Natural Analogue Study on CO2 Geological Storage: An Example of the Geochemical Survey at Matsushiro, Central Japan

Environmental assessments and safety control during and after CO2 injection are essential for CO2 geological storage, and we are required to evaluate long-term environmental changes and safety. However, long-term changes are difficult to detect directly because the leakage of CO2 is expected to be small and the evaluation is, sometimes, requested to cover more than 1,000 years. To solve this problem, a natural-analogue study, which inquires into environmental changes at present through a comparison with past geological phenomena, is one possible approach. When the Matsushiro earthquake swarm began in 1965, a large volume of subsurface water accompanied by CO2 gas was discharged along fracture zones. A natural-analogue study on the CO2 discharge during the earthquake swarm should be helpful to create a scenario of leakage and a guideline for the safety of CO2 geological storage. Surveys of the CO2 content in soil gas and CO2 flux emissions from the surface were carried out with carbon isotope ratio measurements to understand the current state at Matsushiro, and to make a conceptual model for environmental assessments and safety control. From geological and geophysical points of view, it is said that deep water gushing out from the surface caused the swarm of earthquakes. As this deep water is still gushing out, we planned to measure CO2 concentrations in soil gas and CO2 flux to examine present CO2 activities at Matsushiro. Because CO2 in the soil is also produced by activities of microbes, however, we decided to measure the isotope ratio of the carbon to distinguish CO2 in deep groundwater origin from that produced by microbes. We selected five survey lines and three survey areas based on previous geochemical measurements and fissure distribution during the earthquake swarm, and measured CO2 concentration in soil, CO2 flux, and isotope ratio. Although there were survey points on the thick fan deposit where CO2 concentration in the soil and CO2 flux were high, the isotope ratio indicated that the carbon is produced by the activity of microbes. On the other hand, the isotope ratio of the samples collected from the thin fan deposit area shows deep subsurface water as the origin. An investigation well was drilled into the basement. Subsurface water samples were collected near the bottom of the well in the igneous rock formation. Geochemical analyses and carbon isotope ratio measurements show higher concentrations of chloride and abiogenic CO2, indicating that groundwater of a deep origin with CO2 is still rising. We are now making a conceptual model of hydrogeological history at the next step. This natural analogue study of CO2 seepage could indicate the importance of understanding shallow hydrogeological characteristics in a CO2 storage field.