MOSCOW (Reuters) - Oil major ExxonMobil XOM.N and Russia's Rosneft ROSN.M unveiled an offshore exploration partnership on Wednesday that could invest upward of $500 billion in developing Russia's vast energy reserves in the Arctic and Black Sea.

A worker walks in the Rosneft Achinsk oil refinery, one of the biggest Siberian fuel suppliers, near the town of Achinsk, some 188 km (117 miles) west of Krasnoyarsk, September 9, 2011. REUTERS/Ilya Naymushin

The deal, between the world’s largest listed oil firm and the world’s top oil producing nation, was the product of nearly a year of talks and came about despite a history of mutual distrust between Washington and Moscow dating back to the Cold War and recent difficulties for other Western firms in Russia.

“Experts say that this project, in terms of its ambitions, exceeds sending man into outer space or flying to the moon,” Russian Deputy Prime Minister Igor Sechin, a key architect of the partnership, told a briefing for analysts in New York.

Under the deal, signed in Moscow on Monday, Exxon and state-controlled Rosneft will seek to develop three fields in the Arctic with recoverable hydrocarbon reserves estimated at 85 billion barrels in oil-equivalent terms.

A final investment decision on the projects in the Kara Sea, in the centre of Russia’s north coast, is expected in 2016-17, Rosneft said in a video presentation at the event, which was hosted by Exxon CEO Rex Tillerson and Rosneft President Eduard Khudainatov.

Three-dimensional seismic surveys of 20 “promising structures” in the Tuapse block of the Black Sea have, meanwhile, found estimated recoverable reserves of 9 billion barrels of oil, the presentation said.

Tillerson said the Russian government’s commitment to reforming offshore energy taxation by abolishing export duty and slashing mineral extraction tax, and to keep taxation stable for 15 years, had been crucial to unlocking the deal.

“There is a clear commitment to see these areas developed,” he said in webcast remarks.

The deal is likely to prove transformational for Exxon, which like other global oil majors is under pressure to add to its 25 billion barrels of oil-equivalent reserves as resource-rich states seek greater control over their energy resources.

Sechin, a close ally of Russian Prime Minister and President-elect Vladimir Putin, hailed the partnership, saying it had already added $7 billion to Exxon and Rosneft’s combined market capitalization since it was announced.

The deal - by far the largest between Russia and a foreign oil company - was possible, he said, despite a history of “excessive politicization and historic stereotypes” that has long hampered bilateral relations.

Sechin later said that, should the project experience force-majeure - a situation beyond the partners’ control that would render it economically unviable - it would then be exempted from all Russian taxation.

TECHNOLOGY TRANSFER

For Exxon, the partnership secures an Arctic prize that was coveted by British oil major BP BP.L before its own talks with Rosneft collapsed last May.

Exxon moved quickly to seal an outline deal in August to create a joint venture that would plough an initial $3.2 billion into exploring the Kara Sea and Black Sea.

Rosneft will have a two-thirds stake in the venture, while Exxon would own a third and shoulder the initial exploration costs. If the reserve base is confirmed, total investments could exceed $500 billion in the coming decades, Rosneft said.

Under the more comprehensive deal announced on Monday, Rosneft will get 30-percent minority stakes in Exxon-led projects to develop hard-to-recover reserves in West Texas, the Canadian province of Alberta and the U.S. Gulf of Mexico.

Further international projects could follow, Tillerson said.

The partners will also seek to transfer the know-how they gain in North America to western Siberia, where Rosneft controls an estimated 1.7 billion tonnes in reserves of ‘tight’ oil, trapped in non-porous rock.

Rosneft, Russia’s top oil producer, will increase hydrocarbon output by 4 percent per year to 3.7 million barrels per day of oil equivalent in the decade to 2020 as it seeks to develop new projects, Khudainatov said.

Khudainatov said that the forecast rise from 2.5 million barrels per day in 2010 assumed an oil price of $90 per barrel in real terms as well as a favorable tax regime.

The rise was underpinned by a rapid rise in gas output. Including associated gas, state-controlled Rosneft expects gas output to rise to 45-55 billion cubic meters in 2020 from 12 bcm in 2010, Khudainatov said.