CNOOC Ltd, China's biggest offshore oil and gas driller, reported on Thursday flat output in the third quarter, as production on its existing fields dropped.

The State-owned company expects a rebound in the fourth quarter, said chief financial officer Zhong Hua during a conference call. Many of the company's new fields are starting production in the second half.

However, the company added that if the contribution from recently acquired Nexen Inc is included, output rose 17.8 percent year-on-year to 103.4 million barrels of oil equivalent. The Canadian company contributed 16.1 million barrels of oil equivalent.