Gas Origin and Fluid Circulation Regimes Interpreted from Gas Geochemistry in Geothermal Areas Around the Bandung Basin, Indonesia

Abstract

Understanding origin and circulation regime of geothermal fluid in geothermal systems is essential to achieve sustainable utilization of geothermal energy. Gas samples were collected from fumaroles and hot springs in three geothermal areas (Patuha, Tangkuban Perahu, and Tampomas) around the Bandung Basin, west Java, Indonesia. The bulk gas concentrations, noble gas isotope ratios (3He/4He and 4He/20Ne) and carbon isotope ratios of CO2 and CH4 were analyzed to estimate origin of the gas components and circulation regimes of geothermal fluids. Measured noble gas isotope ratios indicated that mantle-source He was dominant in almost all the samples. Mixing ratios of crustal sourced He were relatively higher in the geothermal areas located in southern side of the basin (Patuha) compared to the areas in northern side (Tangkuban Perahu and Tampomas). Carbon isotope ratios of CO2 and CH4 also showed that the gases of the northerly areas are relatively nearer to MORB type compared to the gases from the southerly areas. These gas origins may indicate recharge of fluid that has longer residence time in the crust and/or a longer or slower circulation of geothermal fluid in the southerly fields, meanwhile gas from volcanic systems in the northern areas has less contribution of crustal gas sources.