The purchase, expected to be signed in London Friday, comes after tests confirmed the presence of about one billion barrels of oil in Uganda's Lake Albert region, Dow Jones Newswires said, citing unnamed sources.
UK-based Tullow said last month it would bring in either Total SA or Cnooc -- the Hong-Kong-listed arm of China National Offshore Oil Corp. -- as a partner to develop its Ugandan oil fields.
Tullow Chief Executive Aidan Heavey said late last month its new partner would entirely fund the development of a 1,200-kilometre (740 mile) pipeline to export Uganda's oil to the Kenyan port of Mombasa.
A Tullow spokesman told Dow Jones "no agreement has been signed with any company at this stage.".
"Tullow won't reach an agreement with any company until the process has been completed with the government of Uganda," the spokesman said.
A Cnooc spokeswoman told AFP: "The company doesn't have any comment on this market rumour."
Cnooc has been exploring for crude oil in neighbouring Kenya, while China National Petroleum Corp. and China Petrochemical Corp. have major producing oil assets in Sudan, Dow Jones said.