Anadarko Petroleum Corp. (APC.LN) and Tullow Oil PLC (TLW.LN) said Wednesday they have discovered oil off the coast of Sierra Leone, a find that analysts say signals huge potential for further discoveries in an area stretching east through the waters of Liberia, Cote d'Ivoire into Ghana.

"The Venus discovery confirms the existence of an active petroleum system in the basin and enhances the prospectivity of our vast West Africa acreage position," said Bob Daniels, Anadarko's senior Vice President of Worldwide Exploration.

With the 1.2 billion barrel Jubilee field already discovered in Ghana at the eastern limit of the system, and this success at the Venus well at the western limit in Sierra Leone, "we have established bookends spanning approximately 1,100 kilometers across two of the most exciting and highly prospective basins in the world," he said.

"If you've got hydrocarbons at two extremes, there's a good chance of hydrocarbons being in the middle," said Panmure Gordon analyst Peter Hitchens. "There is a huge amount of acreage and a large number of prospects," with great potential for both Tullow and Anadarko, he added.

"The significance of Venus-B's success is huge," said Evolution Securities analyst Richard Griffith. Tullow has identified six prospects between Ghana and Sierra Leone that could be of similar size to its flagship Jubilee field, he said.

However, NCB Stockbrokers analyst Peter Hutton cautioned that a large amount of upside from future West African oil discoveries is already priced in for Tullow. To get to a share price of 1170 pence, which Tullow hit earlier Wednesday, requires around a billion barrels of reserve upside for Tullow to be de-risked through further exploration, he said.

"Sell into strength, but leave some position for a (takeover) bid, which looks even more inevitable after this confirmation," Hutton said.

The Venus B-1 well was drilled to a total depth of approximately 18,500 feet in about 5,900 feet of water and encountered more than 45 net feet of hydrocarbon pay, Anadarko said. It is the first deepwater test in the Sierra Leone-Liberian Basin.

Anadarko operates the Venus well with a 40% interest. Its co-owners are Woodside Petroleum Ltd (WPL.AU) of Australia with 25%, Spain's Repsol YPF SA (REP.MC) with 25% and Tullow with 10%.

At 1155 GMT, Tullow shares were up 5.6%, or 60 pence, at 1147p. Andarko shares had not yet started trading.

   -By James Herron, Dow Jones Newswires; +44 (0)20 7842 9317; james.herron@dowjones.com