Evaluation of the Mountain Home AFB Geothermal System for the Play Fairway Project

The initial geothermal drilling (MH-1) at Mountain Home Air Force Base (MHAFB) in Idaho was done between November, 1985 and July, 1986. That hole was drilled to a depth of 1342 m, and continuous core was collected below a depth of 305 m. The maximum temperature measured was 93C at a depth of 1207 m, the maximum depth to which the temperature log was run. In 2012, we cored hole MH-2 into an active hydrothermal system at MHAFB, and much of our knowledge of the Mountain Home reservoir is a result of that drilling and subsequent studies of the fluid and rock samples. With the work that has been done on samples from MH-2, we are returning to the more limited published studies on MH-1 and using them to compile a more complete geologic model of the hydrothermal system. As we have emphasized in other papers on the Play Fairway analysis of potential reservoirs in the Snake River Plain, the geologic model and data that addresses critical aspects of the model provides the basis for exploration risk maps. We have observed that the hydrothermal system encountered at 1745 m in MH-2 is hosted by a fault zone containing hydrothermal breccias. Indications are that the well penetrated the footwall of the fault below 1793 m, and temperature decreased. Analysis of fracturing in MH-2 suggests that the fault is steeply dipping (~80) and has a strike of about 300. Using this information, we have constructed a cross section that shows that the fault would intersect the surface about 325 m to the southwest of MH-2. There is a great deal of cultural disruption of the surface, and we have not identified surface faulting in the area. However, there was also extensive faulting documented in MH-1, and we hypothesize that this is part of the same hydrothermal fault intersected in MH-2. This conceptual model is the basis for siting another exploration hole that could be drilled and tested to confirm the viability of the resource.