Present Heat Flow Along a Profile Across the Western Canada Sedimentary Basin: The Extent of Hydrodynamic Influence

Abstract

Compilation of bottom-hole temperature (BHT) data (3,466 readings) and their correction along the 680-km profile from the Rocky Mountain Foothills to the western edge of the Canadian Shield has allowed detailed analysis of the 2D temperature distribution, calculation of the thermal gradient, and estimate of heat flow. Temperature data are available typically from only a section of the depth interval making 2D contouring of the temperature field difficult. Surface temperature values were used in calculations of the thermal gradient. The quality of the BHT data deteriorates towards the shallow part of the basin where larger errors of heat-flow estimates of the profile are calculated. Estimated heat flow increases from the southwest towards the northeast. Anomalously high values between 300 km – 500 km of the profile are questionable.

Thermal gradient values based on BHTs and resulting estimates of heat flow have been compared with calculated thermal gradient and heat flow from 2D steady-state models of coupled fluid flow/heat flow in saturated porous media along the profile for 5 situations of hydraulic conductivity in the major aquifers. The results of this comparison show that, for uniform basement heat flow (assumed 70 mW/m²), it is difficult to explain simultaneously the observed heat flow lows and highs by the same fluid-flow system within the range of acceptable permeabilities.

Other possible explanations of high thermal gradients based on BHTs in the shallow part of the basin need to be sought. It has been determined that the effective thermal conductivity can be a partial explanation especially if the influence of compaction of the shales is considered. An alternative explanation is the systematic overestimate of temperature by BHTs in the shallow northeastern part of the basin. The comparison of the shallow basin temperature data with high-precision temperature measurements in two areas in the shallow part of the basin shows that the industrial temperatures tend to be systematically higher. This disagreement is interpreted to be a result of the systematic error in shallow BHT’s. The validity of thousands of industrial BHT’s from shallow depths (from 500 to 900 m) documenting the regional thermal high is in question. This observation calls for reevaluation of thermal gradient maps based on BHTs only; especially in the shallow part of the basin towards the shield where anomalously high thermal gradients (>40 mK/m) have been reported previously.

Precise temperature measurements in three sites in the shallow part of the basin and the geothermal gradient map based on shut-in well temperatures in oil pools show that the change from low gradients in the Foothills (<25mK/m) to highs in the foreland basin (>35mK/m) takes place over the horizontal distance of less than 300 km and the latter change can be explained easier by the hydrodynamic heat flow coupled system based on a 2D model.