Tullow, which has a reputation for opening up new oil basins, is yet to establish this frontier part of West Africa as a potential new producing territory after classifying the Montserrado well in Liberia as a sub-commercial discovery.

The firm also revised down its 2011 average production to 79-81,000 barrels of oil per day (bopd), after giving guidance in August that full-year output would average 82-84,000 bopd, citing mechanical issues with wells at the Jubilee field in Ghana.

Shares in Tullow tumbled 6.9 percent to 1,337 pence at 0924 GMT, lagging the European oil and gas sector which was 0.8 percent lower.

“It is a premium priced stock and vulnerable to any disappointing well results,” Oriel Securities analysts said, adding that the well result was encouraging for further exploration in Liberia.

Royal Bank of Canada analyst Al Stanton said that disappointment with Montserrado was heightened by the fact that historically a successful well for Tullow has been followed by further successes.

“People were anticipating not just the upside value of Montserrado but Montserrado look-a-like 2, Montserrado look-a-like 3, look-a-like 4,” he said.

Tullow, whose future production targets are underpinned by the giant Jubilee oil field off the coast of Ghana, said the ramp up to the 120,000 bopd plateau production level at Jubilee would be delayed from its previous end-of-year guidance.

“It’ll be sometime during 2012. It depends really upon working through these issues,” Chief Financial Officer Ian Springett said in an interview with Reuters when asked when plateau production would be achieved.

Stanton said the lack of clarity on Jubilee timings was also impacting the stock: “No explicit guidance on the 120,000 bopd plateau will push people’s second half cash flow forecasts down and likewise first half numbers for next year will go down slightly.”

The delay to the ramp up in production in Ghana had already been flagged by analysts following an update on the Jubilee field from Tullow’s partner Anadarko earlier in November.

Tullow also said it was still waiting for a much-delayed deal in Uganda to complete after having predicted in August that it would conclude in September.

On Wednesday, Tullow moved that guidance to say completion, which involves the Ugandan government giving it its final blessing to the deal, was expected “shortly.”

“I think our view is that it absolutely should conclude this year,” Springett said, shedding light on what was meant by shortly.

Tullow, which declined to comment on the reason for the delay, has been waiting since 2010 to finalize a deal to bring in new partners — French oil major Total and Chinese group CNOOC — to start a $10 billion oil development project around Lake Albert.