Heat Flow in the Utah‐Wyoming Thrust Belt From Analysis of Bottom‐Hole Temperature Data Measured in Oil and Gas Wells

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Abstract
Bottom‐hole temperature (BHT) data from oil and gas wells represent a large, albeit low‐quality, data base for heat flow determinations. A method for analyzing BHT data sets that directly addresses the problem of noise in the data is presented and illustrated by application to 303 BHTs from the Anschutz Ranch (AR), Cave Creek (CC), and Anschutz Ranch East (ARE) oil/gas fields in the Utah‐Wyoming thrust belt, located in the Middle Rocky Mountain physiographic province of the western United States. Correction of raw BHT data by means of a Horner plot yields 81 corrected BHTs. An additional 90 BHTs, not suitable for a Horner correction, are corrected by means of a new BHT correction which may be applied to any single BHT for which a depth and time of measurement are known. The average geothermal gradient at both AR‐CC and ARE is found to be 23°C/km, although an average‐gradient analysis fails to explain scatter in the data. Analysis of corrected BHTs, through inversion of a layered‐earth model, yields estimates of formation thermal gradients and temperature fields at AR‐CC and ARE that are stable and unique, with an average precision of 2° to 3°C. Thermal conductivity measurements were made on 360 drill cutting samples representing 14 thrust belt formations; in situ conductivity is estimated using porosity from calibrated well logs. Heat flow is estimated by comparing temperature calculated by a two‐dimensional finite element model with temperature estimated from analysis of BHT data. Analysis of BHT data from the AR‐CC area yields an estimated regional heat flow of 60 mW/m2 (± 8 mW/m2); a similar analysis at ARE results in an estimate of 58 mW/m2 (± 8 mW/m2). These estimates are close to values determined for the Wyoming Basin and the central Colorado Plateau and imply that a thermal transition from the Middle Rocky Mountains to elevated heat flow characteristic of the Basin and Range province (80 to 90 mW/m2) must take place within 50 km.