地球化学からみた天然ガスの成因と熟成度

抄録

Origin and maturity of natural gases are interpreted by the molecular and isotope data. Based on the carbon and helium isotope data, no indication of abiogenic gases has been found in commercial natural gases. Biogenic gases are classified into microbial and thermogenic gases by molecular and isotope compositions. However, secondary alteration (mixing, migration, microbial degradation, etc.) sometimes largely affects the compositions of natural gases. Microbial degradation especially alters both molecular and isotopic signatures for reservoired gases. Thus, although the prime order of isotopic and molecular fractionation in gases is due to genetic phenomena, secondary effects must be taken into account by putting together various pieces of information. If the secondary alteration is small, carbon isotope compositions of thermogenic hydrocarbons are largely controlled by the maturity. Berner and Faber (1996) developed isotope/maturity models for methane, ethane and propane based on open-system dry pyrolysis experiments for Type II and III kerogens, and instantaneous kinetic models. The model for Type II kerogen was successfully applied to natural gases in Northeast Japan. The estimated maturity of gases in the Niigata Basin is generally higher than the Akita-Yamagata Basin. Besides the maturity estimation, the application of the model enables detection of microbial degradation, and mixing between microbial and thermogenic gases.