CNOOC (CEO.NYSE, 0883.HK) and Ghana National Petroleum Corp. (GNPC) have made a joint US$5 billion offer to buy Kosmos Energy’s Ghana assets, Bloomberg reported. According to sources close to the deal, the all-cash bid was received two weeks ago. Exxon Mobil (XOM.NYSE) had previously offered US$4 billion for the Kosmos assets, which include a 23.49% stake in Jubilee, an oil field that holds an estimated 1.8 billion barrels of crude. Dallas-based Kosmos is backed by private equity firms Blackstone and Warburg Pincus. Ratings agency Moody’s said CNOOC’s debt could rise if it continues to increase overseas acquisitions. CNOOC recently said it would pay US$1.08 billion for a one-third stake in a shale oil and gas project in Texas.