Geochemical study on genesis of natural gases accumulated in deep volcaniclastic rocks.

Abstract

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.