China's top offshore oil producer CNOOC posted a 11.4 per cent slide in its 2013 net profit yesterday, lagging analysts' forecasts, as it struggled to deliver production growth and control costs amid weakening crude prices.

CNOOC posted a net profit of 56.5 billion yuan ($9.09 billion) for last year, versus 63.7 billion yuan in 2012. The result compared with a consensus forecast of 63.2 billion yuan from 31 analysts polled by Thomson Reuters.

The state-run firm completed its $15.1 billion acquisition of Canadian energy firm Nexen in February last year. It was China's largest overseas takeover that CNOOC says will boost its annual output by 20 per cent and proven reserves by 30 per cent.

Once an investor darling for its high-growth profile, CNOOC has been struggling to boost its output over the past few years as domestic fields age.

Its production grew 20.2 per cent to 411.7 million barrels of oil equivalent in 2013, thanks to contributions from Nexen.

Its operating expenses soared 40 per cent to 30 billion yuan last year as a result of the Nexen acquisition last year, while its realised crude prices dipped 5.3 per cent to $104.6 per barrel.-Reuters