State-run Russian gas monopoly Gazprom has rubber-stamped its $13.1bn (£7.47bn) takeover of Roman Abramovich's oil firm Sibneft.
Gazprom aims to be a major player in oil as well as gas
The deal gives the Kremlin control over one-third of Russian oil output.
Gazprom chairman Dmitry Medvedev said the deal would increase competitiveness at the world's largest gas producer, and a key supplier to Europe.
It also said the takeover would allow it to compete with global majors on the world's oil markets.
Gazprom, the world's biggest natural gas producer, has been back under Russian state control since June, when the Kremlin spent more than $6bn to increase its stake from 38% to 51%.
The move came after Gazprom became embroiled in the Yukos affair and abandoned plans to merge with another oil firm, Rosneft.
Gazprom had signed a binding agreement to buy 72.663% of Sibneft's shares from Millhouse Capital - an investment vehicle belonging to Mr Abramovich, who also owns Chelsea football club.