Gas geochemistry of the Miravalles, Pailas and Borinquen geothermal areas of Costa Rica, and a comparison with Reykjanes and Theistareykir geothermal fields, Iceland

Abstract

The application of gas geochemistry is an integral tool in geothermal development, providing information that is not obtainable by geological or geophysical surveys. The objectives of this study were to interpret deep fluid gas composition with the purpose of understanding gas behaviour in a geothermal system, and to evaluate the applicability of gas geothermometers for geochemical exploration and exploitation. Both well and fumarole gas samples from three Costa Rican geothermal areas (Miravalles, Pailas and Borinquen) and two Icelandic hightemperature (between 230 and 310°C) geothermal fields (Reykjanes, SW-Iceland, and Theistareykir, NE-Iceland) were considered. About 21 single gas and gas ratio geothermometers were applied. Equilibria of mineral assemblage buffers that could potentially control aquifer partial pressure of hydrogen sulphide (H2S), hydrogen (H2) and carbon dioxide (CO2) were modelled. The origin of gases was identified with a ternary N2-CO2-Ar diagram.

Validation of geothermometers was carried out by comparing the salinity and redox state of the geothermal systems with those conditions in which the gas geothermometers were calibrated. Three mineral assemblage buffers could control the aquifer H2S and H2 partial pressures in Miravalles neutral and Reykjanes geothermal systems. These assemblages are: anhydrite-clinozoisite-magnetite-prehnite-pyrite-quartz, anhydrite-magnetitepyrite- quartz-wollastonite and hematite-magnetite-pyrite. Aquifer H2S and H2 partial pressures in Theistareykir system could be controlled by two mineral buffers: epidote-pyrrhotite-pyrite-prehnite and magnetite-pyrrhotite-pyrite. Single H2S, H2 and CO2 geothermometers calibrated for saline fluids (>500 ppm chloride) are appropriate for Miravalles neutral and Reykjanes systems, whereas those calibrated for dilute fluids (<500 ppm chloride) are suitable for the Theistareykir system. Gas ratio geothermometers are more appropriate for application in fumaroles than single gas thermometers due to steam condensation. Both single gas and ratio geothermometers should be used as complementary tools in a geochemical assessment during geothermal exploration and reservoir management.