There were violent magma activities which have caused rich and abnormal petroleum with chemical properties in igneous rocks region of rift basins.In order to further study the natural gas origin，we did systematic analysis on geochemical characteristics of gas and source rocks in Gubei area.It is shown that，some source rocks are over-matured，and there were more inorganic CO2 and H2S in the gas with heavy alkane carbon isotopes and significant variable properties.Based on the theory of natural gas origin and isotope fractionation，we thought that the source rocks are subjected not only to normal thermal evolution by burial，but also to frequent heating by magma，which have led to the multistage evolution of organic matter，hydrocarbon generation，and decomposition of carbonate rock.In addition，the interfusion of magma liquid also made the gas compositions more complex.Finally，we indicated that the gas possessed the characteristics of universal mixture and multifold mixtures（that is multi-source gas，multi-stage evolution，multi-times hydrocarbon generation，hydrocarbon and non-hydrocarbon mixture），and the difference of mixing types or mixing proportion led to the variability of the gas chemical compositions of intrusive rocks region.