Estimates of terrestrial heat flow in offshore eastern Canada: Discussion

Abstract

In a recent paper, Reiter and Jessop estimated heat flow for offshore eastern Canada based on corrected well temperature data and assunled thermal conductivity values for the continental margin sediments. They concluded that fluid movement from depth was the likely cause for their high and intermediate estimates of heat flow. We do not dispute their corrected temperature data, which are in close agreement with values from Issler (1984) for wells on the Scotian Shelf. Wowever, we believe that their assumed thermal conductivity values are too high and that consequently their estimates of heat flow are also erroneously high. Thermal models incorporating measured sediment thermal properties from offshore eastern Canada (Beaumont et al. 1982: Mackenzie et al. 1985; Issler and Beaumont, in press; Keen and Beaumont, in press) predict values of heat flow that are significantly lower than the estimates of Reiter and Jessop (see Table I). These models, based on variations of McKenzie's ( 1978) kinematic stretching model, assume that heat moves vertically by thermal conduction. Grain matrix values for sediment thermal conductivity and radiogenic heat production were derived from laboratory measurements on borehole samples from these regions. Model predictions are constrained by, and show good agreement with, observed organic geochemical indicators and corrected bottom-hole temperatures. The difference between our interpretation of the heat-flow data and that of Reiter and Jessop is important in that heat-flow and temperature data are often used as indirect evidence of fluid-flow regimes in sedimentary basins. The implication of our results is that fluid flow does not currently play a major role in heat transport in the regions studied. We wish to demonstrate the importance of knowing the in situ thermal properties of a sediment column and the errors that may result from using assumed values. Table 1 shows a comparison between estimates of thermal conductivity, thermal gradient. and heat flow from Reiter and Jessop and computed values from our thermal models for selected wells from the Nova Scotia and Labrador shelves. Reiter and Jessop's estimated and preferred values of heat flow are listed along with thermal gradient and conductivity data used for various depth intervals in each well. Results from the model of Mackenzie et al. (1985) for the Seotian Shelf are also listed in Table 1. For this model, sediment thermal properties were averaged for the entire sediment column so corresponding thermal gradients are uniform with depth. Models A and B differ in the suite of matrix conductivity values used (Mackenzie et al. (1985, Table 3). The Labrador results (Table 1) are from Issler and Beaumont (in press). In this model, thermal properties vary with depth,