Thermal and geochemical structure of the Uenotai geothermal system, Japan

The Uenotai geothermal area is located in southern Akita prefecture of northern Honshu Island. The Uenotai geothermal system is a liquid-dominated system with a central zone of aquifer boiling. The two-phase reservoir has evolved from liquid in the natural state due to exploitation. Gas composition of the vapor phase in the reservoir is nearly in equilibrium and correlates with the vapor fraction in the reservoir and with discharging steam quality. The marginal part of the Uenotai system has cooled with the drop in ground-water level. The chemical characteristics of the geothermal water indicate mixing of the immature high Cl source water with conductively heated or steam-heated shallow water or surface water, as well as boiling and steam gain.