The offshore Taranaki Maari oilfield could ultimately produce more than 100 million barrels of oil, according to field partner Horizon Oil, after announcing the latest remaining- reserves estimates for the field.

The Maari joint venture has already produced more than 19 million barrels of oil since it started in early 2009.

At present prices the oil would be worth more than $2 billion.

Maari is about 80 kilometres off the coast of South Taranaki and has been a bonanza for its partners.

However, production has been in "natural" decline in the past year, and the field has also had problems with its electrical submersible pumps.

The partners have been looking to lift the field's performance, with drilling plans that could add to production from 2014 onwards.

The main partners are Austrian company OMV with 69 per cent, a Todd Energy offshoot with 16 per cent, and Cue Taranaki with 5 per cent.

Horizon has a 10 per cent stake in the Maari field.

Cue, with 5 per cent of the Maari field, said its share of production in the past financial year was just under 270,000 barrels, down from more than 360,000 in the previous year, a drop of about 25 per cent.

Cue expected production of 300,000 barrels at Maari and much of the lost production was down to failed electrical submersible pumps.

The Maari partners have spent about $1.5b on the field since 2005, and said they expected to fully recover that some time this year, just four years after it began producing oil.

Horizon Oil yesterday posted independent reserves audit figures showing probable or "P50" remaining reserves of 88.4 million barrels at the Maari-Manaia field.

P50 indicates reserves estimated to have a better than 50 per cent chance of being technically and economically producible.

That included more than 54 million barrels of developed reserves and another 34 million or so in undeveloped reserves.

When it first started in 2009, Maari was expected to produce about 50 million barrels of oil in 10 to 15 years.

Horizon chief executive Brent Emmett said the company was pleased with the independent verification of 8.8 million barrels as Horizon's share of the remaining proven and probable reserves in the producing zones at Maari and Manaia.

"Given that around 19 million barrels have been produced to date, the reserves audit supports our long-held opinion that these zones will ultimately produce in excess of 100 million barrels," Emmett said.

But more development drilling and enhancement of the water injection system would be needed to extract the reserves.

The partners have designed a work programme to do that in the next two years.

As well as existing producing zones, there was more potential in the deeper Maari zones, and deeper and shallower zones at the nearby Manaia structure and in the undrilled Maari South structure, he said.

Horizon estimated the potential was estimated to be a gross 35 million barrels and at least part of that would be evaluated in the drilling programme planned for 2013-14.

At the more conservative "proved" P90 level, Maari's reasonably certain remaining reserves were estimated to be 40.8 million barrels.

At the outside, there was a 10 per cent chance the reserves could be as high at 132.7 million barrels. Fairfax NZ

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