The acquisition will increase CNOOC's oil production by about 20 percent, which was between 341 million and 343 million barrels oil equivalent in 2012, and reserves by 30 percent, said CEO Li.

"The move certainly helps CNOOC become more geographically diversified," said John Licata, founder and chief energy strategist at New York-based Blue Phoenix Inc, an independent energy-research company.

"But now that the company gains a footprint in new areas, management needs to explore how best to build upon existing relationships to further enhance and foster even more business opportunities in these regions."

"CNOOC has the opportunity to roll up its sleeves and maximize Nexen's ability to break down heavy oil through technological advancement," he said.

"CNOOC needs to beef up exposure to clean fuels, especially since coal is still rather dominant in China, so while the Nexen deal makes sense, the company needs to not make the same mistakes as the US and become too focused on one or two energy sources."

The largest overseas acquisition by a Chinese company was first announced in July. Canadian regulators approved it in December.

In February, the Committee on Foreign Investment in the United States (CFIUS), which reviews sensitive deals by foreign investors in the country, signed off on the deal's part involving Nexen's assets in the Gulf of Mexico.

"The one-year acquisition process was filled with hardships," CNOOC Chairman Wang recently told the Shanghai Securities News.

"Its success has big significance and value to the company."

Kevin Reinhart, an 18-year Nexen veteran who had served as interim CEO for the company since January 2013, remains in charge. Nexen formed a new board chaired by Li, and with members from both companies, including Reinhart.

CNOOC's keeping Nexen autonomous is one of the main reasons the company's workers aren't anxious about the takeover, said Reinhart.

Nexen's oil-sands assets in Canada, the United Kingdom, the North Sea and offshore West Africa will be managed from its Calgary headquarters, as will about $8 billion worth of CNOOC's North and Central American assets, said Patti Lewis, Nexen's spokeswoman.

"For Nexen, it means immediate growth," said Lewis. The Chinese company's financial backing and other support will allow Nexen to "realize the full potential of its opportunities."

"Both companies share many of the same goals and values," she said. "I think our two cultures will help to reinforce each other and make the combined company an even better one."

CNOOC will seek to retain Nexen's current management team and employees and will provide compensation that is not less than what it was before the transaction, Lewis said of the commitments.