Isotopic Analysis- Fluid At Fenton Hill HDR Geothermal Area (Goff & Janik, 2002)

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Exploration Activity: Isotopic Analysis- Fluid At Fenton Hill HDR Geothermal Area (Goff & Janik, 2002)

Exploration Activity Details
Location Fenton Hill HDR Geothermal Area
Exploration Technique Isotopic Analysis- Fluid
Activity Date 1982 - 2002

Usefulness useful
DOE-funding Unknown

Exploration Basis
The objectives of this review were to discuss the geologic setting of gas features with respect to the caldera, to understand variations in gas compositions that occured during drilling and flow testing of the Valles scientific wells, and to compare Valles gases with those at other geothermal sites, including the Yellowstone and Long Valley calderas.
The paper presents geochemical results from approximately 80 gas analyses of samples obtained from fumaroles, springs, and wells over the previous two decades. Gases from the various sample points were collected into 300-mL double-port evacuated glass bottles filled with ~100 mL of high-purity 4 N NaOH using a tygon tube connected to various sampling aparatuses attached to the gas sources (Fahlquist and Janik, 1992). Hot spring gas samples were collected by submerging a 20-cm-diameter plastic funnel into the pool over the bubble stream. Fumarole gas samples were collected by burying either a similar platic funnel aparatus or a 6-cm-diameter pipe into the fumarole vent. Gases sampled from sub-boiling, non-flowing wells were collected by attaching metal and/or plastic fittings to the wellheads, which were then attached to the tygon tubing. In situ fluids were also sampled from well VC-2B using three different down-hole samplers, and gases were subsampled in the field using a specialized gas extraction system that enabled precise measurement of gas volume and pressure (Goff et al., 1994). Gas samples from flowing geothermal wells were collected using a miniature Webre separator attached to the flow line downstream of the wellhead. Collection pressure and temperature data were recorded as the gas samples were fed into sampling bottles. Procedures used for gas sample analysis are described in detailed in several reports, including Giggenbach (1975), Trujillo et al. (1987), Giggenback and Goguel (1989), and Falquist & Janik (1992).
> The study confirmed that Valles gases are chemically and isotopically similar to those in other volcanic-hosted geothermal systems, and that the gases are in apparent equilibrium at temperatures >200°C. Relative proportions of Ar, He, and N2 are similar to those measured at hot spot locations such as Yellowstone and Kilauea. He R/Ra values of 4-6 within the caldera are suggestive of mantle/magmatic degassing, whereas R/Ra values of \leq0.7 outside the caldera reflect an He input dominated by U/Th decay in crustal rocks. Major gas components of the caldera surface discharges have remained relatively constant over the course of the sampling program and generally resemble gas compositions of the geothermal wells, excluding the Footbath acid spring, whose gas composition changed noticeably during 6 years of drilling and flow testing of wells VC-2A and VC-2B. The study also revealed that Valles caldera gases contained relatively little CH4 and N2 compared to other geothermal systems hosted within sedimentary rocks, suggesting that organic carbon and nitrogen in Paleozoic and Miocene strata were depleted during 13 million years of magmatism in the Jemez volcanic field.

Additional References