Heat flow in the Uinta Basin determined from bottom hole temperature (BHT) data

Abstract

The thermal resistance (or Bullard) method is used to judge the utility of petroleum well bottom‐hole temperature data in determining surface heat flow and subsurface temperature patterns in a sedimentary basin. Thermal resistance, defined as the quotient of a depth parameter Δz and thermal conductivity k, governs subsurface temperatures as follows: [Formula: see text] where [Formula: see text] is the temperature at depth z=B, [Formula: see text] is the surface temperature, [Formula: see text] is surface heat flow, and the thermal resistance (Δz/k) is summed for all rock units between the surface and depth B. In practice, bottom‐hole and surface temperatures are combined with a measured or estimated thermal conductivity profile to determine the surface heat flow [Formula: see text] which, in turn, is used for all consequent subsurface temperature computations. The method has been applied to the Tertiary Uinta Basin, northeastern Utah, a basin of intermediate geologic complexity—simple structure but complex facies relationships—where considerable well data are available. Bottom‐hole temperatures were obtained for 97 selected wells where multiple well logs permitted correction of temperatures for drilling effects. Thermal conductivity values, determined for 852 samples from 5 representative wells varying in depth from 670 to 5180 m, together with available geologic data were used to produce conductivity maps for each formation. These maps show intraformational variations across the basin that are associated with lateral facies changes. Formation thicknesses needed for the thermal resistance summation were obtained by utilizing approximately 2000 wells in the WEXPRO Petroleum Information file. Computations were facilitated by describing all formation contacts as fourth‐order polynomial surfaces. Average geothermal gradient and heat flow for the Uinta Basin are [Formula: see text] and [Formula: see text], respectively. Heat flow appears to decrease systematically from 65 to [Formula: see text] from the Duchesne River northward toward the south flank of the Uinta Mountains. This decrease may be the result of refraction of heat into the highly conductive quartzose Precambrian Uinta Mountain Group. More likely, however, it is related to groundwater recharge in late Paleozoic and Mesozoic sandstone and limestone beds that flank the south side of the Uintas. Heat flow values determined for the southeast portion of the basin show some scatter about a mean value of [Formula: see text] but no systematic variation.