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A potential giant oil or gas prospect identified in deep water about 50 kilometres offshore from Dunedin is a promising discovery, reports LINDSAY CLARK.
The Australian-based Origin exploration team, who received little publicity for their announcement at the New Zealand Petroleum Conference in Auckland back in March, are excited by their findings and are now seeking to drill a deep water well within a couple of years. Origin, which holds a controlling 51 percent interest in Contact Energy, has since received a takeover bid from British-based multinational gas company BG Group, and has as part of its response to the bid published some preliminary estimates of its exploration prospects. These estimates indicate the interlinked structures could together contain 700 million barrels of oil or as much as 2800 billion cubic feet of natural gas. This would be three times the size of current New Zealand oil reserves or slightly bigger than current gas reserves of 2000 billion cubic feet. While nothing is really proven about the size or quality of the petroleum reserves until tested with a drill bit, there are a whole series of pieces of information about the area that together add up to an area of great promise. Rob Willink, executive general manager exploration for Origin, says the Carrack and Caravel structures were the largest Origin has come across in its exploration endeavours in Australia and New Zealand in the past decade. “We are now looking for international partners with expertise operating in deep waters. There are only about 15-20 oil exploration companies in the world with such experience,” he says. Origin Energy hopes to make the commitment to drill by the third quarter of this year, though drilling is not likely until late 2009 or 2010 due to the acute shortage of available deep water rigs around the world. The 200-square-kilometre Carrack and 80-square-kilometre Caravel structure are believed to contain Late Cretaceous marine sandstones draped over more ancient upraised fault blocks providing classic large anticlines where oil or gas might be trapped. Origin’s recent extensive seismic surveys across much of the Canterbury Basin between Banks Peninsula to the Great South Basin south of Dunedin made a significant discovery of a new oil-generating ‘kitchen’ – thick hydrocarbon source rocks to the east and north of the prospects. Mid-Cretaceous coal measures up to 800 metres thick are thought to have generated much larger amounts of petroleum than the geologically younger Late Cretaceous rocks found closer to shore. Basin modelling by Origin predicts that some 15 trillion cubic feet of gas and 7000 million barrels of oil may have been generated and expelled from source rocks over many millions of years in the Carrack and Caravel drainage areas, more than enough to completely fill both structures. The offshore Otago area is a proven petroleum production area. In 1985 BP drilled the Galleon 1 discovery well flowed gas but was regarded as uncommercial as the structure was only partly filled with hydrocarbons. Galleon-1 however had very high levels of condensate contained in the gas stream. This ‘wet gas’ containing a large proportion of valuable petrol-like oil in the gas is particularly valuable today with oil at over US$100 a barrel. The seismic data obtained over Caravel and Carrack (about 30 and 50 kilometres away from the Galleon discovery) is almost identical to that over Galleon, suggesting comparable reservoir development can be expected. Origin is predicting either wet gas or oil will be found in the two prospects. The northernmost but smaller Caravel area is believed to gather hydrocarbons which overflow into the higher and bigger Carrack area. A key piece of evidence suggesting the two structures may be charged with hydrocarbons is a map of seismic amplitude anomalies (a technique used by explorers to reflect a picture off hydrocarbon-charged rocks). These anomalies match almost exactly the maps of the closure trap area of the two structures. Seismic maps also show a possible ‘gas chimney’ over the crest of Carrack, where hydrocarbons may have leaked into younger sands. Whether or not hydrocarbons are found off Otago, Origin’s efforts do show that exploration off New Zealand’s extensive Continental shelf can reveal potential oil fields not known before.
Energy NZ No.5 Winter 2008 |
The Carrack-Caravel double structure located by Australian explorer Origin Energy appears to be one of the most promising new petroleum ‘discoveries’ made in New Zealand in decades.