CNOOC announced it encountered an oil pay in its Luda 21-2 prospect with a thickness of about 550 feet, which it said was one of the thickest oil layers ever encountered in the area.
Zhu Weilin, general manager for exploration at CNOOC, said in a statement the prospect has "fairly good economics in exploration and development."
The prospect is located northeast of Bohai Bay, which is estimated to hold as much as 146 billion barrels of oil.
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.