The gas giant will acquire a 72.7% stock controlled by Millhouse Capital, investment instrument of the Chelsea soccer club owner Roman Abramovich.
Given Gazprom owned Sibneft shares already, its stake in the oil company will rise to 75.7%.
Gazprom chief Alexei Miller said the concern had made another step in diversifying its business and transforming Gazprom into a global energy company.
"Gazprom-Sibneft business has a clear synergy and will improve Gazprom's performance both in Russia and on the world oil and gas markets," Miller said in a news release Wednesday.
The concern borrowed a record $12 billion from a syndicate of Western banks, including ABN AMRO, Dresdner Kleinwort, Citigroup, Morgan Stanley and other influential lending organizations last week to finance the Sibneft deal.
Sibneft's market capitalization was estimated at $17 billion at the moment.
Earlier this month Sibneft shareholders decided to pay a 2.3-billion-dollar dividend for 2004.
Gazprom and Sibneft shares fell slightly on the news of the Sibneft takeover.
Experts said the stock market had expected the deal, the expectations had been reflected in the share prices, and, therefore, the decline was quite logical. They predicted the shares would start growing again shortly.
Gazprom board of directors decided Wednesday to increase its 2005 investment program by 92.4 billion rubles ($3.23 billion) to the level of 305 billion rubles ($10.68 billion), the move designed to ensure the commissioning of important facilities in 2006, the company's press release said.
