Aerial photo shows a sunset view of Daqing Oilfield in Daqing, northeast China's Heilongjiang Province, May 24, 2021. (Xinhua/Wang Jianwei)

Daqing Oilfield, China's largest onshore oilfield, has produced 300 million tonnes of crude oil via the tertiary oil recovery technology, according to Daqing Oilfield, a subsidiary of China National Petroleum Corporation.

So far, the annual crude oil output via tertiary recovery at Daqing Oilfield has exceeded 10 million tonnes for 21 consecutive years.

Tertiary recovery, also known as enhanced oil recovery, is the third stage of extracting oil from an oil reserve. Through the injection of chemicals, the method exploits the oil that cannot be collected in the primary and secondary recovery stages.

At present, tertiary oil recovery technology is widely used in Class I and Class II reservoirs in the Daqing Oilfield, and the recovery ratio is increased by 14 to 20 percent on the basis of the secondary oil recovery technology.

The technology is expected to increase China's recoverable crude oil reserves by more than 1 billion tonnes, said the company.

Daqing Oilfield is one of China's largest oil production bases. Located in the northeastern province of Heilongjiang, the oilfield has made a significant contribution to China's modern petroleum industry.

It has produced 2.5 billion tonnes of crude oil since it was discovered in 1959. The oilfield accounts for about 40 percent of China's total domestic onshore crude oil production during the same period.