The Huizhou 19-6 oilfield lies about 170 kilometres (100 miles) from Shenzhen in south China's Guangdong Province, Beijing's Xinhua news agency said.
Test drilling has yielded a daily production of 413 barrels of crude oil and 68,000 cubic meters of natural gas, the CNOOC said.
The state-owned oil giant's CEO, Zhou Xinhuai, hailed "successive breakthroughs in oil and gas exploration in the eastern waters of the South China Sea".
CNOOC "has discovered 100-million-ton oilfields for two consecutive years, which creates a new growth pole for the continuous growth in offshore oil and gas production," he added.
The US Energy Information Administration says the South China Sea is mostly underexplored because of territorial disputes, but most discovered oil and gas are in uncontested areas.
China claims almost all of the South China Sea as its own, but this is disputed by the Philippines, Malaysia, Vietnam, Indonesia and Brunei.
CNOOC Chief Geologist Xu Changgui said the discovery was a "major breakthrough".
"The discovery proved the largest clastic whole oil field in the northern part of the South China Sea in terms of geological reserves, breaking through traditional theoretical understanding," he said.
China remains the world's largest importer of crude oil, receiving 11.1 million barrels a day last year, according to US government analysis.
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