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Geothermal resources of Wyoming sedimentary basins have been defined through analysis of over 14,000 oil well bottom-hole temperatures, thermal logging of 380 wells, measurement of rock thermal conductivities, calculation of 60 heat-flow values, drilling of 9 geothermal exploratory wells, conductive thermal modeling, and the study of existing geologic, hydrologic, and thermal spring data. All data have been integrated into interpretations of the thermal structure of the Big Horn, Wind River, Washakie, Great Divide, Green River, Laramie, Hanna, and Shirley basins of Wyoming.
Controlling factors for the formation of geothermal resources in these basins are regional heat flow, rock thermal conductivity values, depths to regional aquifers, and hydrologic flow directions. Regional basin heatflow values range from about 40 to 80 milliwatts/m2; measured thermal conductivities are in the general range of 1.5 to 4.0 watts/m°K; and depths to aquifers are up to 11,000 m (36,000 ft). This results in regional geothermal gradients for Wyoming basins in the range of 15° to 40°C/km (44° to 116°F/mi) with predicted maximum aquifer temperatures near 300°C (570°F).
Anomalous geothermal areas within the basins contain measured thermal gradients as high as 400°C/km (1,160°F/mi) over shallow depth intervals. These anomalous areas are the combined result of local geologic structures and hydrologic flow. A simplified model for such areas requires water movement through a syncline with subsequent heating due to regional heat flow and thermal conductivities of overlying rock units. Consequent flow of the heated water up over an anticline produces a localized area of anomalous geothermal gradients.
Access to Wyoming basin geothermal resources is primarily through producing oil wells. Fifty two oil fields which account for over 90% of Wyoming's oil field water production, produce 575 million L (152 million gal) of thermal water per day. The temperature of this water ranges from 30° to 110°C (86° to 230°F) with 88% warmer than 38°C (100°F) and 60% warmer than 50°C (122°F). Over 50% of this water is disposed of, generally by discharge to the surface.
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