Compilation of gas geochemistry and isotopic analyses from The Geysers geothermal field: 1978-1991

We present 45 chemical and isotopic analyses from well discharges at The Geysers geothermal field and summarize the most notable geochemical trends. H₂ and H₂S concentrations are highest in the Southeast Geysers, where steam samples have δD and δ¹⁸O values that reflect replenishment by meteoric water. In the Northwest Geysers, samples are enriched in gas/steam, CO₂, CH₄, and N₂/Ar relative to the rest of the field, and contain steam that is elevated in δD and δ18O, most likely due to substantial contributions from Franciscan-derived fluids. The δ13C of CO₂, trends in CH₄ vs. N₂, and abundance of NH₃ indicate that the bulk of the non-condensable gases are derived from thermal breakdown of organic materials in Franciscan meta-sediments.