The Hululais area is one of several Quartenary andesitic-dasitic volcanic centers of the Sumatera volcanic arc. The Hululais prospect area concentrates mostly on the West side of the major zone of crustal weaknesses created by the Northwest - Southeast trending Sumatera fault system. The Northwest - Southeast and Northeast - Southwest structures controll the existense of the Hululais volcanic system and the presence of thermal manifestations. The prospect area is characterized by the appearance of fumaroles, hotsprings and altered rocks.

Fluids chemistry analyses on hotspring waters show a dilution of shallow meteoric groundwater and indicate that some may originate from a single hot water reservoir. However, hotspring waters located in the Southern parts contain higher ratio Cl/HCO3 than those located in the North indicating less shallow ground water contaminations. Higher HCO3/SO4 ratio in the Northern hot springs designates the fluids flows from South to North. The upwelling fluids mostlikely take a place beneath Suban Agung (North) and move laterally Northward to Semelako area (South). The presence of magmatic steam in Suban Agung fumarole is indicated by high content of N2, He Ar and isotopic data. Recharge area is located at about 600 meters above sea level. Mercury anomaly forms a lineation trending Northwest-Southeast that coincides with the main fault pattern in this area.

The subsurface temperature estimates using solute and gas geothermometry were obtain ranges values of 240 - 300 ?C respectively. These are confirmed by isotope data which show enrichments in their isotopic content as a result of evaporation from high temperature fluids of more than 180 ?C. These analyses indicate that Hululais can be categorried as high enthalpy geothermal system.

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