We used bulk rock (BR), solvent extracted BR, isolated kerogen, and bitumen from a mature Salgan Shale as reactants during gold-tube simulation experiments to investigate the contribution of insoluble kerogen to late-stage gas generation at high thermal stress. We measured the gas yields and stable carbon isotopic compositions of methane, ethane, and propane. The high methane yield ( methane yield at 600°C) in BR series demonstrated the huge potential for late methane generation. The maximum methane yield (normalized to total organic compound) in the bitumen series is twice more than that in the BR series. This phenomenon is interpreted by the high availabilities of hydrogen. However, bitumen only takes up a very small proportion of the overall BR, and generated gas from bitumen cracking is limited. Thus, in mature Salgan Shale, kerogen cracking contributes more hydrocarbon gases than that from bitumen cracking. Meanwhile, generated gas from kerogen cracking is very dry and enriched in the isotope, whereas gas cracked from retained oil is much wetter and associated with much lighter values. Mature kerogen cracking is also helpful to enlarge the shale gas reserve at high maturities especially when the retained oil is limited.