Map Making Methodology

The 2004 version of the Geothermal Map of North America was debuted at the American Association of Petroleum Geologists (AAPG) annual meeting in Dallas, Texas April 17-20, 2004. The objective of the 2004 Map was to utilize extensive industry oriented thermal data sets, as well as the new research heat flow data. Four new data sets were used in addition to the previous regional heat flow data. First, the Western Geothermal Data base (previously proprietary industry data) was analyzed. Although it has few initial heat flow values, being mostly raw temperature-depth and gradient information, we determined thermal conductivity, rated quality, and calculated heat flow for as many of the wells as possible. Of the 6,000 points in the data base, about 4,000 points were used in the preparation of the map. The second new data set used was the AAPG BHT data set (AAPG, 1994) for US and Mexico with over 20,000 BHT points. This data was corrected using the Harrison et al. (1983) correction and then an additional correction determined from equilibrium logs (Blackwell and Richards, 2004). The third new data set is the USGS Great Basin Data base. The fourth is the Canadian heat flow and BHT published data set. Other data came from the SMU United States Regional Heat Flow Data base, the 1992 North American geothermal data set and new heat flow information was included for the oceans and Costa Rica. The hot spring locations are from the 1980 Thermal Springs data compiled by the National Geophysical Data Center ( accept Alaska, which is from the "Geothermal Resources of Alaska" map, 1983, by Alaska Geological Survey and NOAA. The volcanoes are of Pleistocene and Holocene age. The U.S. portion of the map data are online at the SMU Geothermal Lab website. 

The previous version of the map was prepared in late 1980’s and published in 1992 by the Geological Society of America (GSA) (Blackwell and Steele, 1992) in conjunction with the GSA sponsored Decade of North American Geology (DNAG). The map featured research data (about 3,000 points) and served as the basis of several geothermal evaluation projects. For this 2004 Map, other users expected to use the map are in fields of oil and gas exploration, global climate change, mineral exploration, and plate reconstruction. 
Sites were assigned a quality ranking based on the drilled depth and the quality of the data. Wells with depths deeper than 25 meters were examined. The deeper wells receive the higher quality rankings and on a regional basis only wells deeper than 100 m were used. The general ranking criteria are described by Blackwell et al. (1991). Wells with “geothermal” scale gradients/heat flow or those drilled within a known geothermal area are given a separate ranking. Plotted heat flow values by symbol on the map are for regional wells. A cut off heat flow of 120 mWm-2 was used to differentiate “geothermal” versus “regional” sites. All of the heat flow values considered regional from the different data sets and countries were merged into a single point file and contoured using a minimum curvature algorithm with a tension factor of 0.5, averaging all points within a 0.02° spacing of each other, and a grid interval of 5’ (0.08333°) of latitude/longitude. Points were added to constrain the contours to follow structural/province trends in areas of sparse data coverage. The gridding was done using Golden Software program Surfer.

The number of heat flow contour levels are increased from the 1992 GSA map of 10 mWm-2 to 5 mWm-2 between each interval. By increasing number of heat flow points, the knowledge on heat flow contouring has improved with them. This increased detail amplifies the information for a region. By using colors and contours that blend from one interval to the next, the map is designed to show the heat flow values for specific areas where detailed information is known, i.e., the Basin and Range, but still allow for the greater generality in places with little to no specific data, i.e., Kentucky. The overall heat flow contouring represents the regional heat flow levels of North America. Although, where the geology, tectonics, and data represent more specific area detail, we have contoured it accordingly.

The Pacific and Atlantic Oceans were contoured based on the oceanic crustal age, rather than the individual heat flow data. Although there are many more ocean data points, the amount of scatter in ocean heat flow and large areas lacking data still makes it difficult to contour according to actual data points for these two oceans.

2004 Map Features compared to 1992 Map 
  • The size of the map decreased from four sheets totaling 70” x 70” to one sheet 48” x 63” to make this map more manageable.
  • Topographic base layer added GTOPO30.
  • Bathymetry base layer added "GEBCO" General Bathymetric Chart of the Oceans 2003.
  • Improved projection from Transverse Mercator to Orthographic.
  • This map uses both industry data and research quality thermal data, as opposed to only the thermal data on the 1992 GSA Map.
  • Heat flow contour levels changed from 10 mWm-2 on the 1992 map to 5 mWm-2 on this map.
  • Entire North American area is contoured for first time in 2004.

  • Formal Map Reference: 
    Blackwell, D. D., and Richards, M. 2004. Geothermal Map of North America. American Assoc. Petroleum Geologist (AAPG), 1 sheet, scale 1:6,500,000.

    Related References: 
    AAPG, American Association of Petroleum Geologist, CSDE, COSUNA, and Geothermal Survey Data_Rom, 1994.

    Blackwell, D.D. and Maria Richards, Calibration of the AAPG Geothermal Survey of North America BHT Data Base, AAPG Annual Meeting, Dallas, TX, Poster session, paper 87616, 2004.

    Blackwell, D. D., and J. L. Steele, DNAG Geothermal Map of North America, scale 1:5,000,000, Geological Society of America, 1992.

    Blackwell, D.D., J.L. Steele and L.S. Carter, Heat flow patterns of the North American continent: A discussion of the DNAG Geothermal Map of North America, pp. 423-437, in D.B. Slemmons, E.R. Engdahl, and D.D. Blackwell eds., Neotectonics of North America, Geological Society of America. DNAG Decade Map Volume 1, pp. 498, 1991.

    Harrison, W.E., K.V. Luza, M.L Prater, and P.K. Chueng, Geothermal resource assessment of Oklahoma, Special Publication 83-1, Oklahoma Geological Survey, 1983.

    Purchase the Map from the AAPG Bookstore as Product Code 423