Existing views on hydrocarbon generation, migration and accumulation in the South-Caspian Basin are analyzed. A secondary nature of hydrocarbon accumulations in the major commercial interval ( Middle Pliocene) is substantiated. It is shown that the main source of oil is the underlying Paleogene-Miocene clay sequences, and that of gas and condensate is the deep Mesozoic section. The geochemical age of hydrocarbons is determined. Dominant role in the hydrocarbon accumulation and placement of the intraformational vertical migration through fractures and faults is established. A thermodynamic model of the formation and distribution of different hydrocarbon phases is developed.