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

Measurement of organic compounds in KarahaTelaga 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. The Coso and Karaha-Telaga Bodas analyses indicate that benzene concentrations and alkane/alkene ratios are directly related to hydrogen fugacity. The large differences in hydrogen fugacity are best explained by boiling, because hydrogen strongly partitions into the vapor phase and is thus stripped from the geothermal fluids. The change in alkane/alkene ratios downhole would be reversed if the reactions were temperature driven. Calculations explain why benzene is a common constituent of geothermal fluids. Methane will react to form benzene at relatively high hydrogen fugacities. The relationship between organic species and hydrogen suggests that equilibrium at Karaha-Telaga Bodas and at Coso is fluid dominated. INTRODUCTION A major goal for developing a dual quadrupole fluid inclusion gas analysis system was to study organic species (Norman, 2002). Organic species in geothermal fluids have received little attention because of their low concentrations that have a minor affect on power production. Based on experience with major aqueous gaseous species, we assume that organic species may be used to interpret fluid source, fluid chemistry, and fluid process. Benzene concentrations are also of concern because benzene is a carcinogen for which the US Environmental Protection Agency (EPA) has set a drinking water maximum contaminant level (MCL) of 0.005 ppm (5 ppb). We previously presented ideas on how the distribution of C1-C4 straight chain (alkane or paraffin compounds) and those with a double carbon bond (alkene or olefin compounds) are related to organic compound source (Norman, 2002). Fractionation of C1-C4 simple organic species as a consequence of fluid boiling and condensation was also introduced then (Norman, 2002). Here we present organic fluid inclusion analyses performed on material from two bores holes, one from the Karaha Telaga-Bodas, Indonesia geothermal field, and one from the Coso, California geothermal field. We concentrate on equilibrium between the organic species, and how equilibrium considerations may be used to interpret fluid source and process. The Coso host rock is comprised of gneiss and granite, whereas the Karaha-Telaga Bodas system host rock has abundant organic material, including lakebeds (Moore, 2002; Moore, 2002; Lutz, 1999). By comparing the analyses from the two bore holes, we will show that changes in organic species with depth is best explained by equilibrium driven reactions, and not by wall rock environment.