Abstract <p>The relationships of Ne isotopic composition with Ne and N₂ partial pressures and N₂/Ne ratio in gas accumulations was analyzed for the first time. The influence of groundwater on the contents and isotopic compositions of noble gases in CO₂, N₂, and CH₄ gas reservoirs was also evaluated. It was shown that Ne partial pressure in different gas accumulations increases in the sequence CO₂–CH₄–N₂. Neon with the mantle isotope signature is typical of CO₂ accumulations, whereas N₂ accumulations are dominated by a mixture of atmospheric and crustal Ne (²⁰Ne/²²Ne is 8.9–10.2, and ²¹Ne/²²Ne is 0.040–0.083). The initial concentration and isotopic composition of Ne and certainly other noble gases depend on the formation conditions of the gas phase that enters the reservoir. Subsequently, Ne partial pressure increases in the CO₂ and CH₄ gas accumulations but decreases in the N₂ gas accumulations, supposedly owing to gas exchange with groundwater. We proposed a model for gas exchange between a gas reservoir and groundwater, which allowed us to estimate the distribution of noble gases between them. (1) CO₂ entering reservoirs has a negligible partial pressure of mantle Ne (∼0.008 Pa). Therefore, the fraction of Ne from groundwater in them may be as high as 90%. (2) In contrast, N₂ reservoirs have the atmospheric Ne isotopic signature with an admixture of a nucleogenic (crustal) component and Ne partial pressure higher than in the atmosphere. The N₂ gas phase was most likely derived from ancient Ca–Cl brines under a formation pressure of less than 300 bar. (3) The isotopic composition of Ne from CH₄ reservoirs is consistent with the assumption that microbial CH₄ is formed under near-surface conditions (atmospheric gases dominate), and thermogenic CH₄ is formed at greater depths. Neon and N₂ in thermogenic CH₄ was probably produced by gas exchange with Ca–Cl brines. In general, the isotopic composition of Ne in CH₄ reservoirs reflects the mixing of microbial and thermogenic CH₄ during migration to the surface.</p>