Chemical and isotopic analyses were applied to characterize genesis of natural gases commercially produced in north-east Japan, so called “Green Tuff” region. Concentrations of He and N₂, isotopic ratios of ³He/⁴He and ⁴He/²⁰Ne, δ¹³C values of CH₄ showed positive correlations with stratigraphy of gas reservoirs. Especially isotopic values of He included in the deepest gases from volcaniclastic rocks in Nanatani and Nishikurosawa Formations were almost equivalent to those of volcanic fumaroles in the Japanese Islands, suggesting high contribution of mantle materials.
On the other hand the genetic diagram of hydrocarbon compositions versus δ¹³C values of methane attributed hydrocarbon compounds exactly to pyrolysis products of organic matters in sedimentary rocks regardless of their reservoir rock types.
No more than 1% of methane seems to be derived from upper mantle, whose contribution may be estimated by comparing CH₄/³He values of Green Tuff gases with those of hydrothermal fluids according to studies of EPR.