China National Offshore Oil Corporation, the country's largest offshore oil and gas producer, warned that it will have its first half-year loss of 8 billion yuan ($1.2 billion) since becoming a listed company, due to a prolonged slump of oil prices and a write-down on its Canadian oil sands assets.

The offshore oil giant expects a loss for the first half - a big swing from a net profit of 14.7 billion yuan in the same period in 2015, according to a filing to the Hong Kong Stock Exchange on Thursday.

Shares of CNOOC in Hong Kong fell 0.28 percent to close at HK$9.28 ($1.19) on Friday. That was less than the benchmark Hang Seng Index's drop of 1.52 percent.