Shares in Tullow Oil, the biggest independent oil company in the City, surged today after a “world class discovery” off the coast of Ghana, west Africa.

The group said that results from an exploration well revealed a deepwater field it found in June was far bigger than expected and could yield 800 million barrels of crude.

Tom Hickey, chief financial officer, told Times Online: “By some distance this is the largest thing that Tullow has ever been involved in.

“Before we drilled Mahogany we thought the field may contain between 250 million to 600 million barrels of oil. Since then, with the two wells we have drilled, we think it has the potential to be bigger."

Merrill Lynch, the company’s broker, immediately raised its target price on the stock to 610p. Tullow’s share price rose 23p to 485.5p, up 5 per cent, valuing the business at nearly £3.1 billion.

Merrill said: “This confirms Tullow has made a world class discovery in Ghana. The company continues to look better positioned than most.”

Speculation of a success had been mounting since the start of the week, prompting a rise in Tullow’s share price .

The oil field would transform Ghana’s $60 billion economy if successfully developed. Previous discoveries in the country have only held as much as 20 million barrels.

The country’s main money-earner is cocoa and it has been banking on a growth in tourism to drive GDP.

Aidan Heavey, Tullow’s chief executive, was due to meet the Ghanian president today.

Mr Hickey said the company plans to continue appraisal work on the field for the rest of the year “to increase our confidence in it.” He said it could be developed as early as 2011.

Today’s drilling success comes six months after Tullow found a major oil field in Uganda. The group’s presence in Africa has been transformed by last year’s £580 million acquisition of Australia-based Hardman Resources.

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