Abstract: Purpose of this study is to reveal the variation characteristics of kerogen during thermal evolution by determining the residual rate of kerogen and the carbon isotopic composition of residual kerogen at (different thermal) (evolution) stages, thus providing evidence of carbon isotope for quantitative identification and assessment of effective gas source rock. The result indicates that, at the early stage of methane (generation) (R_o1.5%), kerogen (obviously) enriches in ¹³C with the increasing of thermal evolution, up to 3.8‰. However, when R_o reaches 1.5%-2%, δ¹³C values of residual kerogen display a trend of slight depletion in ¹³C, varying within about 2‰. When the thermal maturity is higher (R_o2%), the variation of carbon isotopic composition is less than 0.8‰ for residual kerogen.