Geothermal Prospect Review in the Western Part of Salak Volcano, West Java, Indonesia

Abstract

The Ertankian Geothermal Prospect is located in the southern area of Bogor District, West Java, Indonesia. From 1982 through the end of 1983, Unocal Geothermal of Indonesia, Ltd. (UGI) conducted extensive geological, geochemical, and geophysical surveys that outlined Ertankian as a potential geothermal field on the flanks of the Salak Volcano. This volcano forms the north-eastern part of the Salak-Perbakti-Gagak Volcanic Massif, which consists of Upper Pleistocene to Recent stratovolcanoes, parasitic vents, and phreatomagmatic craters. The exploration survey included, magnetotelluric and gravity surveys, water geochemistry, thermal manifestation gas, ion, and isotope analyses, and geological, photo-geological interpretation, and photogrammetry interpretation. Kawah Ertankian is a vigorous fumarole area with numerous superheated and boiling steam vents, boiling acid sulfate hot springs, and large areas of acid-altered ground. Elemental sulfur is common around many of the vents. Steam from the Ertankian Fumaroles contains 7 mol% H2S and the maximum reservoir temperature obtained from the NAH-CO2 gas geothermometer was 525°F. The Ertankian Hot Springs have acid sulfate character and yield little information about the underlying hydrothermal system. Bicarbonate chloride warm springs occur on the northern flanks of the Salak Volcano at northwest and northeast locations and interpreted as the outflow product of Ertankian Geothermal System. Extensive areas of low resistivity are distributed across most of the Ertankian Geothermal Prospects. Low resistivity zones are observed around the Kawah Ertankian thermal manifestation complex and broaden to the far northwestern warm spring and Gunung Perbakti in the southwest. The shallowest base of the conductor is observed in the area near Kawah Ertankian and probably extends towards the northwest in the Alpha-19 well area. Towards the south and southwest, a significant deepening of base of the clay cap is observed. The observed resistivity trend coincides with and parallels to the NW-SE South and North Ertankian Faults, which may be the outflow pathway of the hydrothermal system. The lower rock conductance values and deepening of the clay cap base toward the southwest indicate a lower possibility of connection to the main Salak geothermal system. Considering the geological, geochemical, and geophysical data, the Ertankian Geothermal Prospect is presumed to be an encouraging area for further studies.