One of China's national oil companies has announced its first gas field discovery in the South China Sea, about 150 kilometers south of the country's southernmost island of Hainan.

The find, announced in state media Tuesday, is the first by a $1 billion deep sea exploration rig that China National Offshore Oil Corp. deployed two years ago.

The rig was at center of a tense marine standoff with Vietnam in May when Beijing allowed it to be towed inside Vietnam's exclusive economic zone.

The discovery by CNOOC was made about a month after the rig withdrew in July to far less-contested waters closer to China.

It remains to be seen whether the field will be commercially viable.