Abstract: Based on 283 sets of carbon isotopic data from typical great-medium gas fields in China,the elementary characteristics of gas alkyl hydrocarbon in the great-medium coal formed gas fields of China are discussed by contrasting coal formed gas with oil type gas.The results reveal that the carbon isotope of gas alkyl hydrocarbon has following characteristics:with the increase of carbon number,the δ¹³C frequency range of methane and its homologues reduces,and the main frequency value gets heavier;④compared with oil type gas,coal formed gas has lower δ¹³C₁ main frequency peak values and higher δ¹³C₂ values;㈣the correlation betweenΔ(δ¹³C₂-δ¹³C₁)and δ¹³C₁ is negative;there are only two types of carbon isotope series for coal formed gas,that is,positive and reversed carbon isotope series;the characteristics of carbon isotope series for gas alkyl hydrocarbon depend on microcomponents,evolutionary degree and pool forming mechanism.