A drilling operation by the HYSY 981 rig has been completed off a Xisha island in the South China Sea, China's Foreign Ministry confirmed on Wednesday.

Quoting China National Petroleum Corp. (CNPC) and China Oilfield Services Limited, ministry spokesman Hong Lei said the Chinese companies began the drilling operation off Zhongjian Island, one of the Xisha Islands, on May 2. Work was completed on July 15 as scheduled.

Hong said the companies will analyze and assess the geological data collected and decide on the next step.

The spokesman also noted the Xisha Islands are inherent territory of China and that the operation was conducted in indisputable inshore waters off Xisha, which completely falls within China's sovereignty, sovereign rights and jurisdiction.

"China strongly opposes Vietnam's irrational disruptions and has taken necessary security measures to ensure the operation," Hong added.

CNPC started drilling two wells in May and completed the work on May 27 and Tuesday respectively. The company has been exploring the area since 2004.

The drilling rig provided by China Oilfield Services Ltd. will be relocated to operations in the Hainan Islands.

Qiu Zhongjian, a geologist from the Chinese Academy of Engineering, said that the drilling was well organized, as the CNPC fully considered geological hazards, engineering issues and possible typhoons.

Preliminary analysis of the geological data acquired has shown that the area, or Zhongjian Sag as it is called, has the basic conditions and potential for oil exploration, but extraction testing cannot begin before a comprehensive assessment of the data, said Wang Zhen, deputy director of CNPC Policy Research Office.

For safety reasons, the test operation was not arranged immediately, because July is the beginning of the typhoon season.

While the project may help improve understanding of the engineering and geological issues in the South China Sea, the development of "deep-sea drilling technology" will also benefit, said Wang.