A comparative analysis of components and isotopic compositions of gas inclusion in volcanic reservoirs of the Xujiaweizi fault depression before and after thermal decrepitation was performed by using the vacuum mechanical crushing method.The origin of natural gas was further discussed in combination with the corresponding characteristics of natural gas produced from wells and the results of thermal simulating test.The hydrocarbon content of gas from magma degassing is high

indicating that some hydrocarbon gases may be of inorganic origin

thus it is necessary to further research on the differences between gas from volcanic action and that from magmatic action.The main cause of reversal carbon isotopic sequence of hydrocarbon gas in the study area is the mixing of natural gases generated in different times or from different sources

rather than the component and isotopic fractionation during dissipation of natural gas through the caprock.Therefore

the reversal phenomenon can not be used individually to confirm the genesis of natural gas.In comparison with organic gas

inorganic gas(including CH

4

C

2

H

6

CO

2

)often has a heavier carbon isotopic value.It is applicable to some extent to use the the δ

13

C

1

value over -20‰ as the criteria for determining inorganic gas

but this criteria is over strict.The CO

2

in Xujiaweizi fault depression is mainly sourced from magma degassing.The inorganic CO

2

did not experience primary migration and had no chance to contact with formation water

being favorable for their accumulation.These understandings are helpful to determine the origin of natural gas in Xujiaweizi fault depression.