| Summary: | The geochemical statistics indicate that the wetness (C<sub>2</sub>~C<sub>5</sub>/C<sub>1</sub>~C<sub>5</sub>) of over-mature shale gas with carbon isotope reversal is less than 1.8%. The magnitude of carbon isotope reversal (δ<sup>13</sup>C<sub>1</sub>–δ<sup>13</sup>C<sub>2</sub>) 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 CH<sub>4</sub> polymerization proceeding to CH<sub>4</sub> substantial cracking is an important factor involved in isotope reversal of over-mature shale gas. Moreover, δ<sup>13</sup>C<sub>1</sub>–δ<sup>13</sup>C<sub>2</sub> decreases with an increase in experimental temperature prior to CH<sub>4</sub> substantial cracking. The values of δ<sup>13</sup>C<sub>1</sub> and δ<sup>13</sup>C<sub>2</sub> tend to equalize during CH<sub>4</sub> substantial cracking. The δ<sup>13</sup>C<sub>1</sub>–δ<sup>13</sup>C<sub>2</sub> 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 δ<sup>13</sup>C<sub>1</sub> and δ<sup>13</sup>C<sub>2</sub> for over-mature shale gas and very low wetness could potentially serve as useful criteria to screen CH<sub>4</sub> substantial cracking. Two geochemical indices to indicate CH<sub>4</sub> 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.
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