Prospective resource estimates of the Marantao prospect are based on three scenarios – oil only, gas only and a large gas cap over a 50m oil column.

The low case estimate for oil only is 320MMbbl; the high case is 2682MMbbl; and the mean case is 1323MMbbl, according to Otto’s calculations.

The low case for gas only is 721 billion cubic feet; the high case is 5.6 trillion cubic feet; and the mean case is 2.8Tcf.

The low case for a gas cap over an oil column is 468Bcf and 144MMbbl; the high case is 3.6Tcf and 1.1 billion barrels; and the mean case is 1.8Tc and 567MMbbl.

Otto said preliminary interpretation of 2006 2D seismic data acquired over the prospect and reprocessing of other data in the rest of the block had significantly enhanced the company's understanding of the petroleum system.

This has allowed detailed calculation regarding potential resources in Marantao, according to Otto.

"Marantao is the jewel in our exploration portfolio in the Philippines and we are pleased with prospectivity of this licence from the initial 2D seismic data,” chief executive Alex Parks said.

"Once we have completed the interpretation on the rest of the block, farm-in discussions will be initiated and hopefully successfully concluded in early 2008."

Data from a further 960km of 2D seismic the company shot in June this year to assess additional structures along the deepwater margin is being processed and the results will be available to Otto in late October for interpretation in November.

Marantao is about five times bigger than the Shell-operated Malampaya oil and gas field, which is geologically similar and on trend.

SC 55 also contains a string of similar reefal and turbidite leads that have the combined potential to be as big as Marantao, according to Otto.

The company currently holds a 55% interest in SC55 through its wholly-owned subsidiary NorAsian Energy, in partnership with Trans-Asia Oil and Energy and Vital Resources.