"This is a bloody good prospect and, if Tui really is undisturbed and both the Moki and Kapuni formations work, then it could be huge.

"There is also the potential that it will make a huge impact on the small companies involved and Shell could then find itself facing some real competition," O'Connor told EnergyReview.Net.

Late last year New Zealand Overseas Petroleum, a subsidiary of the Bermuda-registered Transworld Oil group, took a 45% stake in PEP 38460 and is acting as operator for the Tui-1 drilling program.

Sydney-based New Zealand Oil and Gas has a 20% stake, NZOG associate Pan Pacific Petroleum 15% and Australian Worldwide Exploration 20%.

O'Connor said the obvious attraction of Tui - one of several prospects in the licence - was its proximity to the declining but still commercial Maui field and its two platforms and associated processing facilities.

Other positive elements were the presence of bright seismic amplitudes and that the amplitudes were orientated in such as way as to support the concept of a linear shoreline sand belt, added the Diligenz managing director.

However, the presence of seismic amplitudes alone was not sufficient to indicate any definite hydrocarbon columns and some work by interested parties had produced some rather ambiguous results from detailed amplitude studies, he cautioned.

He said the key risk element was the presence of a trap for the Eocene-aged Kapuni group and migration and trap for the Miocene-aged Moki formation.

"If the stratigraphic trap does exist, then this prospect, if a discovery, would completely alleviate the gas supply problem facing New Zealand," he added, referring to the likely faltering of Maui, which supplies about 80% of this country's gas, from mid-2007.

Risked stratigraphic volumes for the Maui sands alone at Tui were equal to the proven size of the undeveloped Kupe field, off south Taranaki, about 260PJ of gas and 15 million barrels of oil.

Diligenz had independently modeled risked reserves of 35 million barrels for the Kapuni group within any Tui prospect structural trap, and a more harshly risked 65 million barrels in the stratigraphic case.

"Clearly, these are very economic indeed and, in the event of a commercial discovery, Tui could be fairly quickly developed," O'Connor added.

Former Tui operator NZOG also believes the prospect has the potential to change this country's energy industry.

The Tui prospect is a very large prospect and NZOG considers it to be gas-condensate prone, as it is closely geologically related to the nearby Maui field.

The main target is the Kapuni D beach sands at approximately 3400m depth. At Tui, the D sands closure is thought to be a remnant of an originally much bigger closure that hosted an oil accumulation extending over both Maui and Tui.

This hypothesis is consistent with oil found in the D sands at Maui B, which may also be a remnant of the larger original oil accumulation.

It is thought Tui has not been affected by the late uplift and faulting that probably caused much of the oil in the original pool at Maui to escape.

NZOG has said the cumulative potential of the D sands is 700 million barrels of recoverable oil; with the secondary objectives of the Moki formation and the Kapuni F sands having the potential to contain 400 million barrels and 50 million barrels respectively.