Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010 1 Evolution of Cisolok – Cisukarame Geothermal System, West Java – Indonesia, Based on Its Surface Manifestation Niniek Rina Herdianita and Wulandari Mandradewi Study Program of Geology, Faculty of Earth Science and Technology, Institut Teknologi Bandung (ITB). Jl. Ganesa 10 Bandung 40125 - Indonesia herdianita@gc.itb.ac.id Keywords: surface manifestation, hot spring, alteration, travertine. ABSTRACT The geothermal surface manifestations of spouting springs and surface alterations indicate that the Cisolok geothermal system has changed. The deep reservoir water has changed from Cl to HCO 3 waters due to subsurface water-rock interactions. Temperature of the reservoir has been cooling down, i.e. from above 220 o C to 160-200 o C. These changes also affect the characteristic and mineralogy of surface alteration deposits, i.e. from silica sinter to travertine deposits. 1. INTRODUCTION The geothermal system of Cisolok and Cisukarame is considered a geothermal prospect area in West Java. The geothermal manifestation appears at 106°27'13.4" E and 6°56'0.5" S in the Cisolok River. It is about 70 km west of Sukabumi or about 170 km from Bandung (Figure 1). The geothermal manifestation of Cisukarame itself is located about 6 km north of Cisolok. At the present, the geothermal manifestation of Cisolok is used as public bathing place. The geothermal system considered a geothermal prospect area as a result of its intense geothermal surface manifestations including hot spring and surface alteration. Thermal water discharging in the Cisolok River has a very high temperature that is at times above the boiling point. The thermal water also has neutral pH and very high discharge rate. Hydrothermal alteration occurs at the surface and the bank of the Cisolok River showing very high alteration intensity that is dominated by the occurrence of thick silica sinter and travertine. The type and characteristic of this surface manifestation indicates that the geothermal system of Cisolok and Cisukarame has a shallow, high temperature reservoir. However, this geothermal resource has yet to be utilized for purposes beyond tourism. One reasons for this is the behavior of Cisolok and Cisukarame geothermal system is still largely unknown. A geothermal system including reservoir conditions and subsurface flow patterns can be understood by studying surface geothermal manifestations such as hot springs, hot pools, steam vents and hydrothermal surface alteration (Hochstein and Browne, 2000, and Browne, 1978). Identification and correlation of an active and fossil geothermal system can aid in the understanding of the evolution of a geothermal system including changes in reservoir conditions and subsurface flow patterns. 2. GEOLOGY Morphology of the study area can be divided into three parts including steep mountain, gentle mountain, and plain. The steep mountain forms at the northern and western parts of the study area. Its elevation is above 200 m with slopes between 40 and 70°. At the eastern part of the study area there is a zone of gentle mountain at elevations less than 150 m and slopes of less than 30°. The last morphology zone is plain and occurs in south coastal area. Figure 1: Map of West Java and Banten showing the location of the study area, Cisolok, Sukabumi.