Geothermal heat flux at the COST B-2 and B-3 Wells, U. S. Atlantic continental margin
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Heat flow estimates at two sites on the U.S. Atlantic continental margin are presented. An estimate of the heat flowing from the basement also has been obtained. About 4.8 km of sediments penetrated at the COST B-2 and 4.0 km at the COST B-3 were deposited since the Upper Jurassic. Well logs were used to evaluate thermal gradients and sedimentation rates, whereas thermal conductivities and radiogenic heat productions were measured on drill cuttings samples. A procedure to estimate in-situ thermal conductivity from drill cuttings and well logs is described. A substantial set of samples, in the form of drill cuttings, were sorted in four major lithologies: sandstones, siltstones, shales and limestones. Laboratory measurements of density, porosity, thermal conductivity, quartz (%), potassium (%), uranium (ppm) and thorium (ppm) were performed on 128 reorganized and pulverized samples. A significant correlation of the matrix thermal conductivity to quartz and potassium content was found. In situ porosity and volume fraction of each lithology, determined mainly from well logs, were used to calculate in situ mean thermal conductivity. Finally the mean in situ vertical component of the thermal conductivity, as required for heat flow values, has been estimated from a correction factor for the anisotropy of each lithology. The in-situ temperature and anisotropy effects substantially decrease estimates of thermal conductivity at depth. Below the uppermost 1 km in both wells the best estimate of the thermal gradient is 26.3°C km- 1 at COST B-2 and 26.1°C km- 1 at COST B-3, whereas in situ mean thermal conductivities range between about 1.8 and 1.9 W m- 1 K- 1 (4.3-4.5 T.C.U.). The average heat flow is estimated as about 45 mwm- 2 (1.07 H.F.U.) at COST B-2 and 44 mWm- 2 (1.06 H.F.U.) at COST B-3, with an uncertainty of about 20-25%. The mean radiogenic production in sediments at the two sites has been estimated as 1.83 (COST B-2) and 1.44 (COST B-3) 10- 6Wm- 3 • With a 12-14 km thick sedimentary sequence a radioactive contribution of 20-25 mWm- 2 can be expected. The effects of sediment deposition, compaction, pore water advection and radiogenic heat production have been combined in a numerical model (Hutchison, 1985) to estimate the undisturbed basement heat flux. Although the sedimentation depresses the basement heat flux by 15-20%, this effect is more than compensated by radioactive heat production in the sediments, so that the surface flux is estimated to be higher than that from the basement. The latter is calculated at about 33-39 mwm- 2 (0.8-0.9 H.F.U.), a relatively low value. The overall uncertainity is about ± 20-25%, and other estimates on continental margins with thick sediments (e.g. Reiter and Jessop, 1985) probably have at least a similar uncertainty.