We have presented the results of a study of core samples which had been recovered from a deep borehole after the second unsealing of subglacial Lake Vostok in 2015 (borehole 5G-3, in which a mixture of kerosene and HCFC-141b densifier had been used as the drilling fluid). The intensive mixing of the subglacial water and the drilling fluid resulted in the formation of a solid plug that filled the volume of the borehole and blocked the access to the lake. We have demonstrated that this solid plug consists of kerosene, ice and clathrate hydrate of HCFC-141b. The obtained data suggests that the drilling fluid presently used at Vostok should be replaced, at least in the bottom section of the hole, by another fluid which does not react with subglacial water.