NEW YORK (

TheStreet

) --

Petroleo Brasileiro

(PBR) - Get Report

, also known as Petrobras, gained Thursday after U.S.-listed shares of the Brazilian oil and gas company were upgraded.

American depositary receipts of Petrobras rose 1.3% in early-afternoon trading on high volume.

Petrobras said it will pay Brazil $42.5 billion in new stock in exchange for being allowed to develop 5 billion barrels of oil in offshore fields below the Atlantic Ocean, off the coast of Rio de Janeiro.

S&P Equity Research upgraded the firm's U.S.-listed shares to buy from hold. It said even though Petrobras' stock-for-oil deal came in at the high end of expectations and is likely to add near-term pressure to its stock, the deal is still viewed in a positive light because of its strong long-term growth potential.

Petrobras' offering is one of the largest stock offerings this year, and is part of its overall plan to invest $224 billion by 2014. The deal is expected to close Sept. 30.

Petrobras investors such as billionaire

George Soros

,

BlackRock

(BLK) - Get Report

and

Banco BTG Pactual

have been part of a recent sell off in Petrobras shares,

Bloomberg News

reported.

Elsewhere in the energy sector, shares of Mariner Energy were among the most widely-traded shares on Thursday after a fire broke out on an offshore platform it owns in the Gulf of Mexico, a region still reeling from the aftereffects of the catastrophic

BP

(BP) - Get Report

oil spill.

Early reports of the explosion seemed to be either mistaken or exaggerated. Workers were sandblasting the platform when the fire occurred, and production had been halted.

A Mariner spokesman said that there was neither evidence of an oil spill, nor any visible sheen on the surface of the water.

>>No Spill at Mariner Energy Platform

Shares of Mariner, in the process of being acquired by

Apache Energy

(APA) - Get Report

, promptly sold off after the news broke, dropping nearly 11% but as more details emerged that allayed fears of a large-scale accident, the stock rebounded somewhat.

Mariner shares were down by 4%, to $22.41, in early-afternoon trading.

-- Written by Miriam Marcus Reimer in New York.

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