Geothermal characteristics of the Vorotilovo deep borehole drilled into the Puchezh-Katunk impact structure

Abstract

The Vorotilovo borehole, 5374 m in depth, was drilled in 1989–1992 in the central part of the large Puchezh-Katunk impact structure, which is located in the East European Platform (lat. 57.1°N, long. 43.6°E). The geothermal studies of the borehole section were based on the temperature logging of the entire borehole (1992–1995), on the thermal conductivity measurements for more than 3700 air-dry and fluid-saturated core samples under normal PT conditions and for 473 samples at a temperature up to 100°C; the rock anisotropy was taken into account in these measurements. Significant local vertical variations (±20–30% even within short depth intervals of 100–200 m) and a regular increase of conductivity from 0.8–1.2 W m −1 K −1 to 2.5–3.0 W m −1 K −1 with depth were established. It was shown that conductivity variations are closely related to the grade of shock-thermal metamorphism. The thermal conductivity of metamorphic rocks from the basement of the Vorotilovo Uplift appeared to be substantially lower (up to 40%) than the conductivity of similar Archaean rocks from Ukrainian and Baltic shields, which we have investigated previously. After the drilling had been completed, the thermal regime of the rock massif was restored significantly faster than was inferred from previous theoretical considerations. By September 1995 it was generally stabilized. Substantial vertical variations were recorded for all geothermal characteristics. The section can be subdivided into three segments with distinct heat flow densities: the interval down to 1200 m with the least heat flow density (22–31 mW/m 2, i.e. 50–60% of the highest value), the 1200–1900 m interval (34–42 mW/m 2, i.e. 75–85% of the highest value), and the 1900–5300 m interval with the highest density (40–56 mW/m 2). Appreciable local variations of the heat flow density obviously are caused by the refraction of heat flow at structural elements of the massif. The character of mutual change of geothermal parameters in certain intervals testifies to the non-steady thermal regime and fluid filtration. The value of terrestrial heat flow density for the drilling site was estimated as 52–58 mW/m 2. Calculations show that in the depth range of 400–2000 m the effect of palaeoclimate causes a reduction of heat flow density of approximately 25%. This cannot exhaustively account of the recorded vertical variation of heat flow density and allows us to suppose an effect of mass transfer or non-steady thermal regime in the upper part of the section.