Geothermometry At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)
Exploration Activity: Geothermometry At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)
|Exploration Activity Details|
|Location||Long Valley Caldera Geothermal Area|
|Activity Date||2000 - 2001|
The study integrates detailed results from pump tests, fluid level monitoring, temperature logging, and fluid sampling/analysis of the LVEW with information obtained from other wells drilled on or near the resurgent dome to provide a comprehensive conceptual model of the different stages of hydrothermal activity, flow, and recharge in the Long Valley caldera groundwater system. Fluids were sampled from LVEW during flow testing in May 2000, July 2000, and September 2001 in order to characterize the composition of aqueous fluids encountered below the resurgent dome, provide data required to apply geothermometers for estimating reservoir temperatures, determine potential sources of water in the wellbore, and identify magmatic gases present in the well fluids.
Water samples were collected from the pump discharge line at the surface during each flow test. Dissolved gases were sampled at the surface using 1-1 glass flow-through tubes and cold-welded copper tubes. Downhole water samples were also collected from 2600 m depth after the flow test in July 2000. The temperature, pH, specific conductance, and alkalinity of each water sample were measured in the field within minutes following collection. Field processing of water samples was conducted in accordance with standard USGS protocols outlined by Wilde et al. (1998), and the chemical constituents of each sample were analyzed at the USGS Central Laboratory in Denver, CO. Chloride and sulfate concentrations are lower in LVEW waters than those encountered in thermal waters of the shallow hydrothermal system, and the alkalinity of waters sampled from the well was also higher. Cation and SiO2 concentrations used to calculate geothermometer temperature estimates yielded temperatures of approximately 152°C and 151°C, respectively. These estimates are about 50°C greater than the maximum temperatures measured in the LVEW wellbore, and about 20°C lower than thermal waters being produced at Casa Diablo. Close agreement between the two geothermometers suggests that waters in LVEW equilibrated with reservoir rocks at about 150°C, then cooled conductively as they migrated from the reservoir to the wellbore, with minimal dilution along their flow path. Together, the chemical and isotopic (see separate activity entries) characteristics of fluids sampled from LVEW suggest that the well is not directly connected with the shallow hydrothermal system that flows laterally from the west around the southern edge of the resurgent dome.
- Christopher D. Farrar, Michael L. Sorey, Evelyn Roeloffs, Devin L. Galloway, James F. Howle, Ronald Jacobson (2003) Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling