Abstract In this paper, we consider the primary controls on gas and liquid geochemistry at The Geysers geothermal field (California) prior to reservoir exploitation and reinjection programs. Well discharges vary considerably in steam/gas ratio, gas composition, and dD and δ 18 O of steam. Many of the variations can be linked to the degree of liquid saturation or steam fraction (Y) within the reservoir. Discharged fluids from the central Northwest Geysers have low molar steam/gas (<200) and are produced from reservoir vapor because little condensed liquid water appears to exist in that part of the system (i.e., they are high Y fluids). The gas is relatively uniform in composition, typically with ~60 mol percent CO 2 and around 10 mol percent NH 3 + CH 4 on an H 2 O-free basis. N 2 /Ar ranges to values >500. Discharges from the central Northwest Geysers are interpreted to contain a mixture of connate and metamorphic gases derived from high-temperature breakdown of carbon- and nitrogen-bearing metasediments, either within or below the geothermal reservoir. Input of volcanic gas from underlying intrusions appears to be present but minor. The gas-rich end member is less evident in the Southeast and Central Geysers where discharged fluids consist primarily of steam boiled from condensed reservoir liquid (i.e., they are low Y fluids). Molar steam/gas in these parts of the field commonly exceeds 3,000; N 2 /Ar approaches that of airsaturated meteoric H 2 O (~38). Isotopes within reservoir steam (dD and δ 18 O) are only slightly shifted from local meteoric waters. Reservoir gases in the Southeast and Central Geysers are thus diluted by the dominant input of meteoric water, which disguises the connate and/or metamorphic signature of the gas. The resulting small proportion of gas is highly variable in composition.

Abstract Thermal/mineral waters of the Clear Lake region, California are among the most challenging geothermal fluids in the world to study because they display enormous chemical and isotopic diversity and do not geochemically resemble fluids in typical, high-temperature (≥200°C) geothermal systems (Goff et al., 1993a, 1993b). The Clear Lake region contains no boiling hot springs, hot fumaroles, or springs actively depositing sinter, features commonly linked with high-temperature reservoirs. Regionally, the fluids display tremendous variations in chemical and isotopic composition, caused more by variations in bedrock composition than by subjacent magmatic heat sources (Goff et al., 1977; Thompson et al., 1981a; 1992; Donnelly-Nolan et al., 1993). The distribution of fluids is roughly coincident with the late Tertiary and Quaternary Clear Lake volcanic field (2.1 Ma to 10 ka; Donnelly-Nolan et al., 1981; Hearn et al., 1981). The region lies northeast of The Geysers steam field, the largest geothermal field in the world, yet drilling of approximately 25 exploration wells has not found a commercially exploitable geothermal system. Because conditions in most of these wells are very hot (≥200°C at 2000 m) but relatively impermeable, the Clear Lake region is rated as one of the best hot dry rock geothermal prospects in the United States (Goff and Decker, 1983). Gas geochemistry is becoming more widely used for geothermal prospecting, especially in areas where spring chemistry is ambiguous or where spring waters are not directly derived from deep reservoirs (Goff et al., 1985; 1991; Janik et al, 1991; 1992). Geothermal gases commonly originate from