GEOTHERMAL CHARACTERISTICS OF THE VOROTILOVO DEEP BOREHOLE DRILLED INTO THE PUCHEZH-KATUNK IMPACT STRUCTURE

The Vorotilovo borehole, 5374 m in depth, was drilled in 1989-1992 in the central part of the large Puchezh-Katunk impact structure, which i s located in the East European Platform (lat. 57.1 degrees N, long. 43 .6 degrees E). The geothermal studies of the borehole section were bas ed on the temperature logging of the entire borehole (1992-1995), on t he thermal conductivity measurements for more than 3700 air-dry and fl uid-saturated core samples under normal P-T conditions and for 473 sam ples at a temperature up to 100 degrees C; the rock anisotropy was tak en into account in these measurements. Significant local vertical vari ations (+/-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 conductivi ty variations are closely related to the grade of shock-thermal metamo rphism. The thermal conductivity of metamorphic rocks from the basemen t of the Vorotilovo Uplift appeared to be substantially lower (up to 4 0%) than the conductivity of similar Archaean rocks from Ukrainian and Baltic shields, which we have investigated previously. After the dril ling had been completed, the thermal regime of the rock massif was res tored significantly faster than was inferred from previous theoretical considerations. By September 1995 it was generally stabilized. Substa ntial vertical variations were recorded for all geothermal characteris tics. The section can be subdivided into three segments with distinct heat flow densities: the interval down to 1200 m with the least heat f low density (22-31 mW/m(2), i.e. 50-60% of the highest value), the 120 0-1900 m interval (34-42 mW/m(2), i.e. 75-85% of the highest value), a nd the 1900-5300 m interval with the highest density (40-56 mW/m(2)). Appreciable local variations of the heat flow density obviously are ca used by the refraction of heat flow at structural elements of the mass if. The character of mutual change of geothermal parameters in certain intervals testifies to the non-steady thermal regime and fluid filtra tion. 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 r ange of 400-2000 m the effect of palaeoclimate causes a reduction of h eat flow density of approximately 25%. This cannot exhaustively accoun t of the recorded vertical variation of heat flow density and allows u s to suppose an effect of mass transfer or non-steady thermal regime i n the upper part of the section. (C) 1998 Elsevier Science B.V. All ri ghts reserved.