Despite more than 100 years of exploration, the East Coast Basin remains a frontier basin and deepwater extensions of the East Coast Basin, including the Raukumara Sub-basin to the north of the Raukumara Peninsula and the Pegasus Sub-basin to the east of Cook Strait (Fig. 1) have only recently begun to be investigated. Northwest of the country, three belts of basins are known; the Northland and Reinga basins occupy the zone closest to the Gondwana margin's volcanic arc and may have originated as fore-arc basins. The head of the New Caledonia Basin contains deepwater parts of the currently productive Taranaki Basin and this basin may have originated in Mesozoic time as a back-arc rift basin. The Northland/Reinga and Deepwater Taranaki sedimentary accumulations are separated by the buried West Norfolk basement ridge and both have extensions beyond New Zealand's jurisdiction and into Australian territory. The western side of the Challenger Plateau may also contain thick sedimentary accumulations. In particular, the Bellona Gap between Challenger Plateau and the Lord Howe Rise and the Monawai Basin along the western flank of the Lord Howe Rise both contain thick sedimentary accumulations, and may have originated as intracontinental basins. To the east of New Zealand, the pattern continues. Basins associated with the fore-arc and trench zone of Gondwana include the Pegasus and Chatham Slope basins. The Chatham Rise basins may have originated as part of the fore-arc, while the Canterbury Basin and the Bounty Trough occupy the back-arc location. The submerged continental mass of New Zealand extends east and southeast of South Island as the Campbell Plateau. The Great South Basin occupies a large area of the Campbell Plateau, where four other named sedimentary accumulations; the Pukaki, Outer Pukaki, Campbell and Outer Campbell basins remain largely unexplored. To the west of Stewart Island, the Solander Basin is the offshore continuation of the onshore Te Anau and Waiau basin system. These southern basins were all created by rift faulting prior to, and during Late Cretaceous opening of the Tasman Sea and Southern Ocean. More than 50 oil and gas seeps are known from the West Coast region of South Island, some associated with faults and others in coal exploration wells. To date, no commercial quantities of petroleum have been discovered within the region, although coal-bearing sedimentary basins extend offshore. Although the petroleum histories of most of the onshore and nearshore areas are considered to begin with Late Cretaceous rifting leading to break-up of Gondwana and basin formation, new data suggests that the rifting history of the 500 km wide Gondwana margin zone that we now know as the New Zealand mini-continent extended further back into the Mesozoic. Basins that were previously considered to be Cretaceous rift basins appear to have developed across older features, probably exploiting preexisting faults and regions of thinned crust. In onshore New Zealand and on the continental shelf, many of the source rocks are coaly and were deposited during the rifting period associated with Late Cretaceous plate separation and formation of the Tasman Sea and Southern Ocean. During basin formation, the earliest sediments to be deposited were commonly fluvial, lacustrine, deltaic and nearshore facies with an increasing marine influence as the region foundered through the latest Cretaceous and Paleogene. The exceptions to this pattern are the basins of the East Coast of North Island which developed near the Gondwana margin and appear to be almost entirely marine. The present plate boundary was initiated near the start of the Neogene as the New Zealand landmass emerged in response to plate collision. Many of the more spectacular structures in the New Zealand sedimentary basins were formed during the Neogene. Meanwhile, the deepwater basins away from the plate margin continued a quieter development. Some inversion occurred, but generally not to the extent of the nearshore and onshore regions. The relatively gentle structural evolution of the deepwater basins increases the likelihood of discovering large hydrocarbon fields in their intact structural traps. (C) 2010 Elsevier Ltd. All rights reserved.