Isotope 18o of Alteration Rocks at Well C-1 Kamojang Geothermal Field, West Java, Indonesia

Abstract

The purpose of this study is to determine the reservoir condition system of Well C-1. Kamojang geothermal field located in the Quaternary volcanic complex caldera system (0.452 to 1.2 Ma). The volcanic complex contributes to the formation of vapor-dominated high-temperature geothermal systems (250oC). The potential of the electricity produced from this field was 260 MWe. This study describes a new method, namely the 18O isotope of alteration rock, to determine reservoir conditions. Another approach is to use a petrographic analytical form, X-ray diffraction, and isotope 18O and D on some core and cutting samples. Isotope abundances are measured using an isotope ratio mass spectrometer (IRMS). Zoning mineral alteration in Well C-1 comprises the cristobalite-montmorillonite zone, illite-montmorillonite zone, and chlorite-epidote zone. Hydrothermal fluids that generate steam in geothermal systems were derived from meteoric water. These fluids interact with rocks based on the ratio of reservoir water-rock interaction, Well C-1. The research shows that the value of the fraction of oxygen atoms of rocks in the system (1-x) was 0.16. That indicated that the reservoir containing fluid 16% or reservoir is rock-dominated. This research on 18O isotope is a novelty because it differs from what has been done by previous researchers who studied reservoir characteristics using water 18O isotope. Meanwhile, this study uses the 18O isotope of alteration rock to identify reservoir conditions. It is hoped that this method can be used in other geothermal fields.