The H₂ generation mechanisms of natural gas in the Sulige gas field, Ordos Basin, China

Due to its renewability, zero-emissions, and low production cost, natural hydrogen (H₂) holds considerable potential as a carbon-free energy resource and represents a key focus for enabling energy transition and achieving carbon neutrality. The generation mechanisms and accumulation patterns of H₂ need further investigation, particularly with regard to the sources of H₂ in sedimentary basin. This knowledge gap hinders the exploration and development of H₂ resources. The study reports the concentrations and isotopic compositions of H₂ and hydrocarbons of natural gas in the Sulige gas field. The results suggest that the H₂ content in natural gas can reach up to 22.98%, with H₂ isotope values (δ²H-H₂) ranging from −809‰ to −700‰. Based on comprehensive analysis of carbon and hydrogen isotopes of hydrocarbons, geological conditions, and hydrogen isotope fractionation mechanisms of H₂, this study reveals that the H₂ in the Upper Paleozoic natural gas is likely derived primarily from organic matter pyrolysis in coal-bearing source rocks, while the H₂ in the Lower Paleozoic natural gas is probably generated mainly through water radiolytic in basement granite and metamorphic rocks. The diffusion fractionation model demonstrates that significant isotopic fractionation occurs during the migration of deep-sourced H₂ to sedimentary reservoirs, resulting in notably depleted δ²H-H₂ values in the Lower Paleozoic natural gas. The H₂ generation through organic matter pyrolysis primarily occurs during the late gas generation stage, with peak production concentrated in the Late Triassic to Early Cretaceous periods. Given the genetic correlation between H₂ and hydrocarbons, the H₂ may accumulate with natural gas in reservoirs. In contrast, H₂ generation through water radiolysis in the study area exhibits multi-source characteristics and prolonged duration, demonstrating significant potential for independent accumulation. This study not only elucidates the generation mechanisms of H₂ but also provides a significant geological case study for understanding the distribution characteristics and resource potential of H₂ in sedimentary basins.