BEIJING (Bloomberg) — Cnooc Ltd., China's biggest offshore oil explorer, agreed to buy a 50 percent stake in rival Argentine producer Bridas Corp. for $3.1 billion, adding 318 million barrels of reserves as Chinese fuel demand surges.

Bridas, controlled by Argentine businessman Carlos Bulgheroni, owns a 40 percent stake in Pan American Energy LLC, the country's largest oil exporter, and also has oil and gas assets in Chile and Bolivia, according to a Cnocc statement sent yesterday to Hong Kong's stock exchange. BP Plc, Europe's largest oil company, owns the remainder of Pan American.