Fourteen companies have qualified to take part in Brazil’s highly-anticipated auction of production-sharing rights on some of the country’s most prolific pre-salt oilfields, scheduled for 6 November.

The offering covers billions of barrels of oil deemed “surplus” to a transfer of rights (ToT) agreement made in favour of state-controlled oil company Petrobras in 2010, and is expected to raise 106.5 billion reais ($25.5 billion) in signature bonuses.

The original ToT agreement entitled Petrobras to produce up to 5 billion barrels from 10 fields including giants such as Buzios and Atapu.

The list of qualified bidders includes: BP; Chevron; China National Oil and Gas Exploration and Development Co, (CNODC); China National Offshore Oil Corp (CNOOC Ltd); Ecopetrol; Equinor; Petrobras; Galp Energia-controlled Petrogal Brasil; Petronas; Qatar Petroleum International (QPI); Shell; Total and Wintershall Dea.

The areas on offer include four of the pre-salt fields: Buzios, Atapu, Itapu and Sepia.

Estimates of “surplus” oil reserves in the area ranged from 6 billion to 15 billion barrels and Brazil’s National Petroleum Agency (ANP) described the areas as offering zero exploration risk.

The highest signature bonus is for the Buzios field, set at 68.2 billion reais.

Some potential participants have expressed concerns about the procedure for compensating Petrobras for investments already sunk into the area, with four big floating production, storage and offloading (FPSO) units already producing oil on the Buzios field.

Additional FPSOs for Buzios, Sepia and Atapu under construction and Petrobras has issued a tender to contract another chartered FPSO for the Itapu field.

Bidding for the federal government’s share of production rights will start at 26.23% for Atapu, at 27.88% for Sepia, at 23.24% for Buzios and at 18.15% for Itapu.

Petrobras decided to exercise preferential bidding rights to guarantee a minimum 30% operating stake on surplus volumes at the Buzios and Itapu fields.

If a rival presents a higher offer in the round for any of the two areas, Petrobras will be entitled to join the winning group with the same 30% operating stake.

Petrobras is also allowed to compete for the other two fields — Atapu and Sepia — but will do so on equal terms with other bidders.

Any bidding is likely to be through multi-partner consortia.