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.

Oil and natural gas sales were at 56.1 billion yuan ($9.16 billion) in the three months ended Sept 30, up 15.9 percent from the third quarter of last year.

The Beijing-based company doesn't report profit on a quarterly basis.

CNOOC's average realized oil price saw a 1.5 percent uptick from the same period last year to $106.26 per barrel, while its average realized gas price went down 6.9 percent to $5.43 per thousand cubic feet, dragged by low gas prices at Nexen.

CNOOC shares fell 0.8 percent to HK$15.28 ($1.97) on Thursday in Hong Kong before the announcement. The company's shares have shed 8.9 percent since the beginning of the year, amid a 0.8 percent gain in the benchmark Hang Seng Index.

In the third quarter, CNOOC discovered five new fields, two of them in China and three overseas, and drilled a total of 15 appraisal wells.

"I am glad to see that the company made significant progress in exploration for the third quarter, particularly the new discovery Luda 5-2 North and successful appraisal Kenli 9-5/9-6 have further enlarged the reserve scale of Bohai and facilitated our sustainable development in the future," said Li Fanrong, CNOOC's CEO.

Last week, CNOOC bought a 10 percent stake in a 35-year production-sharing contract to develop the Libra pre-salt oil discovery in the Santos Basin, offshore Brazil.

China National Petroleum Corp - the country's biggest oil and gas producer - also acquired a 10 percent stake in the project. Exploration work is in the preliminary stage, said CFO Zhong, and total investment on the project can't yet be determined.

Taking out the impact from Nexen, CNOOC's capital expenditure increased 18.2 percent to 17.7 billion yuan in the third quarter, mainly due to increasing costs for project development.