New geothermal data from the Kola superdeep well SG-3

The Kola superdeep well SG-3 reaches a depth of 12,262 m. This provides the opportunity to obtain experimental data on thermal properties of deep hori zons. Since 1985, a 8578-m-deep borehole (with a casing depth of 8278 m) ha s been used as a geolaboratory. The determination of the equilibrium temper ature gradient (Gamma) and heat flow density (HFD) is more reliable than as published before, To estimate HFD, we used measurements of thermal conduct ivity (lambda) from more than 8000 core samples. Below 4000 m, rocks (schis ts, amphibolites, gneisses and plagiogranites) are characterized by signifi cant anisotropy (1.17-2.10). This was taken into account when estimating th e conductive HFD. New experimental data on HFD in the interval of 0.4-0.9 l an are in good agreement with previous results from 470- to 1675-m-deep hol es drilled near the SG-3. The 0.9-2.0-km interval shows abrupt increase in HFD up to an average value of 57 mW/m(2) which differs significantly from t he previously reported values of HFD from SG-3 (from 37 to 44 mW/m(2), as r eported by different researchers). The average HFD value of 63 mW/m(2) in t he 5.2-7.5-km interval is 17-43% higher than the previously estimated value s. Within the depth interval from 7.6 to 8.2 km HFD shows a gradual decreas e from 62 to 51 mW/m(2). Positive correlation between 1/lambda and Gamma is observed only in the intervals of 2.5-4.3 km and 5.0-6.5 lan, as can be ex pected for a stationary, conductive thermal regime. Analysis of the tempora l variations of Gamma during the return of the hole to thermal equilibrium after its completion reveals that this parameter is capable of characterizi ng the permeability of a rock massif. From the analyses of temporal variati ons of Gamma, a lower permeability can be attributed to the part of the mas sif lying in the 3.0-4.5 km depth range. It is proposed that the observed v ertical variations in HFD are due to movement of fluids in the massif in so me cases resulting in non-stationary fields. In particular, the largest geo thermal anomaly at 0-2 km depth is caused by a downward movement of meteori c waters in the zone of active water exchange. The experimental results can be understood quantitatively if a permeability of (1 to 3) x 10(-13) m(2) over a 1-2 km zone of exogenic fracturing is assumed. An abrupt change of c onductive HFD in the depth interval 1.7-2.2 km is attributed to a downward movement of fluids of 2-3 cm/yr along inclined zones of fracturing at the b oundary between igneous and sedimentary sequences. This movement of fluids might be caused by a post-glacial uplift of the Baltic Shield and would con tinue as lateral pressure in deep faults decreases. (C) 1999 Elsevier Scien ce B.V. All rights reserved.