Purpose. Finding the connection between the thermodynamic conditions of hydrocarbon mixtures synthesis and zones of oil and gas accumulation. Methodology. The thermodynamic eguilibrium depths of gas and gas-condensate deposits were fixed by the method of eguilibrium constants of independent reactions based on the chemical composition of hydrocarbons re-counted on chemical elements. The investigations of formation waters, natural gases and water-dissolved gases were based on the methods of chemical, elementary spectral, atomic absorption and gas chromatographic analysis. Results. Received data point to essential differences in depths of a thermodynamic equilibrium for the deposits of Eocene, Maikopian, and Neogene on the one hand, and Lower Paleocene and Cretaceous sediments on the other. The first ones are characterized by values of depths in boundaries from 30 to 50 km while the second ones show a similar parameters in the boundaries from 120 to150 km. The analysis of the geological, hydrogeological and geochemical circumstances of the Pre-Black Sea aquiferous basin (PBSAB) testified to that the gas-vapour mixtures that are the sources of the Upper Cretaceous, Paleocene and Maikopian gas deposits are formed in high-temperature zone (about 300 C) of depth origin. We consider the Lower Paleocene fields as primary formed only if gases rapidly migrated vertically (in free phase) from high-temperature places of their generation. The ways of possible migration were sublatitudinal faults and zones of decompression. The hydrocarbon fields in Maikopian and even Miocene sediments show the further way of vertical migration of gas and its accumulation in the traps throughout the whole way. Based on the analysis of the hydrogeological, hydro-and gas-geochemical investigations and thermodynamic calculations the model of gas fields forming in PBSAB was carried out. Originality. Hydrogeological data and thermodynamic calculations indicate the depth of origin of hydrocarbons. We suppose that differences in equilibrium depths between the fields of various deposits are coherent with peculiarities of processes of their filling with fluids. The maximum localization of gas-condensate fields at depths of 1900-3000 m testifies not to hydrocarbons formation in this range of depths, but to optimum geology-hydrogeological conditions of forming and of preservation deposits. The migration and preservation of hydrocarbons demand essentially various geology-physical and hydrogeological circumstances: the first is favoured by dynamics of the hydrostatic pressure systems, active tectogenesis, high temperatures, low mineralization of waters; guasistagnat environment, moderate temperatures, presence of the reservoirs that overlapped by reliable seals are favourable for the other. The zone of oil and gas accumulation is related to elisional systems; and zone of through migration to thermohydrodynamic water drive systems. Practical significance. The received data allow forecasting composition of a hydrocarbonaceous component of a field proceeding from its proximity to a decompressions zone and depth of occurrence. We guess that with depth the amount of heavy alkanes will be incremented. The gas condensate fields, on depths more than 1900 m, have a plutonic genesisis.
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