Continuous Thermal Structures of the Present-Day and Contact-Metamorphic Geothermal Systems Revealed by Drill Cuttings in the Kakkonda Geothermal Field, Japan

Assessing the thermal structure of a geothermal reservoir and its history is of critical importance when evaluating the energy potential and heat source of the reservoir. However, the present and past thermal structures of res-ervoirs are not easily obtained due to temperature perturbations during drilling and/or the sparse sampling of the reservoir in rock cores. We examined drill cuttings collected from Well-20 of the Kakkonda geothermal reservoir that penetrated to a depth of-2700 m and into the Quaternary Kakkonda Granite, which is thought to be the heat source of the Kakkonda geothermal reservoir. Here we provide the first comprehensive report on the compositions of minerals from the Kakkonda geothermal reservoir from the Kakkonda Granite into its contact metamorphic aureole. The minimum temperature for each depth obtained by chlorite thermometry systemati-cally changes within a range of 265-369 degrees C at depths of 1510-2700 m, and the temperatures are consistent with fluid inclusion thermometry and the temperature of the reservoir undisturbed by drilling. Our method provides a rapid, spatially continuous, and simple way of estimating the past or present thermal profile of a reservoir. The maximum temperature for each depth obtained by biotite thermometry shows a continuous thermal profile for the contact metamorphism with temperatures reaching-760 degrees C at the granite/host-rock interface at a depth of-2700 m, and gradually decreasing to-630 degrees C at a depth of-1700 m. These results suggest that the thermal effect of the Kakkonda Granite on the surrounding lithologies was much larger than estimated previously. Such high contact metamorphic temperatures require effective heat transfer from the Kakkonda Granite, probably enhanced by magma convection in the magma chamber and fluid convection within the supercritical geothermal reservoir.