China National Offshore Oil Corp. (CNOOC), the country's top offshore oil producer, said Monday it has entered into an agreement to buy Canada's Nexen Inc. for $15.1 billion in cash.

The deal will help expand CNOOC's overseas business and resource base in regions such as Canada, Mexico and Nigeria, the company said in a statement to Hong Kong stock exchange.

"The current indebtedness of Nexen of approximately $4.3 billion will remain outstanding. The company intends to fund the proposed acquisition through existing cash resources and external financing," it said.

The transaction, expected to be concluded in the fourth quarter, has received the unanimous approval of Nexen's and CNOOC's boards of directors.

Headquartered in Calgary, Nexen had an average production of 207,000 barrels of oil equivalent per day (mboe/d) in the second quarter of the year.