The temperature or maturity limit of methane (CH4) cracking is very useful for the determination of the most depth or the highest maturity in natural gas exploration owing to the composition of over mature gas. In this work, three series of CH4 cracking experiments were conducted under different conditions of N2 + CH4, N2 + CH4 + montmorillonite and N2 + CH4 + shale, respectively, in a gold tube system. The experimental results show that some heavy gas with negative carbon isotope composition could be generated in the three series experiments and that shale has more intense catalysis for CH4 cracking than montmorillonite. The catalysis of metal elements distributed in the minerals of shale is attributed to CH4 cracking acceleration. The shale catalysis makes the maturity threshold of CH4 substantial cracking decrease from 6.0%Ro under no catalysis, to 4.5%Ro under a shale system in a geological setting. Nevertheless, we suggest not to lightly practice natural gas exploration in shale with the maturity range of 3.5−4.5%Ro, as the maturity threshold of gas generation from oil prone organic matter distributed extensively in shale is 3.5%Ro.