Analyses data on the composition, structure and formation of
various petroleum hydrocarbons: the alkanes, cycloalkanes
and arenes. Attention is paid to biological markers, com-
pounds that may have preserved the main structural features
of the original biogenic molecules. Concepts of chemical
classification of crude oils are reviewed with respect to
the molecular mass distribution of biological markers, and
the genesis and chemical evolution of petroleum hydrocarbons
are discussed.