Noble gas isotopic characteristics and helium dilution of coalbed methane from the third coal seam in southern Qinshui Basin

Helium is a crucial strategic resource with very limited reserves, but its enrichment and dilution mechanisms in gas reservoirs remain unclear. Noble gas isotopes play an important role in characterizing the interactions between gas and groundwater. In this study, noble gas compositions and isotopic signatures of coalbed methane (CBM) from the third coal seam in the southern Qinshui Basin were analyzed to determine the isotope composition characteristics of noble gas and to establish a helium reservoir formation model. Gas samples were collected from 13 CBM production wells. The results showed that the helium (He) content in CBM was generally one order of magnitude higher than in the atmosphere. The 3He/4He ratios were 0.002 9-0.021 8 Ra, with a very low mantle source contribution (0-0.31%). The 20Ne/22Ne ratios (10.09-10.43) and 21Ne/22Ne ratios (0.029 6-0.031 9) were slightly higher than those in the atmosphere, reflecting an excess of 21Ne relative to the atmosphere. The 40Ar/36Ar ratios (295.23-779.44) were overall higher than the atmospheric values, suggesting a significant influence of crustal 40Ar accumulation over time. The isotopic signatures of krypton (Kr) and xenon (Xe) were similar to those of the atmosphere. Quantitative calculations of helium production revealed an external 4He flux into the self-generating and self-preserving CBM system. The linear relationship between 4He and 20Ne indicated that helium dissolved in groundwater before degassing into the gas reservoir, while methane desorbed from coal seams diluted helium (as well as neon and argon) in the groundwater-associated gases. Therefore, gas reservoirs with lower grades were more likely to accumulate helium. Helium was mainly distributed in areas with effective helium source rocks, ancient groundwater systems, efficient migration channels, and appropriate hydrocarbon generation intensity, providing a theoretical basis for exploring helium resources in CBM. Rayleigh fractionation, dilution modelling, and gas production quantification showed that the water output per well during gas production was 8.03×103-1.63×106 m3. CBM exploration affected only the local water around each well, offering a basis for optimizing well spacing design.