Chemical compositions and carbon isotopes of 34 natural gas samples

collected from 13 oil and gas fields in western depression of Qaidam basin

are measured and analyzed. Based on the carbon isotopes of methane

ethane and propane in the gas samples

the natural gas in western depression of Qaidam basin can be divided into 4 different types

including sapropel type oil associated gas

mixed type oil associated gas

coal related (humic) gas and hybrid gas. Most of the gases in western depression are cogenetic and synchronous sapropel type oil associated gas

mixed type oil associated gas and coal related gas. Hybrid gas

consisted of biogas

immature mixed type oil associated gas and low mature mixed type oil associated gas

distribute only in the shallow layers (Neogene) in Gas Hure oilfield in Gas faulted depression

the Yuejin No. 2 west intersection point

Youshashan oilfield and Wunan oilfield in Kunbei fault bench zone.