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The geochemical statistics indicate that the wetness (C2~C5/C1~C5) of over-mature shale gas with carbon isotope reversal is less than 1.8%. The magnitude of carbon isotope reversal (δ13C1–δ13C2) increases with decreasing wetness within a wetness range of 0.9~1.8% and then decreases at wetness <0.9%. The experimental result demonstrates that CH4 polymerization proceeding to CH4 substantial cracking is an important factor involved in isotope reversal of over-mature shale gas. Moreover, δ13C1–δ13C2 decreases with an increase in experimental temperature prior to CH4 substantial cracking. The values of δ13C1 and δ13C2 tend to equalize during CH4 substantial cracking. The δ13C1–δ13C2 of mud gas investigated at different depths during shale gas drilling in the Sichuan Basin increases initially, then decreases with further increase in the depth, and finally tends to zero, with only a trace hydrocarbon gas being detectable. Thus, the approximately equal value between δ13C1 and δ13C2 for over-mature shale gas and very low wetness could potentially serve as useful criteria to screen CH4 substantial cracking. Two geochemical indices to indicate CH4 substantial cracking in a geological setting are proposed according to the variation production data with the geochemistry of over-mature shale gas in the Sichuan Basin, China. [ABSTRACT FROM AUTHOR]