Fluid Inclusion Gas Compositions of an Evolving Vapor-dominated Geothermal System, Karaha - Telaga Bodas, Indonesia

Abstract

Variations in the compositions of gases trapped in fluid inclusions represent a unique source of information on the origins of the geothermal fluids and on the processes that have affected them. In this paper, we present data on fluid inclusion gases from the Karaha-Telaga Bodas geothermal system in west central Java. Wells up to 3 km in depth have encountered temperatures as high as 353oC and the underlying granodiorite that provides heat to the geothermal system. The gas compositions of fluid inclusions trapped in two wells, T-2 located near a vapor-chimney that allows magmatic gases to reach the surface and K-33, located in the central part of the system away from the chimney were studied. Vein filling minerals were analyzed for H2, He, CH4, H2O, N2, O2, H2S, Ar, CO2, SO2 and hydrocarbons (C2-C7). Fluid inclusions in bulk core samples that included the surrounding wall rock were analyzed for 180 masses including those listed above. Gas concentrations commonly range from more than 1.5 to 10 mole percent, although some of the inclusions contain gas contents exceeding 20 mole percent. Concentrations greater than 1.5 mole percent provide evidence of boiling, whereas contents greater than 10 mole percent are interpreted to indicate the presence of a gas cap. N2/Ar ratios suggest fluid inclusions from T-2 contain significant contributions of magmatic gases whereas the gas compositions of fluid inclusions from K-33 suggest a meteoric origin. High gas concentrations in the fluid inclusions from both wells document the presence of a vapor phase and suggest the present-day steam zone was more extensive in the past. In T-2, gas-poor inclusions in the wall rocks underlie the steam zone and are interpreted as representing the early boiled fluid. Low gas fluid inclusions overlie the steam zone in both wells. These inclusions are interpreted to represent descending gas-poor condensate and meteoric waters that deposited calcite and anhydrite.