Two models of hydrocarbon deposits formation in the South Caspian and Middle Kura Depressions

Abstract

The model for the formation of hydrocarbon deposits in the South Caspian and Middle Kura depressions, despite a detailed study of the problem using geophysical and geochemical methods, remains a subject of debate. The model, which is shared by most researchers, assumes that the deposits in these depressions were formed according to the classical scheme of sediment compaction and horizontal-vertical migration of hydrocarbons along a system of bedding and fractures. According to seismic data, such a model of field formation, under some assumptions, corresponds to the conditions of the Middle Kura depression, where there are no vertical channels for the migration of hydrocarbon fluids formed in sediments of the Middle Eocene and partially Maikop (Oligocene-Miocene). Here, hydrocarbon deposits confined to these stratigraphic intervals of the geological section were formed as a result of intrastratal migration of fluids. Data from geological and geophysical studies show that this model does not fit well into the paleotectonic cycles of development of the South Caspian basin, where the source rocks are Oligocene-Miocene (Mayopian) rocks, and hydrocarbon deposits were discovered in Pliocene reservoirs. This depression is characterized by a model developed by the authors of the article and supported by a number of researchers, which suggests that the determining factor in the formation of oil and gas fields is the paragenetic development of mud volcanoes and traps, as well as the main routes for fluid migration are the eruption channels of mud volcanoes. The following facts are cited in favor of this hypothesis: in the South Caspian depression, all large and giant deposits are associated with eruption channels of mud volcanoes, here the development of structures and the formation of mud volcanism are synchronized and conjugate in space and time. The chemical and isotopic composition of fluids found in breccias of mud volcanoes and samples taken from deposits is almost identical. The above opinions of the authors are formulated on the basis of geological interpretation of seismic data and the results of geochemical studies. Key words: migration of hydrocarbons, faults, mud volcanism, oil and gas traps, type of reservoirs