UPDATE 6-CNOOC to buy Nexen for $15.1 bln in China's largest foreign deal

* State oil company to pay $27.50 a share, a 61 pct premium

* Nexen has assets in North Sea, Gulf of Mexico, Nigeria

* Deal represents China's biggest foreign takeover bid

* CNOOC already invested in Canada energy, seeks local listing

* Insiders say chances are good for regulatory approval

* Nexen stock jumps 52 percent (Adds comments from investor, regulatory details)

By Lee Chyen Yee and Jeffrey Jones

HONG KONG/CALGARY, July 23 (Reuters) - State-controlled CNOOC Ltd launched China's richest foreign takeover bid yet on Monday by agreeing to buy Canadian oil producer Nexen Inc for $15.1 billion, forcing Ottawa to decide whether security concerns outweigh its desire for foreign investment in its energy resources.

CNOOC, China's third-largest oil company, hopes to sell the deal to shareholders and the government with a hefty 61 percent premium to Nexen's Friday stock price. It promised to retain all employees and to make Canada home base for its Western Hemisphere operations.