SAO PAULO (Reuters) - Shares in Braskem SA soared 10 percent in their biggest intraday gain in more than two months on Friday after Latin America's largest petrochemical firm said it is in advanced talks with authorities in Brazil and the United States for a leniency agreement related to a massive corruption scandal.

Class-B preferred shares <BRKM5.SA>, Braskem's most widely traded class of stock, rose 9.77 percent to 29.77 reais ($8.62) after the announcement, the highest intraday rise since Sept. 28.

Braskem did not provide additional details of the leniency talks in the statement. On Thursday, Braskem's parent Odebrecht SA [ODBES.UL] signed a $2 billion leniency deal with prosecutors to settle its own liabilities in Brazil's biggest-ever graft case.

The imminent agreement could help disentangle a series of aspects preventing controllers Odebrecht SA and Petroleo Brasileiro SA <PETR4.SA> to sell their stakes in Braskem, as it could eliminate liabilities related to the corruption probe, making it easier to attract a bidder.

The so-called Car Wash probe, started by prosecutors in Brazil in 2014, unveiled a sweeping graft scheme involving several construction companies and state-controlled Petroleo Brasileiro SA, known as Petrobras <PETR4.SA>. Petrobras owns a 47 percent voting stake in Braskem.

Reuters reported in March that Odebrecht, which owns 50.1 percent of the voting shares in Braskem, was considering selling a stake in the petrochemical producer in a joint transaction with Petrobras.

Under terms Odebrecht's agreement, the firm will pay the hefty fine over 20 years. The money will be divided between authorities in the United States, Switzerland and Brazil, which would be the main beneficiary.

(Reporting by Ana Mano; Editing by Jane Merriman and Marguerita Choy)