State-owned CNOOC said in a statement that the acquisition was completed on Monday.
China's biggest overseas energy deal was finalized after winning approval from a U.S. agency that reviews takeovers by foreign companies for national security implications. The agency had a say because Nexen has Gulf of Mexico oil and gas fields.
Canada approved the deal despite worries it could lead to a flood of takeovers in the country's oil sands sector. Canada's prime minister said approval of similar deals in the future would be unlikely.
Calgary-based Nexen also operates in western Canada, the North Sea, Africa and the Middle East.
Copyright 2013 The Associated
Press. All rights reserved. This material may not be published,
broadcast, rewritten or redistributed.