New geothermal data from the Kola superdeep well SG-3

Abstract

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 horizons. Since 1985, a 8578-m-deep borehole (with a casing depth of 8278 m) has been used as a geolaboratory. The determination of the equilibrium temperature gradient (Γ) and heat flow density (HFD) is more reliable than as published before. To estimate HFD, we used measurements of thermal conductivity ( λ) from more than 8000 core samples. Below 4000 m, rocks (schists, amphibolites, gneisses and plagiogranites) are characterized by significant anisotropy (1.17–2.10). This was taken into account when estimating the conductive HFD. New experimental data on HFD in the interval of 0.4–0.9 km are in good agreement with previous results from 470- to 1675-m-deep holes 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 the previously reported values of HFD from SG-3 (from 37 to 44 mW/m 2, as reported by different researchers). The average HFD value of 63 mW/m 2 in the 5.2–7.5-km interval is 17–43% higher than the previously estimated values. Within the depth interval from 7.6 to 8.2 km HFD shows a gradual decrease from 62 to 51 mW/m 2. Positive correlation between 1/ λ and Γ is observed only in the intervals of 2.5–4.3 km and 5.0–6.5 km, as can be expected for a stationary, conductive thermal regime. Analysis of the temporal variations of Γ during the return of the hole to thermal equilibrium after its completion reveals that this parameter is capable of characterizing the permeability of a rock massif. From the analyses of temporal variations of Γ, a lower permeability can be attributed to the part of the massif lying in the 3.0–4.5 km depth range. It is proposed that the observed vertical variations in HFD are due to movement of fluids in the massif in some cases resulting in non-stationary fields. In particular, the largest geothermal anomaly at 0–2 km depth is caused by a downward movement of meteoric waters in the zone of active water exchange. The experimental results can be understood quantitatively if a permeability of (1 to 3) × 10 −13 m 2 over a 1–2 km zone of exogenic fracturing is assumed. An abrupt change of conductive 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 boundary between igneous and sedimentary sequences. This movement of fluids might be caused by a post-glacial uplift of the Baltic Shield and would continue as lateral pressure in deep faults decreases.