CNOOC Ltd's acquisition of Canadian oil and gas company Nexen Inc for $15.1 billion — China's largest overseas takeover — was officially closed on Tuesday, seven months after it was announced.

The nation's biggest offshore oil explorer CNOOC is now in control of Calgary-headquartered Nexen, which owns assets in western Canada, the North Sea, West Africa and the Gulf of Mexico.

"The deal helps CNOOC come into the oil and gas businesses in the North Sea for the first time, in accordance with the company's long-term strategy," said Yang Hua, general manager of the company, on Tuesday in Beijing.

After the acquisition, the proven oil and gas reserves of the Chinese company will increase by 30 percent and the output will grow by 20 percent. Meanwhile, the company's assets will become more diversified and will include projects in the oil-rich North Sea area and the shale gas-rich Horn River in Canada.

The deal has provided a platform for CNOOC to expand its businesses in North America, said Yang.

Kevin Reinhart, who served as interim CEO since January 2012, will remain in charge of Nexen, and Li Fanrong, CEO of CNOOC, will be the chairman of Nexen's new board of directors.

"The company is delighted to acquire a leading international platform through the acquisition of Nexen," said Wang Yilin, chairman of CNOOC. "We strongly believe that this acquisition is a good strategic fit for us and will create long-term value for our shareholders."

Yang said that Chinese companies will likely seek an increasing number of M&A deals in overseas markets.

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