Organic Species in Geothermal Waters in Light of Fluid Inclusion Gas Analyses

Abstract

Measurement of organic compounds in Karaha-Telaga Bodas and Coso fluid inclusions shows there are strong relationships between H2 concentrations and alkane/alkene ratios and benzene concentrations. Inclusion analyses that indicate H2 concentrations > 0.001 mol % typically have ethane > ethylene, propane > propylene, and butane > butylene. There are three end member fluid compositions: type 1 fluids in which alkane compounds predominate, type 2 fluids that have ethane and propylene and no ethylene and propane, and type 3 fluids that have propylene and butylene and no propane or butane. Alkane/alkene ratios increase regularly with depth in Karaha-Telaga Bodas anhydrite and quartz inclusions. Coso analyses show a similar change in benzene and alkane/alkene ratios with sample depth.

Equilibrium calculations show that high-hydrogen fugacities and low temperatures favor alkane compounds, whereas high temperatures and low hydrogen fugacities favor alkene compounds. Benzene, propylene, butylene, and ethylene should form in that order as hydrogen fugacity decreases.