Do the sedimentary strata have power to neutralize leaking CO_2? : a natural analogue study on past CO_2 invasion and carbonate precipitation in the Cretaceous Izumi Group, SW Japan.

摘要

Dawsonite, which frequently plays an important role in geochemical modelling of CO_2 storage complex, is a rare mineral in the nature. This study reports mode of occurrences of natural dawsonite recently re-discovered in a classical locality of Japan (the Izumi Group, SW Japan), in comparison with our geochemical modelling of CO_2 storage in sedimentary strata. The simulation has shown that dawsonite is only stable in the early stage of the evolution of CO_2 storage complex even though an extensive precipitation of this mineral in the CO_2-affected part within. Natural dawsonite forms mineral veins in a compact mudstone layer, and is also found to be a product of early stage of carbonate veining. The observation is concordant with the simulation result suggesting a temporal limit on the stability of dawsonite in the evolution of storage fluid with time. It is also probable that the precipitation occurred associated with the formation of fractures in mudstone to self-seal them effectively. The relation can be a support to assure the storage security, showing how a cap rock mudstone respond to pressurized CO_2 which could be expected as "pressure build up" during the injection phase, and how a cap rock mudstone confine CO_2 through selfsealing by mineral precipitation. The presence of self-sealed veins composed of carbonate(s) only stable in the early stage of the CO_2 plume evolution may imply that some of the geochemical trapping mechanisms could be operative in the early stage of the CO_2 trapping, a scenario a little bit different from that shown by the famous figure (i.e., Fig. 5.9) in the IPCC Special Report.