China's first self-designed deep-sea exploration vessel returned to port on Sunday after completing a deep-sea geological examination in the Pacific.

The Ocean No. 6 returned to port in south China's Guangdong Province after spending 138 days conducting deep-sea surveys over about 30,000 km, according to Yang Shengxiong, chief engineer with Guangzhou Marine Geological Survey.

The surveys were conducted in areas with cobalt-bearing crusts in West Pacific. It found out about the conditions of the crusts in more than 30 areas, Yang said.

"Samples of the crusts more than 30-cm thick were fetched for the first time," Yang said.

The vessel also realized scale collection of cobalt-bearing crusts, while obtaining samples of microplastics along the sea routes in West Pacific. It also analyzed the amount, types and composition of microplastics in the sea, which laid a foundation for the international treatment of plastic waste in the sea.

It is the eighth deep-sea expedition of the vessel, which has conducted multiple tasks in the South China Sea, the Pacific and the waters around the Antarctic since 2009.