The reserve is located at CNOOC's deepwater Kaiping South oilfield in the Pearl River Delta near Guangdong province, and contains light crude, the statement said.
CNOOC has invested heavily in developing China's offshore oil and gas reserves as part of a broader push to offset declining output from aging onshore fields.
The company lifted its 2024 production target by about 8% to a record 700 million to 720 million barrels of oil equivalent. Domestic production, much of this offshore reserves in eastern China's Bohai Sea and the South China Sea.