Effect of Methane Cracking on Carbon Isotope Reversal and the Production of Over-Mature Shale Gas

The geochemical statistics indicate that the wetness (C-2 similar to C-5/C-1 similar to C-5) of over-mature shale gas with carbon isotope reversal is less than 1.8%. The magnitude of carbon isotope reversal (delta C-13(1)-delta C-13(2)) increases with decreasing wetness within a wetness range of 0.9 similar to 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, delta C-13(1)-delta C-13(2) decreases with an increase in experimental temperature prior to CH4 substantial cracking. The values of delta C-13(1) and delta C-13(2) tend to equalize during CH4 substantial cracking. The delta C-13(1)-delta C-13(2) 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 delta C-13(1) and delta C-13(2) 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.

Automated summary

The geochemical statistics suggest that the wetness of over-mature shale gas is related to carbon isotope reversal, and the substantial cracking of CH4 plays a crucial role in this process. The experimental results also show that temperature affects the delta C-13(1)-delta C-13(2) values. Furthermore, two geochemical indices have been proposed to indicate CH4 substantial cracking based on the study of over-mature shale gas in the Sichuan Basin.