Thermal conductivity analysis of Cenozoic, Mesozoic and Paleozoic core samples, Beaufort-Mackenzie Basin, northern Canada

Abstract

One hundred and sixty-two vertically-oriented (parallel to core axis and direction of heat flow), one-inch (2.54 cm) diameter core plugs were obtained from conventional cores from 43 petroleum exploration wells in the Beaufort-Mackenzie Basin. Samples were collected from Upper Cretaceous-Oligocene post-rift and Jurassic-Lower Cretaceous syn-rift sandstone and shale, and Paleozoic pre-rift sandstone, shale and carbonate. Multiple disk samples (approximately one cm in thickness) were cut from these plugs and used for thermal conductivity analysis. Samples were measured dry and saturated with heptane (three samples were water-saturated) using the steadystate divided-bar apparatus at the Geological Survey of Canada in Calgary. In general, each disk sample was measured in triplicate by two different analysts and results were averaged for each set of replicate disk samples to yield 136 dry, 129 heptane-saturated and three water-saturated bulk conductivity values for samples from 39 wells (some samples disintegrated during preparation and analysis). Sample porosity values for the first analyst (initial measurements) are systematically higher than those for the second analyst. A comparison of dry and saturated density data suggests that residual heptane saturation (from incomplete oven drying) has resulted in higher dry density values and lower porosity values for the second analyst and therefore the values of the first analyst are considered to represent the true porosity. Rock matrix thermal conductivity values were calculated for each sample using six different conductivity models: series, parallel, Sugawara and Yoshizawa, self-consistent effective medium theory, Maxwell/Hashin-Shtrikman and weighted geometric mean. The weighted geometric mean model provides the most consistent fit to the thermal conductivity data for all lithologies and for the full range of porosity values (0 to 25 %). Average geometric mean rock matrix values for dry and heptane-saturated thermal conductivity measurements generally agree to within 0.1 W/mK or better for sandstone, shale and carbonate samples from different age/tectonic groupings. Post-rift lithic sandstone and pre-rift sandstone samples are relatively homogeneous with average rock matrix thermal conductivity values of 2.5 W/mK and 3.1 W/mK, respectively. The syn-rift sandstone samples are more heterogeneous with matrix conductivity values ranging from approximately 2.2- 3.5 W/mK. The lowest matrix conductivity (0.8 W/mK) was determined for organic-rich, post-rift shale samples from the Smoking Hills sequence. Organically-lean post-rift and syn-rift shale samples have similar matrix conductivity values of 1.65 W/mK and 1.6 W/mK, respectively, whereas pre-rift shale samples have a higher average matrix value (2.1 W/mK). The average matrix conductivity for pre-rift carbonate samples is approximately 2.6 W/mK with dolomite samples having a higher average matrix conductivity (2.7 W/mK) than the limestone samples (2.5 W/mK). Water-saturated bulk thermal conductivity (Kwater) values were calculated using the geometric mean matrix conductivity values in order to estimate in situ thermal conductivity for the various rock successions examined. Post-rift sandstone and shale samples have Kwater values ranging from 1.3- 2.5 W/mK and 0.7-2.1 W/mK, respectively. Kwater values range from 1.5-3.4 W/mK and 1.0-2.3 W/mK for syn-rift sandstone and shale samples, respectively. For the pre-rift succession, Kwater values range from 2.1-3.4 W/mK for sandstone, 1.7-2.3 W/mK for shale and 2.0-3.2 W/mK for carbonate samples.