GEOTHERMAL EXPLORATION USING PLAY-FAIRWAY ANALYSIS IN WASHINGTON STATE

The goal of the Play Fairway Analysis in Washington is to develop a method that effectively identifies favorable regions of enhanced permeability near viable heat sources with the intention of supporting commercial geothermal exploration and development. Three areas were selected for detailed analyses based on the results of a statewide geothermal assessment: Mount Baker, Mount St. Helens shear zone, and Wind River Valley. All available and newly collected data constrain geothermal favorability maps for the three plays; data include geologic (faults, fractures, volcanic vents, intrusives, seismicity), geophysical (gravity, magnetotelluric, ground-based magnetic, passive seismic, and electrical resistivity), temperature at depth (temperature-gradient measurements and geothermometry), and surface features (hot springs and fumaroles) in each play area. Expert opinion using the Analytical Hierarchy Process (AHP) determined the weighting of input layers. Confidence is addressed by assigning certainty values to input data based on the density of observation points weighted by a quality value. In general, confidence is highest where observations are closely spaced and the model errors are low; confidence is low where model errors are high or where observations are sparse. Exploration risk is addressed by scaling the favorability model by the confidence model to highlight areas with high favorability and high confidence in data. An AHP process also determined an infrastructure favorability model which reflects an array of factors that affect development feasibility. Favorability and confidence models combined with the infrastructure model identify targets for future exploration and areas with the highest potential for hosting a geothermal resource.

Newly collected data and the development of geothermal favorability maps provide testable hypotheses regarding the heat sources, permeability, and occurrence of fluid-filled fractures that together constitute a viable geothermal system. Several favorable locations were selected for drilling temperature-gradient (TG) holes based on the model results. TG drilling commences in summer 2018; results will be integrated into a revised favorability model and will help to reduce future exploration risk and encourage geothermal development in Washington.