Various indexes such as CO

2

carbon isotope

3

He/

4

He and CO

2

/

3

He ratios show that CO

2

with high content in the Songliao Basin is mantle-derived gas.Distribution of CO

2

in the Songliao Basin is highly uneven.The content of CO

2

in the Songliao Basin shows a kind of "two-end member" distribution.Vertically

CO

2

are mainly concentrated in the Yingcheng Formation and the fourth member of the Quantou Formation and their distribution is under the control of regional caprock and reservoir characteristics.While laterally

they are locally distributed in several patchy or narrow-and-long zonal areas

and their distribution is obviously under the control of deep major faults and has relatively close genetic relations with the basement faults and volcanic rock bodies.Conventional hydrocarbon gas and mantle-derived CO

2

occur in hybrid

as both of them are controlled by the major basement faults and sharing some trap and reservoir elements.It was pointed out that

to a great extent

the combination type of pathways for mantle-derived CO

2

migration and accumulation determined the relative content of CO

2

and hydrocarbon gas in volcanic reservoirs and accumulating horizons of CO

2

-bearing natural gas.