Geochemistry Monitoring of Surface Manifestation in Sibayak, North Sumatra, Indonesia

Abstract

Sibayak was the first geothermal power plant where operated in Sumatra Island, Indonesia. This two-phase geothermal field has been produced 12 MW electricity since 1994 from 10 wells. During the exploitation stage, a geochemical study on geothermal surface manifestation conducted to monitor the effect of reservoir production and injection to the surface manifestation. The geothermal surface manifestation found in the crater and the flank of Mt. Sibayak and Barus, North Sumatra, Indonesia. The Sibayak manifestation is located near Sibayak geothermal power plant and it consists of solfatara, kaipohan, hot spring, and warm spring. The hot spring in Sibayak mainly as steam-heated water, acid sulfate fluid. In Barus, most of the thermal manifestation consists of neutral bicarbonate water which located on 12 km east to Mt. Sibayak. Based on the Cl vs B diagram, it can be concluded that geothermal water from Sibayak and Barus came from different heat sources. Geochemistry data monitoring acquired based on sampling data in 2011 and 2018 during exploitation stage of Sibayak geothermal field. The data showed the change of temperature, pH, and some geothermal geochemistry of ion and cations from water and shifting in gas geochemistry diagram. Nevertheless, the variation of data from 2011 and 2018 did not show the effect of reservoir exploitation, while the weather condition might be as the main cause of the change of water geochemistry.