By Paul Kiernan And Luciana Magalhães 

RIO DE JANEIRO--Shares of Brazilian state-controlled oil firm Petróleo Brasileiro SA plunged at the opening bell Thursday after the company wrote off a staggering 50.8 billion Brazilian reais ($16.8 billion) in 2014 due to losses from alleged graft and overvalued assets.

Petrobras' preferred shares recently traded 5% lower at BRL12.47 on São Paulo's stock exchange.

The write-offs caused Petrobras' first year in the red since 1991, and the third-largest net loss of any publicly traded Brazilian firm since 1986, according to local research firm Economatica. The oil company reported Wednesday evening a loss of BRL21.6 billion for 2014.

Chief Executive Aldemir Bendine told journalists Wednesday night that Petrobras won't pay dividends for only the second time since 1986.

Investors had expected the impairments to be smaller: between BRL15 billion and BRL30 billion, according to political risk consultancy Eurasia Group.

There is a potential silver lining to such a large number, analysts say. Independent auditor PricewaterhouseCoopers, which had refused to sign off on Petrobras' financial statements for months because of uncertainty surrounding a corruption investigation, approved the 2014 earnings report "without any reservations," Mr. Bendine said Wednesday.

But one large investor in Brazilian stocks, who didn't want to be identified, said he didn't see much upside to Petrobras shares.

"The relief from the earnings release is already priced in today," the investor said Wednesday night, after Petrobras released its report.

Bruno Gonçalves, an equity analyst at São Paulo-based Alpes brokerage, said investors are now focusing on "excessive leverage," lower-than-expected production guidance and weak oil prices that threaten the company's investments and planned asset sales alike.

"Petrobras managed to get a BRL50 billion problem out of its way," Mr. Gonçalves said. "Now the market is looking at the fundamentals."

Write to Paul Kiernan at paul.kiernan@wsj.com

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