Abstract: In order to determine the carbon isotope characteristics and geological significance of shale gas in Sichuan Basin, shale gas samples in Fuling, Changning, Pengshui and Weiyuan were analyzed with gas spectrometer and other analytical methods. As found in the results, the shale gas in the study area was mainly composed of methane with a content ranging from 96.100% to 99.400%, and mixed with a small amount of ethane and propane; the non-hydrocarbon gases were dominated by N₂ and CO₂, and traces of helium from 0.01% to 0.03% were present; the carbon isotope of CO₂ was -12.5‰ to 8.9‰, indicating that CO₂ was sourced from organic and inorganic genesis. At present, carbon isotopic inversion sequence (δ¹³C₃ <δ¹³C₂ <δ¹³C₁) has been found in shale gas fields in Sichuan Basin, which is mainly caused by mixing of hydrocarbon gases from different sources, reaction between gases and minerals, and fractional distillation in alkane desorption/diffusion. The degree of carbon isotope inversion in shale gas is bound up with the confinement of the shale strata, as the self-confinement of the shale strata directly affects the degree of hydrocarbon expulsion and gas diffusion. In addition, there is a positive correlation between shale gas production and inversion degree, indicating that the carbon isotope characteristics of shale gas are important indicators for shale gas production prediction and preservation conditions and enrichment pattern evaluation. The results of the study are of guidance for the further exploration of marine shale gas in Sichuan Basin.

Key words: shale gas, carbon isotope inversion, marine facies, enrichment pattern, Rayleigh fractionation, Sichuan Basin