Petroleum systems of the oldest gas field in China: Neoproterozoic gas pools in the Weiyuan gas field, Sichuan Basin

Weiyuan, the oldest gas field in China, is a rare example of the giant Neoproterozoic gas pools. This field is located in southwestern Sichuan Basin, with a proven in place gas reserve around 40 billion cubic meters (bcm) or 1.41 trillion cubic feet (tcf) in reservoirs of the Sinian Dengying Formation. This field is almost depleted now, with a cumulative gas production of 14.4 bcm and a recovery factor of around 36%. The Neoproterozoic gas pools are also geographically overlapped by gas pools in the Lower Permian Maokou Formation, with an additional proven in place gas reserve about 0.861 bcm or 30.2 bcf. Gases in the Sinian reservoirs are relatively rich in nitrogen, CO2, H2S, helium and argon, with gaseous alkanes showing delta C-13(1)