CARBON ISOTOPIC COMPOSITIONS OF CH4 AND CO2 FROM THE MATSUKAWA GEOTHERMAL SYSTEM, JAPAN

The changes with time in carbon isotopic compositions of CH4 and CO2 in steam discharged from wells in the Matsukawa geothermal system were examined. DeltaC-13(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 deltaC-13(CO2) values. The deltaC-13(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 parts per thousand relative to PDB) as compared with those of the central region (-5 to -7 parts per thousand 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 deltaC-13(CH4) values remain constant after the mixing with the isotopically light CO2.