"Access to new resources is the life blood of oil companies," Fadel Gheit, an analyst at Oppenheimer & Co. in New York, told Bloomberg. "Russia is one of the largest resources that's still available. It's like Exxon is now dating the girlfriend BP had a few months ago."

As global oil giants compete for new oil and gas reserves, many of the easiest-to-reach hydrocarbons are exhausted. The Kara Sea fields are about the size of the UK's North Sea and may hold as much as 100 billion barrels of oil equivalent, according to BP. London-based BP spokesman Mark Salt declined to comment to Bloomberg on the new deal, however.

Russia's government will support the Exxon-Rosneft alliance "in every way," Putin said. Houston-based Exxon Mobil will be a "reliable, strategic and good partner."

The agreement allows Rosneft a chance to participate in Exxon's unconventional fields in the U.S., where oil and gas are extracted by grinding rocks with sand, water and chemicals. The two companies said they'll study similar fields in western Siberia.