London: Billionaire investor George Soros bought an $811 million stake in Petroleo Brasileiro (Petrobras) in the second quarter, making the Brazilian state-controlled oil company his investment fund's largest holding.

As of June 30, the stake in Petrobras, as the Rio de Janeiro-based oil producer is known, made up 22 per cent of the $3.68 billion of stocks and American depositary receipts held by Soros Fund Management, according to a filing with the US Securities and Exchange Commission. Petrobras has since slumped 28 per cent.

Soros has increased his mining and commodities holdings, a move that accelerated in the first quarter with purchases of such companies as Cia. Vale do Rio Doce, the world's largest iron-ore producer, and Talisman Energy, a Canadian oil and gas company. In November, Petrobras announced the discovery of Tupi, a field with as much as 8 billion barrels of reserves, making it the largest find in the Americas since 1976.

"Petrobras has something that other oil companies don't have: oil - lots of it and they're going to find more," said Ricardo Kob-ayashi, equity fund manager with UBS Pactual in Rio de Janeiro.

"If you can buy now and hang on, if you have the staying power, it's great."

Tupi is part of a new deepwater offshore region known as the pre-salt that may contain as much as 50 billion barrels, according to Peter Wells, oil analyst with the UK's Neftex Petroleum Consultants.

The drop in Petrobras' US-traded common shares since June 30 would have reduced the value of Soros's disclosed stake by $235 million.

Soros Fund Management didn't report holding any Petrobras shares at the end of the first quarter. It did disclose much smaller stakes in the Brazilian oil company during 2007, including 150,000 depositary shares, with a market value of about $17.3 million at December 31.

The hedge fund company also had calls on another 35,000 shares at December 31. Petrobras shares traded at an average closing price of $64.83 each during the second quarter, when Soros bought the stake.

The shares on Friday dropped 91 centavos, or 1.8 per cent, to $50.68 in New York, valuing Petrobras at $204.8 billion, the world's 11th-biggest company by market capitalisation.