China's offshore oilfield cluster Bohai, run by state-run CNOOC, has become the country's largest crude oil producer with output hitting 30.132 million tonnes (602,640 barrels per day) in 2021, state news agency Xinhua reported.
Bohai field, off north China, overtook the country's flagship onshore producer Daqing, in northeast China, which pumped 30 million tonnes last year as reported by state media a week earlier. 
CNOOC Ltd, the listed vehicle of China National Offshore Oil Company, produced 48.64 million tonnes of crude oil last year, up by 3.23 million tonnes, which accounted for up 80 per cent of the national increment in crude oil production. 
Developed in 1965, the Bohai cluster fields are considered marginal assets with relatively high development cost and poor crude oil quality. 
But CNOOC has in the past two decades sharpened its exploration and development know-how, such as shortening the average drilling time to under 10 days from 57 days, and has made several major discoveries like Kenli 10-2 and Bozhong 19-6 fields.
In response to Beijing's call to boost energy supply security, national producers - CNOOC, PetroChina and Sinopec - have in the past several years accelerated drilling more challenging terrains at home, including shale oil, to make up for depleting mature fields like Daqing. 
China, the world's second-largest oil consumer which imports three quarters its oil needs, eked out a 2.5 per cent increase in domestic crude output in the first 11 months of 2021 over a year earlier, official data showed. --Reuters