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The Origins of Reservoir Liquids and Vapors from The Geysers Geothermal Field, California

By
Jacob B. Lowenstern
Jacob B. Lowenstern
U.S. Geological Survey, Mail Stop 910, 345 Middlefield Road, Menlo Park, California 94025
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Cathy J. Janik
Cathy J. Janik
U.S. Geological Survey, Mail Stop 910, 345 Middlefield Road, Menlo Park, California 94025
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Published:
January 01, 2005

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 δ18O 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 CO2 and around 10 mol percent NH3 + CH4 on an H2O-free basis. N2/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; N2/Ar approaches that of airsaturated meteoric H2O (~38). Isotopes within reservoir steam (dD and δ18O) 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.

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Special Publications of the Society of Economic Geologists

Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth

Stuart F. Simmons
Stuart F. Simmons
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Ian Graham
Ian Graham
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Society of Economic Geologists
Volume
10
ISBN electronic:
9781629490342
Publication date:
January 01, 2005

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