The Obama geothermal field (OGF) is one of the most interesting geothermal areas in Kyushu Island, southwestern Japan, because of its large number of high-temperature springs. Thermal waters from Quaternary volcanic rocks (predominantly andesites) discharge along faults in the OGF of southwestern Japan. We have applied different gravity gradient interpretation techniques for geological mapping purposes. The chemistry of more than 100 thermal and ground water samples collected between 1936 and 2005 indicate that the Na-Cl hot spring waters are a mixture of "andesitic" magmatic, sea and meteoric waters. Mixing models and silica and cation geothermometry were used to estimate the SiO2 and Cl composition and the temperature (~200 deg. Celsius) of the reservoir fluids deep in the geothermal system. A 3D numerical simulation study using HYDROTHERM was carried out to obtain a comprehensive hydrothermal model of the field. In the future, the OGF may be suitable for electric power generation, although heat and fluid extraction will require careful management to prevent or minimize reservoir cooling.