Chinese oil and gas giant CNOOC Ltd will buy Canadian oil company Nexen for US$15.1 billion, the two companies announced Monday.

They said CNOOC will acquire all of the outstanding common shares in Nexen for US$27.50 per share in cash.

The transaction is expected to close in the fourth quarter of 2012.

"The acquisition reflects our strong belief in Nexen's rich and diverse portfolio of assets and world-class management and employees,"CNOOC Chairman Wang Yilin said in a statement.

"This is an exciting opportunity for us to build on our existing joint venture relationship with Nexen in Canada, and to acquire a leading international platform in the process,"he added. "We strongly believe that this acquisition will create long-term value for CNOOC Ltd.'s shareholders."