CNOOC said its offshore oil field Penglai 19-3, in the Bohai Sea, is China's second largest oil field after the onshore Daqing field and is estimated to hold 2.6 billion barrels of oil.
CNOOC, which pumps out 370,000 barrels per day, said that under phase one of its development, maximum output is expected to reach 40,000 barrels per day.
The project is a joint venture between US giant ConocoPhillips and CNOOC, which has a 51% working interest in Penglai 19-3 field.