Brazil plans to sell $5-9 billion in offshore oil rights to state-run oil company Petrobras, one of the main owners of chemical producer Braskem.

The move is expected to saddle the Brazilian company with new costs even as its debt soars and it struggles to meet existing production targets.

Shares of Petrobras plunged as some investors said the plan to sell the oil rights to the company was an attempt to transfer future Petrobras profit to government coffers if an expected bonanza from offshore oil fails to materialize as quickly as hoped.

Under the plan approved by the country's national energy council, the government will receive 15 billion reais ($6.82 billion) by 2018. Two billion reais are due this year, said Marco Antonio Almeida, secretary of oil and gas at the mines and energy ministry.

Petrobras will buy the rights in areas where it bought 5 billion barrels of still unproduced oil from the government in 2010 for $43.5 billion of Petrobras stock. Those areas are believed to hold more oil than first expected, though there is no certainty any of the oil will ever be produced from the high-risk, high-cost, deepwater oil areas.

Petrobras CEO Maria das Gracas Foster called the government's plan to sell new oil rights "an excellent opportunity."

Foster said the company will increase future capital spending plans by 3% above its $221 billion 2014-2017 plan and build nine new offshore oil platforms to increase output under the new deal.

"There is no way for Petrobras to anticipate the production of any of those barrels unless it postpones other projects," analysts at Itau BBA in Sao Paulo, the investment-banking and brokerage arm of Banco Itau-Unibanco Holding, wrote in a note quoted by the news agency Reuters.

"In other words," they said, Petrobras will in fact pay 15 billion reais to the government in five years for barrels to be produced in the long term."

The government will also get 76.5% of all oil produced after development costs are paid, money it will be able to spend or see transferred to local governments with little congressional oversight.

Brazilian President Dilma Rousseff, who promised billions of dollars in oil revenue from offshore fields when she ran for and won her first term in 2010, has been slipping in the polls ahead of an October election.

Despite about $200 billion of Petrobras investment since then, little of the expected new revenue has materialized, and stagnant production has made Petrobras the world's most-indebted and least-profitable major oil company, market watchers said.