The Subsurface Geology and Hydrothermal Alteration of the Dieng Geothermal Field, Central Java: A Progress Report

The Dieng geothermal system is volcano-hosted, and its reservoir is liquid-dominated. Thermal manifestations lay at about 2,000 m asl. Previous studies suggest that the field consists of three prospect areas, namely Sileri, Sikidang-Merdada, and Pakuwaja. Dieng Geothermal Field has serious problems with mineral scaling and corrosion in the production facility. The problems, to some extent, are related to the natural characteristics of the system. In our study, cuttings from 4 of 47 wells drilled in Dieng were examined using petrography and X-Ray Diffractometry techniques to better understand the subsurface geology of the Dieng Geothermal Field, including different types of hydrothermal alteration mineralogy. The wells MG-1, MG-2 and MG-3, and MG-4 are chosen to represent the prospect areas, respectively. In general, typical hydrothermal minerals in Dieng Geothermal Field formed by near-neutral pH fluids characterized by clays (smectite, illite, chlorite), silica (quartz, cristobalite), calcite, wairakite, pyrite, epidote, and actinolite. In wells MG-3 which are located in the Sikidang-Merdada area, acidic alteration such as anhydrite, pyrophyllite, and native sulfur are present. Most notably, anhydrite occurs from the near-surface down to depths >2,000 m. These findings indicate the existence of acidic fluids at the deeper parts of the Sikidang-Merdada area. Furthermore, we utilize subsurface geological data from other wells within all the three prospect areas to complement our review. © 2020 IOP Publishing Ltd.