Hydrothermal alterations in the composition of heavy petroleum and organic matter of rocks from producing carbonate reservoirs of the Middle-Carboniferous Bashkirian stage upon heat treatment at 360 degrees C in the presence of water and hydrogen were studied. It was shown that the soluble portion of organic matter remaining in the rocks after thermal treatment became prevalent as a result of intense degradation of the insoluble organic part of kerogen. This leads to an increase in the production index of these rocks. In the group composition of petroleum and rock extracts, the amount of saturated and aromatic hydrocarbons increases and that of resinous-asphaltic components and solid n-alkanes decreases. Among biomarker hydrocarbons, the relative amount of tricyclic terpanes increases with a simultaneous decrease in the proportion of steranes and pentacyclic triterpanes; the amount of pregnane relative to C-27 alpha-sterane (20R) and the amount of rearranged steranes relative to regular steranes also increase. The results of investigations suggest that, by the action of hydrothermal processes, hydrocarbons of petroleum and organic matter from carbonate collectors undergo structural transformations and isomerization, along with thermal degradation, characteristic of natural catagenetic processes catalyzed by clay materials. Thus, it was concluded that the heat treatment may facilitate oil recovery from carbonate rocks and improve the quality of heavy petroleum.