HRT agreed to sell a 45 percent stake in 21 Amazon oil blocks to Anglo-Russian TNK-BP for $1 billion (622 million pound), the Brazilian start-up company said on Monday.

The deal represents a step forward in TNK-BP's expansion beyond Russia, where the state has curbed private companies' access to promising offshore areas. It also accelerates the rise of HRT in Brazil, where state-run Petrobras
still dominates the oil industry.

The agreement may bring renewed attention to oil reserves in Brazil's Amazon at a time when the spotlight has been on the deep-water offshore region known as the subsalt. The New York-state-sized area near Rio de Janeiro and Sao Paulo is believed to hold more than 50 billion barrels of crude, enough to provide all the world's needs for more than 1 1/2 years.

For HRT it's a very good sign, because it puts it in another league of companies and shows their capacity to negotiate, said independent Brazil energy analyst Francois Moreau, who is also a former oil industry executive.

HRT will remain operator of the blocks, it said in a press release. A company spokeswoman later said TNK-BP would have the right to become operator and to buy another 10 percent stake 30 months after Brazil energy regulator ANP approves the transaction. HRT will receive payment over two years.

HRT will first acquire the 45 percent stake from partner company Petra Energia, and then transfer it to TNK-BP, which is Russia's No. 3 oil producer.

HRT has the right to receive additional payments of up to $5 billion over 10 years. The payments would include compensation for past expenses and payment of 73 cents per barrel of proven and probable reserves above 500 million barrels of oil equivalent.

Run by geologist Marcio Mello, HRT raised $1.5 billion in a 2010 stock offering, wooing investors with prospects in Brazil and Namibia. TNK-BP is a 50-50 joint venture between BP Plc and four Soviet-born billionaires.

Despite growing concerns about the Amazon environment, oil production there remains low on the list of perceived environmental threats, compared with cattle ranching and soy farming, which are key causes of deforestation.

LOGISTICS CHALLENGE

The biggest challenge will be transporting the natural gas produced in the isolated area via a pipeline Petrobras built for its own Amazon project.

While river tankers can carry the high-quality light oil to be produced there, HRT's ability to move the natural gas will depend almost entirely on Petrobras, Moreau said.

Clearly bringing partners will let them negotiate with Petrobras in a different context, he said. They have other bargaining chips to put on the table, and I'm sure this is the reason they are bringing them in.

Twenty-five years ago, Petrobras made the Urucu oil and gas discovery in the Amazon and pioneered efforts to operate in the rainforest with limited environmental impact.

But the area has been largely overlooked since then as the company focussed on offshore operations that now account for the vast majority of its oil production.

Petrobras' 2007 deep-water discoveries in the subsalt region opened a new frontier for oil exploration and drew the attention of oil companies from around the world.

But new projects there are on hold until Brazilian politicians can agree on a scheme to distribute oil royalties among different states.

TNK-BP's decision to move into Brazil marks a break from the company's traditional strategy of seeking out assets that are already in production. The blocks HRT holds are in the exploration phase.

TNK-BP pumped 1.5 million barrels per day of crude oil in September and accounts for a fifth of BP's equity production. TNK-BP wants to boost the international share of its output to 25 percent over the next 30 years.

Growth is crucial for TNK-BP outside Russia, where its portfolio of reserves is dominated by ageing fields that have been producing for a long time. Russia's government has given preference to state controlled state-controlled Rosneft for development of top offshore areas.

(Reporting by Brian Ellsworth in Rio de Janeiro; Additional reporting by Sabrina Lorenzi; Editing by Gerald E. McCormick and Lisa Von Ahn)