Brazilian state-run oil firm Petroleo Brasileiro SA (PETR4.SA) said on Friday that its Chinese partners in the Buzios pre-salt oil field will have to pay it $2.94 billion for its prior investment in exploration under the TOR rights to surplus oil found there.

Petrobras said in a securities filing that Brazilian oil regulator ANP still has to approve the estimates of payment by China National Offshore Oil Corp (CNOOC) (0883.HK) and China National Oil and Gas Exploration and Development Corp (CNODC), which have a 10% stake in the surplus output of the productive Santos basin field.

TOR, short for transfer of rights, covers a roughly 2,800-square-km (1,080-square-mile) zone off the coast of southeastern Brazil, where billions of barrels of oil are trapped beneath a thick layer of salt under the ocean floor, according to surveys.

It is part of a larger oil-producing zone known as the "pre-salt," which has emerged as one of the world's most promising conventional oil plays.