China's biggest oil firm, PetroChina, says it has made the country's largest crude discovery in a decade.
China is the world's second largest oil consumer
PetroChina said the find off China's north eastern coast had proven reserves of almost three billion barrels.
Western analysts said that while the field, called Jidong Nanpu, would probably produce less than a quarter of that amount, it was still a major find.
China, the world's second-biggest oil consumer, has been seeing supplies slow at its existing domestic wells.
Energy consultant Andrew Hayman of IHS said that under Western standards, Jidong Nanpu would probably have nearer to 650 million barrels of commercially recoverable reserves.
"This field obviously will be a major asset to the Chinese economy given its proximity to domestic markets," he said.
"And since it is located in [just] three metres of water the costs of development and exploitation will be relatively cheap."
PetroChina is 90% owned by the state-run China National Petroleum Corporation.