Distribution of Oxygen Isotopes and Non-Condensible Gas in Steam at The Geysers

Abstract

The earliest available isotopic and noncondensable gas analyses have been compiled from around the Geysers geothermal field to estimate preexploitation conditions. The observed fieldwide gradients in isotopes and gases provide evidence for several different physical processes which combine to control reservoir fluid compositions. These processes include water rock, water steam, and rock gas interaction. Oxygen isotopes of Geysers steam generally become heavier from southeast to northwest and from the margins of the field toward the center. Throughout the field there is a good correlation between steam isotopic composition and reservoir rock isotopic composition. It is inferred from this that water/rock interaction exerts the predominant control on steam isotopic composition. Fieldwide lateral and vertical gradients in steam isotopes suggest steam condensation and/or meteoric recharge at the top and margins of the field. The noncondensable gas content of early Geysers steam generally increased concentrically outward from a low in the south central part of field. This pattern is consistent with a fieldwide Rayleigh condensation cell, but because of a strong correlation with reservoir rock type is thought to be related to lithology as well. Lithologic factors which may affect gas content include reservoir rock type, alteration mineralogy, and the "thermal maturity" of organic material in the reservoir graywacke.