The equilibrium thermodynamie calculations made for C-H and C-H-N-O-S systems show that the mantle might contain a fluid consisting of heavy (high-molecular) hydrocarbons (HHC) and their derivatives, as well as of inorganic gases (H_2O, CO_2, N_2, NH_4, H_2S, etc.). In the range of diamond-graphite phase transition, the HHC are decomposed to form methane, its nearest homologues, and graphite. The geochemical and experimental data by different authors point to possible HHC occurrence in the upper mantle. These results confirm the concept by Dmitrievskii (2006) that the deep fluids might be lifted to the crustal waveguide to fill its fractures and cavities. Owing to compaction, the fluids are squeezed from the waveguide and lifted into sedimentary layers to form the commercial hydrocarbon deposits. Resulting from the "washing" of sedimentary rocks with the fluids, hydrocarbon gases and oil of organic origin might be formed.
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