BP, BG Group and Exxon will not bid in the planned Oct. 21 auction of the giant offshore Libra field, Brazil's largest, the head of the National Petroleum Agency, or ANP, said Thursday.

The companies decided not to participate for various reasons, Magda Chambriard told reporters. She added that a dozen companies had paid the fee to reserve a spot at the auction, but did not say which ones.

The oil regulator had expected the auction would attract nearly 40 energy companies, according to Chambriard, who said interest was low due to the "global context" and each company's specific situation.

The Libra prospect holds between 8 billion and 12 billion barrels of recoverable oil, or enough to double Brazil's proved reserves, according to the ANP.

Located 185 kilometers (115 miles) off the coast of Rio de Janeiro state, Libra will be the first field in the pre-salt region to be put up for auction.

The winning consortium will be the one that pledges to give the government the biggest share of future oil production, according to the auction rules.

Under Brazil's current regulatory framework, state-controlled oil company Petrobras must be the operator and have a minimum 30 percent stake in all projects in the pre-salt blocks, so-named because their reserves are located under water, rocks and a shifting layer of salt at depths of up to 7,000 meters (22,950 feet) below the surface of the Atlantic.

The winner of the auction also will have to pay 15 billion reais ($6.6 billion) for the operating license.

The winning consortium will need to operate between 12 and 18 rigs at Libra, whose output is expected to reach nearly 1 million barrels per day, according to ANP's studies.

Oil production in Brazil currently amounts to nearly 2 million barrels of oil equivalent per day. EFE