China's CNOOC, the country's largest offshore oil and gas producer, announced Wednesday that it has signed an agreement to buy OPTI Canada Inc. (OPTI) for about 2.1 billion U.S. dollars.

CNOOC Luxembourg S.a.r.l, a wholly-owned subsidiary of CNOOC, will make the acquisition, said the company.

According to the agreement, the transaction, to be conducted under Canadian laws, includes 34 million U.S. dollars to OPTI's shareholders, as well as 825 million and 1.22 billion U.S. dollars payable to holders of its first lien notes and second lien notes, respectively.

Before being reviewed by Chinese and Canadian regulators for approval, the transaction must get consent from holders of OPTI's second lien notes at a special meeting to be held in September. About 55.2 percent of these holders have signed an agreement committing to support the takeover.

The transaction is expected to be completed in the fourth quarter of the year, the company said.

According to the company, the main asset of the Canadian oil sands developer OPTI is a 35-percent stake in Nexen Inc.'s Long Lake Project in Alberta, Canada, which is designed to produce 58,500 barrels of oil per day.