The influence of the heat carrier on the thermal stability and composition of high-viscosity crude oil under formation conditions has been determined for the case of Permian productive deposits of the Mordovo-Karmal＇ field with the use of core material from zones of heat effects, including steam and heat treatment and in situ combustion. Modelling of petroleum displacement under near-formation conditions was carried out on an experimental flow-type unit with the use of an oil-saturated core sample. Gaseous media exhibiting different chemical activity towards the organic matter of rock were used as the heat carriers: an oxygen-containing atmosphere, and eluants of similar composition to the steam-gas mixtures produced by a well turbine steam-gas generator. Features of the change in the composition of crude oils extracted and the increase in their thermal stability with the use of steam and gas heat carriers exhibiting lower chemical activity towards the organic matter of rock have been established.