On the example of oils in the Apsheron oil- and gas-bearing region and Apsheron archipelago located in the western part of the Southern Caspian depression, the authors have developed mathematical models of a group hydrocarbon composition; interconnections between oil density and content of asphalt-resin materials, benzine, and ligroin; interconnections between oil density and viscosity and temperature; and interconnections between content of asphalt-resin properties and low-temperature fractions. The models obtained enabled us to extrapolate factual data on composition and properties of oils beyond the limits of fixed depths of burial of oil-saturated reservoirs both to a zone of great depths and increased temperatures where hydrocarbons were in a gaseous or oil and gaseous state, and to a zone of near-surface conditions where oils acquire the consistency of asphalts.