Thermodynamic Model of Deep Oil Origin and its Phase “Freezing”

In most modern studies of lithospheric (petrogenic) carbonreservoirs in the earth’s crust, it is assumed that crude oil and natural gas (petroleum) are thermal generation products from the relics of biological organic matter accumulated in sedimentary rocks during geological time and deeply buried in a region of high pressure and temperature. In this sedimentary-migration (“biogenic”) concept of the origin of oil, the direction of the proposed evolutionary process of carbon transformation was determined: buried biological material → kerogen → oil → gas as a manifestation of progressive metamorphism (pressure and temperature increase). However, the discovery of kerogen in the meteorite’s composition does not allow us to suggest a biological source of carbon for the formation of this polymeric “organic” substance, but in turn allows us to suggest inorganic sources of kerogen, namely “oil” and “gas"non methane hydrocarbons (HCs), originated in the depths of their parent bodies (icy planetesimals). The genetic relationship of oil, natural gas and carbon matter of black shale formations (kerogen) on Earth is also beyond doubt, and therefore, in this paper, the evolution of petrogenic carbon reservoirs, including oil shale rocks in the lithosphere, is considered on the basis of a deep inorganic concept, in which the direction of the carbon transformation process is the opposite of the biogenic concept and is represented as HCs → gas → oil → kerogen. The analysis of phase diagrams and experimental data made it possible to determine two trends in the evolution of non-methane hydrocarbons in the Earth’s interior. In the upper mantle, the “metastability” of heavy (with a lower H/C ratio) HCs increases with depth. However, at temperatures and pressures corresponding to the surface mantle-crustal hydrothermal conditions, the “relative metastability” of heavy hydrocarbons increases with approach to the surface. When deep HCs fluids rise to the surface, petrogenic oil reservoirs are formed as a result of the decreases in hydrogen fugacity and a phase transition: gas HCs → liquid oil. At the physical and chemical conditions of an oil reservoir, metastable reversible phase equilibria are established between liquid oiland H2O, gas HCs and CO2, and solid (pseudocrystalline) “mature” and “immature” kerogens of “oil source” rocks. A decrease in hydrogen pressure and temperature leads to a stoichiometric phase transition (“freezing”) of liquid oil into solid kerogens. This occurs as a result of oil dehydrogenation in the processes of high-temperature CO2 fixation and low-temperature hydration of oil hydrocarbons, which are the main geochemical pathways for its transformation into kerogen. Thus, the formation of carbon matter in petrogenic reservoirs is the result of regressive (retrograde) metamorphism of deep hydrocarbon fluids, natural gas, liquid oil, and naphthide accumulations.