However, Roc chief executive John Doran said the significance of this potential discovery would not be known until next week after electric logs were acquired and analysed.

“On the basis of currently available drill data, the well is likely to be classified as a discovery,” he said.

“While that is technically encouraging, it is not the key commercial point. The real question the well needs to answer is whether or not the accumulation is large enough to merit further appraisal and we won’t have the first part of that answer until early next week.”

At a depth of 1956m, the well encountered a three-to-five metre sand with good reservoir characteristics, strong oil shows and an associated increase in wet gas readings.

Pre-drill interpretation suggests this sand is at the top of an inter-bedded sand-shale sequence expected to be several hundred metres thick, according to Roc.

At a slightly deeper depth of 1985m, a sandy interval, with an apparent drilled thickness of about 16m, was encountered without any obvious oil shows but with very high wet gas readings, the Sydney-based company said.

On Friday, the well was drilling ahead to a total depth of 2530m.

Located in Block 22/12, the well was designed to test the 3D mapped 6-12 South Prospect, which has the potential to contain 10-20 million barrels of recoverable oil.

“The Wei-6-12S prospect represents an unusual structural target,” Doran said.

“Within a small, one square kilometre area of closure lies a thick, hundreds of metres gross prospective sedimentary sequence, which together creates a potential trap capacity in the order of a few to many tens of millions of barrels of oil.”

While Roc is still waiting for log results, Doran said information from wells within and adjacent to Block 22/12 suggests that the reservoir quality in Wei-6-12S-1 would be good to excellent, the hydrocarbon type was likely to be oil rather than gas, recovery factors should be high, and oil viscosity was unlikely to be a problem in the targeted reservoir section.