SAO PAULO, June 26 (Reuters) - Brazil's Petrobras may have to pay as much as 22 billion reais ($10 billion) by 2018, or 50 percent more than first announced, for new offshore oil rights that a cash-hungry government decided to sell the state-run oil company on Tuesday.
The analysis of the transaction by Reuters is based on the top of a range of oil price forecasts by the U.S. Energy Information Agency (EIA) and exchange-rate estimates collected by Brazil's central bank.
On Tuesday, the government said that Petrobras will pay 15 billion reais by 2018, with 2 billion reais coming into its coffers this year for 9.8 billion to 15.2 billion barrels oil and equivalent natural gas rights in four offshore prospects. The upper level is equal to Brazil's total proven oil reserves.
That oil is in addition to the 5 billion barrels of oil rights Petrobras bought from the government in an oil for stock swap in 2010, but estimates of the amount of oil in the areas has grown since then.
The government's estimate is based on a average exchange rate of 2.20 reais to the dollar and Brent crude oil priced at $105 per barrel.
Even though Petrobras is getting the new rights for far less than it paid in 2010, investors reacted negatively. Since Tuesday, Petrobras shares have fallen 5.4 percent. The company, the world's most indebted and least profitable oil major, has seen production fall despite spending more than any other oil company on expansion in recent years.
The amounts coming to the government after the initial 2 billion reais payment, would be considered pre-payments of the government's share of all oil produced and would be equal to 61 million barrels of oil, the government said. All those payments will come years before any of the new oil is produced.
The value of that oil would be based on the price of Brent crude in the month before the payment and converted at the current exchange rate.
If the price of oil rose to $148 a barrel in 2018, as the top EIA estimate predicts, and the exchange rate weakens to 2.60 to the dollar in 2018, as indicated by Brazil central bank estimates, Petrobras pays about 22 billion reais.
At the lowest EIA forecast of about 69 dollars a barrel in 2018, Petrobras would see its projected payment to the government fall to 12.1 billion reais.
Adriano Pires, head of the Brazilian Infrastructure Institute considers the lower estimate more likely, but still worries that the contract with the government links its financial fortunes to variables it cannot control.
"Petrobras is a hostage to the price of oil and the exchange rate," he said. "The government is betting that oil output will grow and that the excess will finance the company, but there are doubts about the how much Petrobras production will advance."
$1 = 2.20 Brazilian reais Additional reporting by Marta Nogueira; Writing by Jeb Blount; Editing by Eric Walsh