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The source potential and organic maturity of the shales alternating with the reservoir beds in the Lower Pliocene Productive Series -the major oil-bearing unit in the South Caspian Basin -are inadequate to have formed the observed petroleum accumulations. A geochemical study of rocks collected from wells and outcrop localities has been carried out to assess the oil-and gas-generative properties of the Middle Jurassic to Lower Pliocene sediments of the basin. On the whole the section under review is characterized by poor to good organic richness and low hydrogen index, suggesting type 2 and 3 kerogens. Organic-rich sediments are relatively frequent in parts of the Oligocene-Miocene interval. Source-to-oil correlation based on carbon isotope signatures points to a largely epigenetic origin for the oils reservoired in the Lower Pliocene. Isotopically, the oils in these Productive Series reservoirs are best correlated with organic matter contained in the Miocene sediments.

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For the Caspian Sea region an analysis is given of the relationship between gas field parameters for the near-surface lithospheric layer, hydrosphere and surface atmosphere. Strong distinctions were established between mobile, highly dislocated, folded regions and tectonically stable ancient cratons, linked through tectonic control on the intensity and distance of subvertical migration of gases. Tectonic control becomes evident in studying the efficiency of near-surface (1-2 m) gas surveys-about 80% in folded regions and about 60% within ancient cratons. The efficiency of gas surveys in cratonic regions becomes comparable with that of near-surface (1-2 m) gas surveys in mobile folded regions only at depths greater than 50 m.

Two opposing points of view concerning the sources of hydrocarbons (HC) in the main reservoirs of the South Caspian Basin (SCB) Productive Series (PSLower Pliocene) reflect syngenetic or epigenetic genesis. Based on recent research results the nature of HCs in the PS is shown to be epigenetic. In the elevated onshore parts of the SCB PS Oligocene-Lower Miocene (Maikop series) rocks play a prevailing role in the formation of oil fields but in the central buried part of the SCB Middle-Upper Miocene rocks (Diatom suite) play a more dominant generation role. Нydrocarbon generation induces overpressure as shown by direct measurements in wells. The formation of HC pools in the PS occurred as a consequence of subvertical migration from underlying OligoceneMiocene deposits, characterized by impulsive injection. Two main phases of oil migration occurred during the development of commercial accumulations in the PS. The distance of subvertical migration of HCs in the SCB is estimated to be about 4 km for oils and 10-11 km for gases.