ABSTRACT

The changes with time in carbon isotopic compositions of CH₄ and CO₂ in steam discharged from wells in the Matsukawa geothermal system were examined. δ¹³C(CH₄) values are comparatively uniform for all wells, and stable over a period of seven years. On the other hand, two groups of wells classified based on gas composition, degree of rock alteration and reservoir pressure have distinct features in δ¹³C(CO₂) values. The δ¹³C(CO₂) values of samples from wells drilled into the north and east regions of the exploitation area, which are weakly altered and have low reservoir pressures, are lower and variable (−4 to −11‰ relative to PDB) as compared with those of the central region (−5 to −7‰ relative to PDB). This is assumed to be caused by the incursion of isotopically light CO₂ of organic origin into the low pressure geothermal reservoir in the peripheral regions. The carbon isotope exchange reaction between CO₂ and CH₄ may not be fast, so that the δ¹³C(CH₄) values remain constant after the mixing with the isotopically light CO₂.