"Oil shales are defined according to economic criteria they are rocks yielding commercial amounts of liquid hydrocarbons upon destructive distillation. They contain an organic matter (kerogen) similar to that of petroleum source-rocks. Specific geochemical characteristics of oil shales, as compared to petroleum source-rocks are presented hereunder: low natural thermal evolution, high carbon content, high hydrogen amount of the organic matter. The evolution of some geochemical parameters during pyrolysis is shown for different oil shales. Finally, specific features of the composition of shale oils compared to that of natural petroleum are indicated: presence of unsaturated hydrocarbons , higher amount of hetroatomic compounds."