Oil production by Brazilian state-led Petroleo Brasileiro SA in the Campos basin fell 1.4 percent in June over the previous month to 1.042 million barrels a day, its lowest level since 2001, as mature fields decline, according to company data.

Output has dropped 15.8 percent in 12 months due to the ageing of fields off-shore from Rio de Janeiro and Espirito Santo that account for almost half of the crude pumped by Petrobras. The decline has offset rising output from new platforms in the pre-salt region of the Santos basin.

Petrobras has looked at creative ways to handle mature fields by either selling them or entering partnerships to boost recovery efforts.

On June 14 it concluded the sale of a 25 percent stake worth $2.9 billion in the Roncador field to Equinor.

The partnership with the Norwegian company formerly known as Statoil will include measures to slow the decline of production in Roncador and raise the recovery factor.

In a move by private equity firms to gain a foothold in Brazil, Warburg Pincus and EIG Global Energy have placed bids for shallow water mature oilfields being sold by Petrobras, industry sources told Reuters last month.

The clusters located in the Campos basin off the coast of Rio de Janeiro state are likely to fetch proposals of around $1 billion in total, which would help boost a wider effort by Petrobras to sell assets and reduce debt.

(Reporting by Marta Nogueira Editing by Leslie Adler)