Regional permeability estimates from investigations of coupled heat and groundwater flow, north slope of alaska

Abstract

Earlier studies of terrestrial heat flow in the North Slope Basin, Alaska, found that heat flow varies systematically in a trend perpendicular to the strike of basin strata and the neighboring Brooks Range. Heat flow (∼±20%) increases from a low of 27 mW/m2 in the foothills of the Brooks Range in the south to a high of 90 mW/m2 on the coastal plain to the north. The thermal pattern can be explained by a regional‐scale (∼330 km) groundwater flow system which transports heat by advection from regions of high elevation in the Brooks Range and its foothills to lower elevations on the Arctic coastal plain. Permeability data from 2031 core measurements made parallel to bedding and 15 well tests were compiled for 10 geologic units. Arithmetic‐mean permeabilities derived from measurements on core samples range from 2.2 × 10−13 m2 for sandstones of the Endicott Group to 1.1 × 10−16 m2 for limestones of the Lisburne Group. The arithmetic‐mean permeability derived from all 2031 core measurements made parallel to bedding is 6.1 × 10−14 m2. A numerical model of coupled heat and fluid flow in the North Slope Basin was constructed and a series of model simulations were conducted. A model simulation incorporating permeability data obtained from core measurements resulted in a good match to observed heat flow data, apparently suggesting that permeability in the North Slope Basin does not increase significantly from the core scale (∼10−2‐ 10−1 m) to the basin scale (∼105 –106 m). This inference, however, is complicated by the possible effects of factors such as sample bias in measurements and choice of an appropriate averaging algorithm. A further series of model simulations were done in which the specified model permeability was homogeneous and anisotropic. Comparisons of heat flow predicted by these simulations with heat flow determined in field studies suggested that the effective basin‐scale permeability parallel to bedding (kx) is in the range of 2.5 × 10−14 ≤ Kx ≤ 2.5 × 10−13 m2 and permeability perpendicular to bedding (kz) is in the range of 1.0 ×10−16 ≤ kz ≤ 5.0 × 10−16 m2. These constraints depend upon the explicit assumption that groundwater flow is the only mechanism responsible for heat flow variations across the North Slope Basin.