China National Offshore Oil Corp Ltd, China's largest offshore oil and gas developer, announced on Friday a net profit decline last year of 11.4 percent year-on-year to 56.46 billion yuan ($9.11 billion).

The company said the decline is mainly due to lower realized oil prices and cost increases. According to the company's annual results for the year that ended Dec 31, 2013, the company exceeded its production target set at the beginning of the year.

Net oil and gas production reached 411.7 million barrels of oil equivalent, representing a 20.2 percent increase year-on-year, with 60.8 million barrels of oil equivalent contributed by Nexen Inc, which CNOOC acquired in 2012.

Li Fanrong, CEO of CNOOC, said in a statement that the company made progress in production and operational activities while stepping up its pace of overseas development last year.

The company's oil and gas sales revenue reached 226.45 billion yuan, representing an increase of 16.3 percent year-on-year.

In 2013, the company continued to increase its investment in oil exploration, the company said. It also began seven new projects that began producing oil. Its Liuhua 19-5 in the Pearl River Mouth basin of the South China Sea came online earlier this year.

By the end of 2013, the company owned net proved reserves of approximately 4.43 billion barrels of oil equivalent.

In 2013, the company's average realized oil price was $104.60 a barrel, representing a drop of 5.3 percent year-on-year and average realized gas price was $5.78 a thousand cubic feet, representing an increase of 0.2 percent year-on-year.