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This study is based on the isotope-geochemical characteristics of oils from 53 fields in the South Caspian Basin – from Upper Cretaceous, Eocene, Oligocene–Lower Miocene (Maykop), Middle and the Upper Miocene (Diatom) and Pliocene. The isotopic composition of carbon (δ 13 C) allows two main groups of oils to be identified: (1) isotopically heavy and (2) isotopically light oils. The first group includes oils assumed to be generated by Upper Cretaceous, Eocene and Oligocene–Lower Miocene sources. The second group consists of oils assumed to be generated by Diatom (Middle and Upper Miocene) sources. Isotope-geochemical and biomarker parameters demonstrated that oils in the Pliocene reservoirs are not syngenetic to their enclosing deposits. The isotopic composition of carbon in oils and hydrocarbon gases in mud volcanoes and fields and the biomarker parameters of oils allow several isolated stratigraphic oil- and gas-producing complexes (source rocks) in the Mesozoic, Palaeogene–Lower Miocene and Middle–Upper Miocene to be postulated. Oils in the Pliocene reservoirs and in mud volcanoes consist of mixed oils generated by the Pre-Diatom and Diatom deposits and of oils from only one of the above-mentioned complexes.

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