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In order to study the gas generation process of different coaly source rocks, thermal simulation experiments were performed on coal, dark mudstone, and carbonaceous mudstone. Additionally, a series of geochemical analysis including chemical composition, carbon and hydrocarbon isotope composition, as well as light hydrocarbon analysis on thermal simulation gas were also carried out in this study. It revealed that coal has the highest gas production rate; in addition, it yields a large amount of CO2 gas in the early stage of the simulation experiment. The carbon and hydrocarbon isotope of the coal thermal simulation gas is slightly heavier than that of coaly mudstone and carbon mudstone gas. Prior to reaching 500 °C, the carbon and hydrogen isotopes of these three kinds of gas became lighter and higher with the increase of temperature, but after 500 °C, they became lighter all throughout. Carbon isotope series reversal is observed in this process. The coal thermal simulation gas has the highest content of benzene, toluene, and 2,3-methylpentane. This study specified three highlights on the Sulige gas field: (1) CO2, formed in the early stage of hydrocarbon generation, may have a direct relationship with the reservoir densification in the Sulige gas field. (2) According to the analysis of light hydrocarbon, coal should be the dominant contributor to the accumulation of the Sulige natural gas. However, the contribution of coaly mudstone and carbon mudstone should not be ignored. Keywords: Different coaly source rocks, Carbon isotope, Hydrogen isotope, Light hydrocarbon, Sulige gas field