China National Offshore Oil Corporation, the country's largest offshore oil producer, announced on Monday that it has signed an agreement worth $1.93 billion to buy liquefied natural gas from BG Group and increased its stake in BG's LNG project in Australia.

Under the contract, CNOOC will purchase five million metric tons of LNG annually from BG Group, starting from 2015 for 20 years.

The Chinese company will own mid- and long-term LNG contracted volumes of about 21.6 million tons annually through the deal, according to the company.

CNOOC will acquire a 40 percent equity interest in the Queensland Curtis LNG project, increasing its equity stake from 10 percent to 50 percent.

It will acquire 20 percent more interest in BG's reserves and resources in the Walloons Fairway region of the Surat Basin in Queensland, rising to 25 percent, and increase its interest shares in BG's upstream blocks in Surat and Bowen Basins in Queensland from 5 percent to 25 percent.

In addition, CNOOC and BG Group will jointly invest in building two LNG cargo ships in China.

As one of the top three oil and gas giants in China, CNOOC has been exploring overseas resources and markets as a key strategy. Its LNG business has developed fast in recent years.

The company imported 11 million tons of LNG in 2012.

It has four LNG receiving terminals in Guangdong, Fujian, Zhejiang and Shanghai at present and more are under construction in Zhuhai, Hainan and Shenzhen.