Unique chemical and isotopic characteristics and origins of natural gases in the Paleozoic marine formations in the Sichuan Basin, SW China: Isotope fractionation of deep and high mature carbonate reservoir gases

Abstract

Large quantities of natural gases have recently been discovered in the marine carbonate formations in the Sichuan Basin, southern China. The origins and isotopic compositions of the gases are poorly understood and highly controversial. We examined the chemical and isotopic compositions of the carbonate reservoir gases in marine formations from the Sinian to Lower Triassic as well as shale gas from the Silurian and Cambrian Formation in the Sichuan Basin. It is found that the carbon isotopes of the carbonate reservoir gases had been evolved through three stages corresponding to pre-rollover, rollover and post-rollover in the plot of δ13C2 versus wetness, which is consistent with that found in shale gas by Tilley and Muehlenbachs (2013). The unique carbon isotopic compositions indicate that both kerogen and oil-cracking gas have contributed to the accumulation of the carbonate reservoir gases in the Sichuan Basin. A mixing of two thermogenic gases with different maturities can cause the δ13C reversal between methane and ethane, which is commonly observed in the carbonate reservoir gases in the Carboniferous, Permian and Triassic formations, and the shale gas in the Silurian and Cambrian formations in the Sichuan Basin. We established two mixing models for quantitatively determining the contribution of each type gas for the natural gases in different formations. Kinetic extrapolation based on thermal history of the Anyue gas field demonstrates that ethane cracking indeed occurred in the deep reservoir formations, which may explain the disappearance of the 13C reversal for methane and ethane in the carbonate reservoir gases in the post-rollover zone. It is proposed that organic-inorganic interactions may be responsible for the presence of D isotope rollover for methane in carbonate reservoir gases with ultra-high maturity. The organic-inorganic interactions include thermal reactions between water and residual organic matters or carbons and hydrogen isotope exchange between water and methane, and their mechanism needs further investigations in future.