Producing Practical and Useful State Geothermal Maps

Abstract

Geothermal maps have many uses including geothermal and petroleum exploration, planning subsurface mining operations, and understanding volcanic and earthquake hazards. The display of relevant information on geothermal maps is not simple, however, as the mode of heat transport may change from conduction to convection both laterally and vertically, the geothermal gradient typically varies vertically and laterally in areas with both mountains and basins, and measureable heat is generated by decay of unstable isotopes of uranium, thorium and potassium in silicic crystalline rocks. For example, traditionally geothermal maps are presented as geothermal gradient maps or heat flow maps. A geothermal gradient map is given in units of °F/100 feet or °C/km, and would be useful if the values on the map could be taken to calculate temperatures at depth. Assuming a surface temperature of 50°F, a geothermal gradient of 2.0°F/100 feet suggests a temperature of 70°F at 1,000 feet, 90°F at 2,000 feet, etc. However, as lithology generally changes with depth, including rock thermal conductivity, the geothermal gradient changes with depth. If the change were from a higher thermal conductivity sandstone to a low thermal conductivity shale at 1,000 feet, the geothermal gradient could double to 4.0°F/100 feet and the temperature at 2,000 feet would be 110°F. Thus, a geothermal gradient map without a contour map of thermal conductivity structure cannot be used to calculate temperatures at depth. Assuming that lateral changes are gentle relative to vertical changes, heat flow is constant with depth. However, heat flow can only be converted to temperature with a knowledge of thermal conductivity. Thus, a heat flow map also requires a contour map of thermal conductivity structure to calculate temperatures at depth. Both heat flow and geothermal gradient determinations require temperature measurements at depth – as temperature is typically the primary quantity of interest, it may be represented as contours of depth to isotherms (lines of constant temperature). Only areas and depths where temperatures are measured are shown definitively on the map. In sedimentary basins there may be thousands of corrected bottom-hole temperature measurements and detailed temperature information. In mountains, detailed temperature logs may be relatively sparse, but rock thermal conductivities more homogeneous. Revisions of geothermal maps from Colorado will be demonstrated.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016