Brazilian state oil company Petrobras will sell up to $64.5 billion in new stock -- one of the largest in capital markets history -- to raise funds for the world's biggest oil exploration investment plan.

The company will offer 1.59 billion new preferred shares

and 2.17 billion new common shares

, Petrobras said in a statement published in Valor Economico newspaper, for an operation that includes a $43 billion state-backed oil-for-shares swap.

At Thursday's closing price for the stocks, the company would raise 67.8 billion reais ($39.2 billion) from the sale of common shares and another 43.8 billion reais ($25.4 billion) from the preferred shares, making it one of the world's largest- ever stock offerings.

Petrobras expects to begin bookbuilding September 3, and price the share sale on September 23.

The plan has become the financial cornerstone of the company's $224 billion five-year investment plan meant to turn Brazil into a major oil exporter by tapping oil buried deep under the ocean floor in a region known as the subsalt.

The company faces skepticism by investors who have questioned the high price for the oil reserves to be used in the state-backed oil-for-shares swap arrangement, which has sparked fears Petrobras may be overpaying for the assets and diluting shares.

Under the terms of the $43 billion oil-for-shares swap, Petrobras said on Wednesday it agreed to exchange part of the stock to be issued for development rights to 5 billion barrels of offshore oil at a price of $8.51 per barrel -- far above the $5 to $6 per barrel that markets saw as fair.

The swap will endow Petrobras' assets with valuable oil reserves.

The segment of the share sale outside the oil-for-share swap is primarily targeted at non-government and minority shareholders, who will provide the only cash in the operation.

Their participation will be crucial for Petrobras to shore up its balance sheet, which has been stretched by heavy borrowing to finance its ambitious offshore plans.

Government leaders have also said they plan to boost the state's participation in the company's capital to around 40 percent current levels near 30 percent, which has left some investors nervous about greater state sway in the company.

Analysts said the large size of the share swap with respect to the entire operation -- authorized by shareholders for up to $85 billion -- shows the government expects it will be able to pick up a considerable number of shares not subscribed by private investors.

Petrobras' preferred shares, its most widely traded class of stock, closed at 27.60 reais on Thursday while common shares closed at 31.24 reais.

INVESTOR INTEREST

But some investors say those concerns have already been priced into the shares, and that the company is still a compelling investment given its unique access to quality oil reserves in a world that is quickly running out of them.

I think people are going to be surprised by the number of investors that subscribe, said Marc Fogassa, a managing partner at Hedgefort Capital Management which owns shares in Petrobras.

Following months of uncertainty over the plan that pushed the stock down by as much as 25 percent from the start of the year, progress advancing the issue has heartened some shareholders.

The worst of the slaughter is behind us, said Marcio Macedo, who oversees about $40 million in assets at Humaita Investimentos in Sao Paulo. The stock's value is now attractive.

He added that many investors spent the last several months selling or shorting Petrobras to go long in other sectors such as Brazilian banks or retail, and he expected many would take advantage of this offer to get back into the company.

Oil for the exchange will come from at least six fields in the subsalt region, most of which are adjacent to major offshore discoveries such as Franco and Tupi finds.

(Additional reporting by Elzio Barreto, Editing by W Simon )

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