The two pilots and eight off-duty workers were being shuttled from the Bohai Petroleum Corp. platform to the city of Tianjin when the U.S.-made Bell-412 helicopter crashed at the city's port, the agency said.
The Chengbei field, developed jointly by China's Bohai Oil Corp. and a Japanese consortium called Chengbei Oil Development Corp., produced about 1.2 million barrels.
CNOOC itself continues to be active in its efforts to lift production at its own wells in areas such as the Bohai Sea, in China's northern water, where it brought onstream a new field in September.
The spill was caused by the June 8 collision of two Japanese-owned freighters in the Bohai Gulf, east of Tianjin.