Isotope geochemical study of carbonate formation in the 84N-3t well in the Oku-Aizu geothermal area: Natural analog study of fluid origin during caprock formation

•Carbonate compounds in rocks from well 84N-3t in the Oku-Aizu field were analyzed for their isotopic composition.•The fluid was a mixture of meteoric water and fossil seawater in the shallow part and a mixture of magmatic fluid and fossil seawater in the deeper part.•Carbon isotope composition indicates that the carbonate carbon source is derived from organic matter in the shallow part and magmatic CO2 in the deeper part.•The carbon content in the rocks increased sharply in the interval from 500 m to 900 m depth suggesting the caprock.•When supercritical CO2 is injected into the subsurface, it is proposed that some CO2 will be sequestered as carbonate minerals in shallow, low temperature areas. In Japan, a method of power generation is under consideration, which involves the injection of supercritical CO2, serving as a heat transfer fluid, into high-temperature geothermal reservoirs to extract thermal energy from the subsurface. During this process, it is anticipated that some of the CO2 will dissolve in the reservoir fluid and react with the surrounding rocks, leading to the precipitation of carbonates and clay minerals. As part of a natural analog study, we investigated the origin of the fluid and carbon that formed the carbonate minerals by analyzing the carbon content and isotope composition of rocks in the 84N-3t well, which is drilled at the Yanaizu-Nishiyama geothermal power plant in Oku-Aizu. Using the results and downhole temperature data, we estimated the oxygen isotope composition of the fluid and carbon isotope composition of HCO3− and CO2 in the fluid. The carbon content in the rocks increased sharply in the depth interval from 400 to 900 m, which was attributed to the precipitation of large amounts of carbonate minerals. The origin of the fluid responsible for the precipitation of the carbonate minerals was determined to be a mixture of meteoric water and fossil seawater at a depth of