Cooling Indications of the Ulubelu Geothermal System, South Sumatra, Indonesia

Abstract

The Ulubelu geothermal field is located about 100 km northwest of the Southern Sumatra capital city of Bandarlampung. Pertamina has conducted exploration surveys in the area since 1990 and drilled three slim wells, i.e. UBL-1, UBL-2, and UBL-3, to determine its geothermal potential. Hydrothermal minerals such as limonite, kaolin and anhydrite were met in well UBL-1 at depths of about 650-800 m.This indicates mixing of deep hot water with cooler water. A zone of high temperature is indicated by wollastonite and epidote at a depth of about 940 m, while wairakite, prehnite and laumontite show that a lower temperature regime once occurred at the same depth. Fluid inclusions in quartz and anhydrite have a homogenization temperature of about 250- 300?C and 270-305?C at both shallow and deep levels. The homogenization temperatures of fluid inclusions are much higher than measured temperatures but are consistent with the occurrence of hydrothermal minerals such as epidote in veins, prehnite, and wairakite. It is likely that the geothermal system of Ulubelu has already cooled but its deeper regions are still hot, at least 240?C.