The Origins of Reservoir Liquids and Vapors from The Geysers Geothermal Field, California
Jacob B. Lowenstern, Cathy J. Janik, 2005. "The Origins of Reservoir Liquids and Vapors from The Geysers Geothermal Field, California", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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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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To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.