Natural oil- and bitumen-bearing rock samples from Tatarstan multiage deposits with different concentrations of organic matter (OM) and different compositions and types of hydrocarbon fluids have been investigated. It has been shown that hydrothermal treatment in the flow mode in a reducing atmosphere leads to a decrease in the amount of soluble and insoluble OM in the rocks and improves the quality of recovered hydrocarbon fluids owing to a regular decrease in the concentration of the resinous-asphaltenic components, total sulfur, vanadyl porphyrin complexes, and trace elements, and an increase in the proportion of lighter hydrocarbons. Characteristic features of the hydrothermal transformation of the OM of domanik bituminous rock associated with the degradation of insoluble kerogen, which is an additional source of hydrocarbons and heteroatomic compounds, have been revealed.