This paper discusses the kinetic fractionation, composition and distribution characteristics of carbon and hydrogen isotopes for various alkane gases formed in different environments, by different mechanisms and from different sources in nature.It is demonstrated that the biodegradation or thermodegradation of complex high-molecule sedimentary organic material can form microbial gas or thermogenic gas. The δ 13 C 1 value ranges from −110‰ to −50‰ for microbial gases but from −50‰ to −35‰ (even heavier) for thermogenic gases. Controlled by the kinetic isotope fractionation, both microbial and thermogenic gases have δ 13 C and δ D values characterized by normal distribution, i.e. δ 13 C 1 < δ 13 C 2 < δ 13 C 3 < δ 13 C 4 and δ D CH 4 < δ D C 2 H 6 < δ D C 3 H 8 <δ D C 4 H 10 , and by a positive correlation between the δ 13 C and δ D values. Simple carbon-bearing molecules (CH 4 , CO and CO 2 ) can form abiogenic alkane gases via polymerization in the abiological chemical process in nature, with δ 13 C 1 heavier than −30‰.Moreover, controlled by the kinetic isotope fractionation, abiogenic alkane gases are characterized by a reverse distribution of δ 13 C values and a normal trend of δ D values, namely δ 13 C 1 > δ 13 C 2 > δ 13 C 3 > δ 13 C 4 and δ D CH 4 <δ D C 2 H 6 < δ D C 3 H 8 < δ D C 4 H 10 . The δ 13 C values and δ D values are negatively correlated. Natural gases from 26 commercial gas wells distributed in the Xujiaweizi and Yingshan-Miaotaizi faulted depressions in the Songliao Basin, China, show δ 13 C 1 values ranging from −30.5‰ to −16.7‰ with a very narrow δ D range between −203‰-−196‰. These gases are characterized by a reverse distribution of δ 13 C values but a normal distribution of δ D values, and a negative correlation between their δ 13 C and δ D values, indicating an abiological origin. The present study has revealed that abiogenic hydrocarbons not only exist in nature but also can make significant contribution to commercial gas reserviors. It is estimated that the reserve volume of alkane gases with abiogenic characteristics in these 26 gas wells in the Songliao Basin is over 500×10 8 m 3 . The prospecting practice in the Songliao Basin has demonstrated that abiogenic alkane gases are of a promising resource, and it provides an example for the investigation of and search for abiogenic commercial natural gases worldwide. generation mechanism of natural gases, kinetic isotope fractionation, the Songliao Basin of China, commercial abiogenic hydrocarbon gaspool
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