The theoretical foundations and methods for searching of unique and large hydrocarbon deposits at great depths of the sedimentary cover have not been sufficiently developed. In earlier publications, the authors proposed a geofluid dynamic concept for the formation of industrially significant accumulations of oil and gas at depths below 6 km. According to the authors, there is an isolated autoclave-type hydrocarbon system into the Devonian-Lower Permian seismic stratigraphical sequence within the central part of the Pre-Caspian oil-and-gas province. This system’s areal boundaries are determined by the areas of distribution of deep-sea sediments. The processes of generation and accumulation of hydrocarbons have their own specific features within the autoclave hydrocarbon system. The most important of them is the spatial-temporal coupling of the foci of generation and zones of accumulation of hydrocarbons. This feature excludes secondary migration of hydrocarbons from the ontogenetic chain of deposits formation. As a consequence, the question arises as to how the reservoirs are filled with liquid and gaseous hydrocarbons and the water is displaced out of the reservoirs. The article substantiates the mechanisms of accumulation of hydrocarbons for two types of oil and gas localizing objects – underwater fans (slope and deep-water basins) and inter-basin carbonate platforms. It is shown that at the initial stage of oil and gas accumulation in the terrigenous reservoirs of the submarine fan, two interrelated factors played a significant role in the reservation of the reservoir void space by hydrocarbons: the physically conditioned possibility of gas-hydrate’s formation and the pressure of the overlying water column in the basins as a factor preventing the destruction of gas hydrate accumulations. In carbonate reservoirs, an important factor is the generation of hydrocarbons due to the realization of the own oil and gas generation potential of the host rocks. This is accompanied by the processes of secondary epigenesis of reservoir rocks and seals under the aggressive influence of pore waters and gases generated in early catagenesis, including acidic ones. Detailing of ideas about the mechanisms of reservation of the void spaces of reservoirs located within the autoclave HCS, important in explaining the large-scale localization of oil and gas in deep-submerged reservoirs without involving the mechanism of secondary migration, which takes place in a number of salt basins. The article discusses the initial stage of the void space reservation of large-scale oil and gas accumulation in intra-basin carbonate structures, as well as deep-water slopes of the fan. The issue is of great importance for the development of geofluid dynamic exploration of large oil and gas fields at great depths.
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