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

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 <400 m and a mixture of magmatic fluid and fossil seawater at deeper levels. The carbon content originated from organic sources in the shallow part at a depth of <900 m and from magmatic CO₂ in the deep part. Based on these results, the depth interval from 400 to 900 m was identified as the caprock and the formation temperature of the caprock layer was estimated to be 90−150 °C. Carbonate minerals were also detected at depths of >900 m but in lower amounts. This suggests that when supercritical CO₂ is injected into the subsurface from an injection well, some CO₂ is fixed as carbonate minerals in shallow depths where the temperature is relatively low, forming an artificial caprock.