Roc said a 500 metre gross interval in the target Weizhou Formation was found to contain several oil-bearing sands.

These sands collectively represent about 100 metres of net oil pay with generally good reservoir characteristics, the Perth-based company said.

“Any time you get 100 metres of good quality net oil pay, way beyond pre-drill expectations, you can be forgiven for being cautiously optimistic,” managing director John Doran said.

“Consistent with such a thick net oil pay interval, reservoir characteristics vary in fine detail, but generally the majority of the oil-bearing sands display good reservoir quality.”

Before drilling, Roc’s two main technical concerns were trap integrity and the structure’s relatively small areal closure. Now drilling is complete, Doran says Roc no longer has these concerns.

“The integrity of the trap is self-evident and although the structure does have a limited areal extent, that factor is more than offset by the thickness of the multiple reservoir sand sequence,” he said.

“It will take many weeks to collect and analyse the data to the point where we can begin to offer detailed comments on field size commercial implications.

“In the meantime, we can safely say this well has delivered a handsome discovery that certainly merits further appraisal.”

The Wei-6-12S prospect is described by Roc as an unusual structural target. Within a small, one square kilometre area of closure lies a thick, hundreds of metres gross prospective sedimentary sequence, which creates culmative potential trap capacity of a few to many tens of millions of barrels of oil, according to the company.

The joint venture comprises operator Roc Oil (40%), Horizon Oil (30%), Petsec Energy (25%) and First Australian Resources (5%).