NEW YORK ( TheStreet) -- China's Cnooc ( CEO) said Monday it reached an agreement to buy Nexen ( NXY), the Canadian oil and gas producer, for $15.1 billion in cash. Cnooc's offer of $27.50 a share is a premium of about 60% to Nexen's closing price Friday of $17.06. The deal is expected to close in the fourth quarter. Cnooc is China's largest offshore oil company. It said the deal would allow it to expand to expand to Canada, Nigeria and the Gulf of Mexico. -- Written by Joseph Woelfel >To contact the writer of this article, click here: Joseph Woelfel >To submit a news tip, send an email to: email@example.com.