Joint venture partner Voyager said drilling would continue through to total depth before running wireline logs and evaluating the status of the well.

Roc said the well could have come in low to prognosis either because of adverse seismic velocity variations which would have an adverse impact on field reserves, or due to drilling inadvertently on the wrong side of a nearby fault, which would probably have a less severe impact on field reserves.

“If the full data set indicates that the CH-5 location was less than structurally optimal, the joint venture may consider sidetracking the well,” said Roc chairman John Doran.

Cliff Head-5 had been aptly described the well as a ‘pathfinder’, according to Doran.

“If we have ended up on the 'wrong side' of the fault, then the situation may possibly be rectified by a sidetrack to the east,” he said.

“If, however, we are on the 'right' side of the fault, the fact that the top reservoir has been encountered substantially lower than prognosed means that we need to greatly improve our understanding of the East Ridge portion of the Cliff Head structure andthis will impact on the details and timing of the final investment decision for the development of the field."

Cliff Head-5 is being drilled by the Ensco 56 rig. It is a development/appraisal oil well designed to confirm oil reserves and reservoir composition in the East Ridge part of the Cliff Head oil field and has never been intended to be kept for future production purposes.

A development well – Cliff Head-6 – is currently planned to follow Cliff Head-5. The partners plan to case and retain Cliff Head-6 as a future oil producer.

Participants in WA-286-P are: Roc Oil (WA) Pty Limited 37.5%, AWE Oil (Western Australia ) Pty Ltd 27.5%, Wandoo Petroleum Pty Ltd 24%, Voyager (PB) Limited 6% and Cieco Exploration and Production (Australia) Pty Ltd 5%.