Shale gas geochemistry in the Sichuan Basin, China

The unique self-generation and self-storage characteristics of shale gas allow for its geochemical characteristics to better reflect the gas generation process inside the original rock under geological conditions. Based on a systematic analysis of 541 typical shale gas components and stable isotope compositions worldwide, this review provides an in-depth elucidation of the geochemical characteristics of Silurian shale gas in the Sichuan Basin, including an explanation of its special isotope anomalies. The over-mature Longmaxi Formation shale gas is dominated by alkane gas components (> 99%), which almost entirely (98.7%) consist of methane (CH4). The distinctive characteristics of high CH4 and low C2+ contents result in extremely low wetness of <0.6%, which is typical of dry gas. In view of the sapropelic kerogen origin, the Longmaxi shale gas belongs to late-mature thermogenic gases (LMT). Geochemical anomalies of Longmaxi shale gas mainly refer to abnormally enriched δ13C1 values and full reversal, resulting in deviations in carbon isotope composition and light hydrocarbon data for shale gas in the Changning-Zhaotong and Fuling areas, unlike the slightly less thermally evolved Weiyuan area. Noble gases of the Longmaxi shale gas exhibit characteristics of crustal origin and marine carbonate, and the light hydrocarbons in Longmaxi shale gas have been seriously cracked and destroyed, and the residual fraction is more reflective of its chemical stability. At extremely high maturity, the Rayleigh fractionation effect of ethane and the involvement of inorganic material in redox reactions are important causes of isotopic anomalies in Longmaxi shale gas. The variation of δ13C1 and δ13C2 values of shale gas in the producing area suggests that fluid-rock interactions related to CO2 may have occurred, but not to a significant degree, and the cumulative effects of diffusion are similarly observed. The δ13C2 values and magnitude (δ13C1 − δ13C2) of Weiyuan shale gas are negatively and positively correlated with daily production, respectively, which proves that the methane in shale gas mainly comes from ethane cracking during the over-maturation stage. By comparing with typical coal-derived gas, new discriminative trend lines in the reversal zone (δ13C1 > δ13C2) have a certain degree of credibility and applicability. Unraveling the geological processes that led to the formation of geochemical anomalies in the Longmaxi shale gas can provide valuable insights for in-depth analysis of the late-stage gas generation.