China’s China National Offshore Oil Corp (CNOOC) said it had tapped country’s first ultra-deepwater natural gas discovery in the northwestern part of the South China Sea. An “ultra-deep” well is categorized as one that is located more than 1,500 meters under the sea.

The state-owned offshore oil and gas explorer started to drill the Lingshui 18-1-1 exploration well last October in water depths of 1,688 meters and a test of the well in December was successful, the company said in a report published on its website, RigZone reported. CNOOC deployed flagship deepwater rig, the Haiyang Shiyou 981, to drill the well. The company’s report did not provide an estimate of gas flows at the Lingshui 18-1-1 well. However, it did say that the Ministry of Land & Resources certified the discovery.

In addition, the rig that is being used to drill the well has a controversial history. According to OilPro it is the same rig that was involved in a May 2014 dispute with Vietnam, when the latter condemned the operation of Haiyang Shiyou 981 in what Vietnam claimed was its territorial water in the South China Sea as illegal. In 2015, China moved the Haiyang Shiyou 981 closer to the coast of Vietnam in the disputed South China Sea.