FINE‐SCALE CORRELATION BETWEEN TEMPERATURE GRADIENT LOGS AND LITHOLOGY

Abstract

Two high‐precision continuous temperature gradient logs have been produced in a 600-m partially cased, water‐filled borehole for which a nearly complete core is available. The agreement between these gradient logs, obtained using a logging speed of 8 m/minute, is good; there is virtually no dc offset (less than 0.03°C/km), and the mean absolute error is approximately 0.5°C/km. A comparison of these profiles with detailed geologic logs of the core shows excellent precision and resolution. In situations where the thermal resistivity contrast is 50–100 percent, isolated strata as thin as 0.5 m are clearly indicated on the temperature gradient log. More subtle lithologic changes extending over larger depth intervals are also detectable. In several instances gradient contrasts on the order of 1°C/km extending over several meters have been clearly resolved. Comparison of the gradient logs with a thermal resistivity profile from laboratory measurements on the core material indicates that the steel well casing has no observable effect on the temperature gradient profile.