Blackwell, David1, Maria Richards1
(1) SMU, Dallas, TX
ABSTRACT: Calibration of the AAPG Geothermal Survey of North America BHT Data Base
The AAPG Geothermal Survey of North America (GSNA) was a large effort carried out in the early 1970’s that culminated in the production of a massive data base (over 20,000 BHT points from over 10,000 wells in the US, Canada, and Mexico), and the publication of two continent scale maps based on the data. AAPG made the data available on a CD-ROM in 1994. The data set does not appear to have been extensively used, in part because of the uncertain error associated with the raw measurements and the lack of calibration comparisons to accurate equilibrium temperature measurements. We have calibrated the data for the 0.6 to 3.0 km depth range in the Midcontinent using accurate equilibrium temperature measurements in 33 wells (compared to the average of about 500 BHT measurements in wells within 50 km of the equilibrium logs). We used the Harrison et al. (1983) correction, similar to one of the early corrections suggested for the GSNA. After correction there is a bias related to depth/gradient/temperature difference so we calculated a secondary correction that is a function of the average gradient in the well. Using the final correction the test gradient data set had a standard deviation of ±2°C/km as opposed to a SD of ±5°C/km for the single correction. For a well in the 2-3 km depth range the smaller SD corresponds to a gradient variation of only ±1-3°C/km, or about 10 to 15% of the measured value in a normal heat flow environment.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.