PetroChina and Chevron began production at an onshore natural gas field in China's southwest at the end of 2015, PetroChina's parent said on Jan. 11, after years of delays.

The Luojiazhai Field, where the ‘A’ well began operating on Dec. 30, has an annual production capacity of 3 billion cubic meters (Bcm) of gas, the state-controlled China National Petroleum Corp. said in a statement on its website.

Luojiazhai is part of the $6.4 billion Chuanbeidong project, a tricky sour gas development in the Sichuan Basin that contains a high level of hydrogen sulphide, which made for higher operational risk and higher standards for technical processes.

Chevron, an experienced sour gas developer, is the operator of the project, and holds a 49 percent stake. PetroChina holds the rest.

PetroChina initially expected first gas to be delivered in 2010, after a deal was signed with Chevron in 2007.

The project, a 2,000-sq-km block, has proven reserves of 176 Bcm, the two companies have said.