Ulubelu geothermal field is located in the southern part of Sumatra Island, Indonesia. Geophysical studies including gravity, electrical resistivity and self-potential methods have been carried out to explore promising geothermal reservoir conditions within the area. Integrated geophysical data interpretation supported by geology, geochemistry, remote sensing and drilling data has been synthesized to delineate the lateral extent of the reservoir, to assess hydrology within the area and to estimate the reserve potential of the geothermal reservoir. Geophysical studies suggest that the Ulubelu geothermal field could be associated with a graben structure as indicated by the residual gravity anomaly map of the area, supported by two-dimensional gravity modeling. The studies have also revealed two separate up-flow zones with different phase of the reservoir fluids covering a total of 25 km2 and by an outflow zone in the southern area. The top of the main reservoir is probably 650 m below the surface, where the reservoir temperature is about 2600C, as indicated by gas geothermometer and drilling data. The reserve potential of the geothermal reservoir was estimated to be about 100 MWe for 25 years of electric power generation. These studies have given a good understanding of the Ulubelu geothermal field as an attractive geothermal resource for the future development.

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