CNOOC Ltd., China's biggest offshore oil explorer, agreed to buy a 50-percent stake in Argentine producer Bridas Corp. for $3.1 billion to meet the demand from the world's second biggest energy consumer.

The deal marked China's latest move to increase its oil reserves. It will also raise CNOOC's reserves by 12 percent, boosting its average daily production and providing the company entry into Latin America.

In addition, the investment will bring CNOOC closer to a partnership with BP Plc, Europe's largest oil company. While Bridas owns a 40-percent stake in Pan American Energy LLC, Argentina's largest crude oil exporter, BP owns the remainder of Pan American.