"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.