Formation mechanism of deep natural hydrogen in the Sichuan Basin

As a potential renewable and clean energy source, the distribution and exploration prospects of natural hydrogen have garnered widespread attention. However, the limitations in understanding the formation mechanisms and accumulation patterns of underground natural hydrogen have significantly constrained its efficient exploration and utilization. This study reports H2 content and isotopes in natural gas from deep formations in sedimentary basins through geochemical analysis of 58 conventional gas samples from the Sinian to Triassic strata in the central Sichuan Basin. We discovered that H2 content in deep natural gas in central Sichuan generally exceeds 0.1%, with a maximum of 1.6%, and hydrogen isotopes(2HH2) range from-850‰ to-586‰. Analysis of the geological conditions, hydrogen generation mechanisms, and isotope fractionation characteristics in the study area confirms that microbial fermentation or in-situ organic matter cracking are not effective pathways for deep H2 generation. Combined with helium content and isotope analysis of the natural gas, it is inferred that the volcanic basement in the central Sichuan area is the potential source rock for natural H2 in the Sinian to Triassic strata. Specifically, H2 originates from the radiolysis of water and hydrothermal reactions of Fe（Ⅱ） in the basement granite. Additionally, a diffusion fractionation model reveals significant isotope fractionation during the migration and accumulation of deep H2 into sedimentary reservoirs, which lead to anomalously light 2HH2 in deep natural gas in central Sichuan. This indicates that the underground natural hydrogen accumulation is in a long-term dynamic process, with simultaneous H2 accumulation and dissipation in the gas reservoirs. This research not only elucidates the formation mechanism of deep H2 but also provides insights for the resource potential of natural hydrogen in sedimentary basins.