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The Pilgrim Springs geothermal area, located about 75 km north of Nome, was the subject of an intensive, reconnaissance-level geophysical and geological study during a 90-day period in the summer of 1979. The thermal springs are located in a northeast-oriented, oval area of thawed ground approximately 1.5 km2 in size, bordered on the north by the Pilgrim River. A second, much smaller, thermal anomaly was discovered about 3 km northeast of the main thawed area. Continuous permafrost in the surrounding region is on the order of 100 m thick. Present surface thermal spring discharge is (almost equal to) 4.2 x 10−3 m3 s−1 (67 gallons/minute) of alkali-chloride-type water at a temperature of 81 C. The reason for its high salinity is not yet understood because of conflicting evidence for seawater vs. other possible water sources. Preliminary Na-K-Ca geothermometry suggests deep reservoir temperatures approaching 150 C, but interpretation of these results is difficult because of their dependence on an unknown water mixing history. Based on these estimates, and present surface and drill hole water temperatures, Pilgrim Springs would be classified as an intermediate-temperature, liquid-dominated geothermal system.
This work has developed a conceptual geological model for the Pilgrim Hot Springs geothermal system supporting the exploration, assessment and potential development of this resource for direct use and electric power production. The development of this model involved the analysis of a variety of subsurface and geophysical data and the construction of a 3D lithostratigraphic block model. Interpretation of the data and block model aimed to establish the most likely scenario for subsurface geothermal fluid flow. As part of this work well cuttings were analyzed for permeability and correlated with geophysical logs from well to well to constrain the stratigraphic architecture of the unconsolidated sediments. Hydrothermal alteration of the sediments and bedrock core was also studied through reflectance spectroscopy and methylene blue titration in order to investigate past fluid migration pathways. The structure of the basin was interpreted through geophysical surveys including aeromagnetic resistivity, isostatic gravity, and magnetotelIuric resistivity. Based on temperature, well logs, geophysical surveys, and lithologic data, the system is subdivided into a shallow outflow aquifer and a deeper reservoir beneath a clay cap connected by a conduit with 91°C hydrothermal fluid upflow. Stratigraphic correlations indicate several clay layers throughout the section with a dominant clay cap at 200-275 m depth. Extensive pyritization and the clay mineral assemblage suggest an argillic-style alteration facies indicative of past temperatures at or slightly elevated above current conditions of hydrothermal activity at Pilgrim Hot Springs. The conceptual model Supports production from this resource in those subsurface zones where there is sufficient permeability and connectivity with the upflow zone.

Report

University of Alaska (College). Geophysical Institute

Published: 1979

Total Pages: 526


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