Geochemical study of fluid origin and caprock formation with carbonate mineral precipitation in the Okuaizu geothermal area

To elucidate the origin and mechanism of carbonate mineral precipitation controlling fluid movement in geothermal active areas, analyses were conducted in the Okuaizu geothermal area (with a maximum temperature of 340 °C). Carbon and oxygen isotopes and chemical compositions were analyzed for carbonate minerals in rocks from four newly drilled geothermal wells and one existing well. Carbon content in the rocks increased gradually from 200 to 500 m above sea level (mASL; above sea level), sharply increasing to a maximum of 3 wt% near the study area's center. Stable isotope composition data were combined with well temperature data to compute oxygen and carbon isotope compositions of the fluids. Results suggest that the fluids precipitating carbonate minerals originate from a mixture of meteoric water, fossil seawater, and magmatic fluids at depths <−500 mASL. Carbon primarily originates from shallow organic matter sources and deep magmatic CO₂. The caprock formation in the Okuaizu geothermal area occurs at shallow depths, with temperatures ranging from 100 °C to 150 °C, and comprises carbonate and clay minerals. This indicates that conditions conducive to CO₂ interaction with rocks and subsequent fixation as carbonate minerals in the formation develop at relatively low temperatures and shallow depths.