The leak occurred at the Suizhong 36-1 oilfield at 1:30 am on Tuesday due to a malfunction at the central control system, China National Offshore Oil Corp Ltd (CNOOC), the field's operator, said in a statement.
Technicians managed to seal the leak, which covered one square kilometer. Mats and chemicals were used to disperse the sheen which was likely to be cleaned up by Tuesday evening, the company said.
Production at the oilfield was suspended and emergency response procedures were activated immediately after the incident, CNOOC said.
The cause of the leak, a control system malfunction, had been repaired, it said.
Suizhong 36-1 is the country's biggest independent oilfield, annually producing about 5 million tons.
CNOOC's Tianjin branch is responsible for daily operations.
The State Oceanic Administration (SOA) said in a statement on Tuesday that it sent teams to monitor the situation after being alerted by CNOOC and dispatched a helicopter and used satellite remote sensing to monitor the spill.
The incident occurred on the heels of two other spills, in June, at the Penglai 19-3 oilfield in Bohai Bay, which polluted more than 840 sq km of coastal waters.
A cleanup operation is under way, according to ConocoPhillips China, the field's operator.
The US energy company holds a 49 percent stake in the project, while CNOOC holds the rest.
The two companies and the SOA faced widespread public criticism for covering up the Penglai spill for almost a month.
Both CNOOC and the SOA responded quickly to the latest incident on Tuesday.
The SOA said it will improve monitoring of offshore oil and gas activities and release information "accurately and promptly". Read More
Why China Kept Seoul in the Dark Over Oil Spill
Beijing failed to inform Korea, Japan and Russia of a massive oil spill last month in Bohai Bay because it technically lies outside the jurisdiction of a cooperation agreement among the four countries.
The Action Plan for the Protection, Management and Development of the Marine and Coastal Environment of the Northwest Pacific Region (NOWPAP) was signed in 1994.
Supervised by the UN Environment Programme, NOWPAP aims to respond to pollution in the East and West Seas shared by the four countries and protect the marine ecosystems there. Read More