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The Ennis geothermal area is believed to be an elliptical field, roughly ½ mi long in a north-south orientation and ¼ mi wide, located 1½ mi north of Ennis, Montana. The valley is bounded by Precambrian (X) rocks to the west, and by Paleozoic and Mesozoic rocks to the east. Geophysical data indicate a major north-south valley-bounding fault, and the presence of minor east-west cross-valley faults with minor displacement. The geothermal fluids occur with the quartzofeldspathic gneiss and hornblende gneiss which is overlain by a thin (465 to 650 ft, 140 to 200 m) layer of Tertiary and Quaternary sediments along the major axis of the field. Production is from a highly fractured zone within bedrock. Fluids produced average 189°F (87°C), with a total d ssolved solids content of 1,000 mg/L.
Two wells penetrate the fracture zone, and a third well is completed in the Precambrian gneiss but is not believed to intersect the major fracture zone. The southern deep well, TX-12, intercepted the top of the fracture zone at a depth of 492 ft (150 m), 25 ft (8 m) below bedrock contact; it penetrated the bottom of this zone at a maximum depth of 615 ft (187 m), 148 ft (45 m) below top of bedrock. The northern well, MAC-1, intersected the fracture zone between the depths of 1,100 to 1,200 ft (335 to 365 m). The nonflowing temperature log on TX-12 shows a maximum of 198°F (92°C) at a depth of 500 ft (150 m), whereas the shut-in log on MAC-1 is reported to indicate a maximum temperature of 206°F (97°C) at about 1,100 ft (335 m).
Hydraulic connection between the two deep wells was established during a pump test on MAC-1, with transmissivity and storativity values of 4,000 ft2/day (370 m2/day) and 2.5 × 10-4, respectively. The two deep wells are 1,010 ft (308 m) apart along a north-south line, and the third well is 260 ft (80 m) south-southwest from the pumped well. The rate of draw-down during pumping was greater in the distant well, which fully penetrates the fractured zone, than in the third well; this indicates the hydraulic conductivity of the Precambrian gneiss is considerably smaller outside of the fracture or shear zone.
With only two wells, a three-point problem solution to the orientation of the fracture zone cannot be solved. However, we have established a north-dipping fracture zone (minimum dip 30°) which does not fit either into Precambrian or Tertiary tectonic domains. Could the structural control on this geothermal system be related to the Overthrust belt?
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