Heat flow and subsurface temperature as evidence for basin-scale ground-water flow, North Slope of Alaska
In conjunction with the U.S. Geological Survey's exploration program in the National Petroleum Reserve, Alaska (NPRA) several high-resolution temperature logs were made in each of 21 drillholes between 1977 and 1984. These time-series of shallow (average 600-m depth) temperature profiles were extrapolated to infinite time to yield equilibrium temperature profiles (±0.1 °C). Thermal gradients are inversely correlated with elevation, and vary from 22 °C/km in the foothills of the Brooks Range to as high as 53 °C/km on the coastal plain to the north. Shallow temperature data were supplemented with 24 equilibrium temperatures (±3-5 °C) estimated from series of bottom-hole temperatures (BHTs) measured near the bottom of petroleum exploration wells. A total of 601 thermal conductivity measurements were made on drill cuttings and cores. Near-surface heat flow (±20%) is inversely correlated with elevation and ranges from a low of 27 mW/m2 in the foothills of the Brooks Range in the south, to a high of 90 mW/m2 near the north coast. Subsurface temperatures and thermal gradients estimated from corrected BHTs are similarly much higher on the coastal plain than in the foothills province to the south. Significant east-west variation in heat flow and subsurface temperature is also observed; higher heat flow and temperature coincide with higher basement topography. The observed thermal pattern is consistent with forced convection by a topographically driven ground-water flow system; alternative explanations are largely unsatisfactory. Average ground-water (Darcy) velocity in the postulated flow system is estimated to be of the order of 0.1 m/yr; the effective basin-scale permeability is estimated to be of the order of 10-14 m2. Organic maturation data collected in other studies indicate that systematic variations in thermal state may have persisted for tens of millions of years. The ground-water flow system thought to be responsible for present heat-flow variations conceivably has existed for the same period of time, possibly providing the driving mechanism for petroleum migration and accumulation at Prudhoe Bay.