Abstract

The classification of crude oils is based on their content of paraffins, naphthenes and aromatic compounds (aromatic hydrocarbons plus resins and asphaltenes). Six classes are distinguished. The main classes of normal crude oils are the paraffinic and paraffinic-naphthenic oils, containing both less than 1% sulfur, and the aromatic-intermediate oils, containing more than 1% sulfur.

Marine organic matter usually generates paraffinic-naphthenic or aromatic-intermediate crude oils. Terrestrial organic matter, derived from plants, generates paraffinic, or sometimes paraffinic-naphthenic, crude oils.

Thermal evolution may alter the composition of crude oils and change, for instance, a paraffinic-naphthenic oil into a paraffinic oil. Heavy degraded oils usually belong to the aromatic-naphthenic or to the aromatic-asphaltic classes. The aromatic-asphaltic oils have a high sulfur content (more than 1%), whereas the aromatic-naphthenic oils usually have a moderate sulfur content (less than 1%).