Ghana's Jubilee oil field has recoverable reserves of 800 million barrels, Energy Minister Jospeh Oteng-Adjei told a conference in Accra on Tuesday.

Jubilee was discovered in 2007 by a partership of the Ghanaian government and international firms.

Tullow Oil said last month it expects to begin pumping the first oil from Jubilee in the fourth quarter of this year.

Ghana, already a major exporter of cocoa and gold, expects to receive a major economic boost from oil revenues.

"The conviction of President (John Atta) Mills and his government is that Ghanaians should be the biggest beneficiary of the oil resources," Oteng-Adjei said.

Ghana's state-run oil firm Ghana National Petroleum Company (GNPC) is in protracted talks to buy the stake owned by Kosmos Energy, another project partner.

A Ghanaian government source said on Tuesday the government wants to block a reported deal for Kosmos Energy to sell its interest in the field to Exxon Mobil XOM.L for $4 billion.

Kosmos, which is backed by private equity groups Blackstone and Warburg Pinncus, has a 30.875 stake in Jubilee's West Cape Three Points block and an 18 pct interest in the deepwater Taino block.