Geology and Thermal Features of the Sarulla Contract Area, North Sumatra, Indonesia

Abstract

Unocal has signed a contract with the Indonesian government to explore for and develop geothermal energy in the Sarulla area of North Sumatra, within the Sumatra volcanic arc and along the active Sumatra Fault System (SFS). There are no active volcanoes within the contract area, but there are extensive andesite-rhyolite lavas and dacite-rhyolite ash flow tuffs dated between 1.8 and 0.12 Ma. The area contains many surface expressions of hydrothermal activity, including fumaroles, hot springs, gas seeps, and altered rock. The hydrothermal features are clustered into four groups that are, from to SE: Namora-I-Langit, Silangkitang, Donotasik, and Sibualbuali; each is associated with a Quaternary volcanic eruptive center. Springs from the Silangkitang and Donotasik groups have neutral waters with cation geothermometry temperatures of 270?C and respectively. The Langit and Sibualbuali groups have less direct evidence of high temperature geothermal systems.