China's biggest oil discovery in the past decade is a field near Dalian in the northern part of Bohai Bay, run independently by the country's top oil company PetroChina, an industry insider revealed Thursday.

"I know they have taken six drilling vessels there ... What kind of a field needs six vessels to drill at the same time?" said the source - who did not want to be named - implying that it's a gigantic project.

PetroChina, China's top oil and gas producer, recently announced that it discovered a "very rich" oilfield at Bohai Bay. With an initial daily output of 500 tons, the field is "the largest finding in China in the past 10 years", according to Jiang Jiemin, vice-chairman and president of PetroChina.

He did not reveal the total reserves, saying only that a detailed announcement would be made in the first half of this year.

This "very rich" oilfield, as the insider said, is near Dalian, a coastal city in Northeast China, and is located in shallow waters, about five meters below the sea level.

"As a traditional onshore oil powerhouse, it is natural for PetroChina to explore near the shoreline," the source said.

Before the discovery of the new Bohai oilfield, China's largest fossil-fuel reserve find in 10 years was a 400-million-ton oilfield in Northwest China, also operated by PetroChina.

Han Xuegong, a veteran consultant with China National Petroleum Company (CNPC), PetroChina's parent firm, said: "I don't have a clue as to where the new field is located. But based on the information available, the discovery was very likely made by PetroChina's new offshore drilling branch," Han said.

To tap the potentially ample reserves near the coastline, PetroChina established an offshore exploration company by transferring personnel and resources from its Dagang and Liaohe oilfields around the Bohai Bay area, Han said.

The discovery is a result of the efforts by Chinese oil companies focusing on local block exploration and production, said the CNPC consultant.

"Major oil firms in China follow a consistent strategy give priority to domestic oil and gas exploration and production. That's why they come up with such rich strikes like the new Bohai field," Han said.

Another Chinese energy giant, Sinopec, Asia's largest refiner, is reported to have discovered a huge gas field in Southwest China's Sichuan Province.

Sinopec is also tight-lipped about its latest discovery. According to local media reports, the reserves of this field very likely exceeds that of Puguang, a gas field also in Sichuan Province. Puguang's proven reserves, according to the Xinhua News Agency, is over 500 billion cubic meters.