A conceptual model of the Pilgrim Hot Springs geothermal system, Seward Peninsula, Alaska

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 magnetotelluric 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.