China's largest oilfield, located in Bohai Bay, has produced more than 30 million tons of crude oil in 2021, contributing to half of the country's crude oil output increase last year, according to its operator China National Offshore Oil Corp, the country's top offshore oil and gas driller.

The annual crude output of Bohai Oilfield is expected to increase by 2 million tons in the next three years and its annual oil and gas output in 2025 is set to surpass 40 million tons, to ensure sufficient oil and gas supply in the country, said the company.

According to CNOOC, its domestic crude output reached 48.64 million tons last year, 3.23 million tons more than that of the previous year and accounting for 80 percent of total national oil growth.

It was also the third consecutive year that offshore oil production accounted for over half of the national oil growth, signaling the exploration offshore for natural resources has become crucial to the country's drive for production, it said.

An analyst said China has been making progress in increasing domestic oil and gas production over the past few years, with significant results in terms of domestic oil and gas exploration as well as unconventional gas, including shale gas.

With the joint efforts of the country's oil majors, including China National Petroleum Corp, China Petroleum and Chemical Corp and CNOOC, China has achieved several major breakthroughs in not only conventional oilfields but also unconventional oil and gas sectors, said Luo Zuoxian, head of intelligence and research at the Sinopec Economics and Development Research Institute.

Major breakthroughs in Daqing Oilfield in China's northeastern Heilongjiang province and Shengli Oilfield in Dongying, East China's Shandong province, have helped ensure the country's energy security, he said.

According to Cao Xinjian, assistant general manager of CNOOC, the company has invested over 50 billion yuan ($7.85 billion) annually over the past three years and the technological breakthroughs for exploiting deep sea resources opened up new opportunities for the industry.

Bohai Oilfield started its development in 1965. Over 4.4 billion tons of crude oil and nearly 500 billion cubic meters of natural gas have been discovered since the field became active, with more than 493 million tons of oil and natural gas having been extracted, it said.

Wang Dongjin, chairman of CNOOC, vowed to continue oil exploration, focusing on increasing capacity and storage of oil and gas, adopting advanced technologies and conducting environmental protection projects, to bolster China's energy security and decrease reliance on foreign energy sources.