China National Offshore Oil Corporation
CNOOC Ltd's parent, China National Offshore Oil Corporation (CNOOC), is the third largest of the "Big Three" Chinese oil companies, and traditionally specialised in offshore upstream exploration and production, whereas CNPC Group specialised in onshore upstream exploration and production, and Sinopec Group specialised in refining and marketing. These distinctions are now increasingly blurred as the three companies compete in all areas. All of the "Big Three" have vested the majority of revenue producing assets in listed Hong Kong subsidiaries. CNPC into PetroChina, Sinopec Group into Sinopec Limited, and CNOOC Group into CNOOC Limited.