Reservoir pressures, temperatures and chloride concentrations are interpreted from well measurements in the Kakkonda geothermal field (Japan), to obtain a natural state conceptual model and to determine changes in the reservoir subsequent to production and reinjection. In this model the system is viewed as two reservoirs, a shallow reservoir extending from the surface to about 800 m below sea level and, partially isolated from this, a deep reservoir extending down to 3000 m below sea level. The modelling of the changes has taken two forms, one a lumped-parameter mathematical model leading to an expression for the average reservoir chloride concentration as a function of time, and the other a three-dimensional numerical simulation model. The mathematical model is fitted to pressure and chloride concentration changes. The numerical model is fitted to the natural state pressures, temperatures and chloride concentrations, and is verified by comparing the predicted variation of production enthalpy with the measured changes. The numerical model is also used to explore a simplified development scenario, involving a production/injection flow doublet, thus allowing a clearer interpretation of thermal and chemical travel times across the reservoir.