Chief executive Brent Emmett said the partners had a predrill target of about 30mm bbl.

"But we've achieved quite a bit better result than that," he said.

"We can't say just yet by how much we have exceeded it, but it's certainly a very good discovery.

"We have now drilled our second side-track and have gained a high level of appraisal."

In a release on Tuesday, operator Roc Oil said the well encountered four oil-filled sands, totalling 22m gross and 16m net, in the Upper Sand Package, more than 45 metres downdip from the original well and outside structural closure as presently mapped. Two of these sands are not present in the discovery well.

Information from the two sidetrack wells, taken in conjunction with the latest analysis of data from the discovery well, has also confirmed that the Upper Sand Package is not a single reservoir system, Roc said.

Instead, it comprises up to seven distinct oil sands, all with different oil-water contacts and separate oil - and in two cases, oil and gas - columns.

Six of these individual oil columns range in height from 40 metres to 60 metres, while one column, in a five-metre sand, was interpreted to be up to 180 metres.

Emmett told the conference that there was probably about 100m of net pay across the various reservoirs.

While Roc managing director John Doran described the results as encouraging, he was reluctant to speculate on a potential reservoir size.

“The fact that the field seems to be at least partly defined by stratigraphic elements, means that although the data gathered to date is expected to confirm the commerciality of the discovery, more appraisal and/or development drilling will be required before accurate recoverable reserve numbers can be provided for the field as a whole,” he said.

“This is not to say that the field will necessarily prove to be huge, just that it is likely to be bigger than if it was limited by structural closure.”

Doran further added that “current probabilities” suggests that the oil-in-place was likely to be measured in terms of many tens of millions of barrels, which if correct would warrant a commercial development.

Apart from the Upper Sand Package, Roc said the Middle and Lower Sand packages were also encountered between 100 metres and 230 metres down-dip from the discovery well and were water-wet.

Drill and pressure data from the two sidetracks and the discovery well established a 60 metre oil column in the Middle Sand package and infer a possible 105-metre oil column in the Lower Sand Package, updip from the second sidetrack.

In addition, the second sidetrack determined that a deep stratigraphic exploration target did not contain hydrocarbons, Roc said.

Emmett told the Excellence in Upstream Energy conference that the discovery would probably be developed in conjunction with neighbouring finds that were as yet undeveloped.

Interests in Beibu Gulf Block 22/12 are Roc (operator and 40%), Horizon Oil (30%), Petsec Energy (25%) and First Australian Resources (5%).

Horizon is focused on the South China Sea with blocks in China, Vietnam, Cambodia and Thailand.

"The region has high demand with a good deal flow but relatively little competition," he said.

Elsewhere in China, Horizon has acreage covering the

whole of the 3700sq.km Lei Dong Basin.

"It hasn't been drilled yet but I think it is an oil man's dream," Emmett said.

"It has a lot of large structures and simple geology."