Brazil's scandal-plagued state oil company Petrobras said Friday it will sell off its first "pre-salt" deep-water oil field to Norway's Statoil in a bid to pay off its massive debt.

The Petrobras board approved the $2.5 billion sale of its 66-percent stake in the offshore exploration block known as BM-S-8, the company said in a statement.

It said the sale was part of its strategy to prioritize investment in "assets with greater potential for short-term cash generation, capital optimization and economies of scale."

Petrobras had said in December it would start selling off its prized assets, the deep-water offshore fields found in what is known as the pre-salt layer some five kilometers (three miles) below the Atlantic seabed.

It said it needed the money to get its books back in order after its debt soared by more than 40 percent in 2015.

The pre-salt fields hold potentially massive amounts of oil, but it is technically challenging -- and eye-wateringly expensive -- to reach.

Brazil's Congress must still approve the pre-salt sales. Currently, Brazilian law gives Petrobras exclusive rights to the offshore fields, setting the company's minimum stake at 30 percent.

Petrobras says it is confident lawmakers will give their approval.

The company has been battered by plunging oil prices and revelations that corrupt executives, contractors and politicians bilked the company out of billions of dollars over the course of a decade.