CNOOC Ltd. confirmed a 2010 discovery, Penglai 9-1, or PL9-1, was a large oil field. One appraisal well in the structure encountered more than 650 feet of oil pay and tested at around 700 barrels of oil per day.
"I believe the successful appraisal on PL9-1 will open up a new prospect for exploration and development in Eastern Bohai waters as well as make great contribution to maintain steady production in Bohai region," CNOOC Chief Executive Officer Li Fanrong said in a statement.
A discovery made 5 miles south of the structure measures 270 feet thick and yielded more than 1,200 bpd.
CNOOC said the results confirm the structure is the largest oilfield among recent discoveries in Bohai.
ConocoPhillips said last year it observed "several small seeps" from a production platform in Bohai Bay associated with a June 17 release. Conoco said it had about 13,000 cubic feet of oil-based drilling mud cleaned up, which it said represented about 90 percent of the total volume released.
The company in January reached an agreement, along with CNOOC, to pay around $160 million in compensation to settle public and private claims related to the June incident.