Organic materials consisting of aromatic and naphthenic ring compounds containing nitrogen, sulphur and oxygen molecules. The asphaltene fraction of crude is defined as the organic part of the oil that is not soluble in straight-chain solvents such as pentane or heptane.

Asphaltenes exist as a colloidal suspension stabilised by resin molecules (aromatic ring systems) in the oil. The stability of asphaltic dispersions depends on the ratio of resin to asphaltene molecules. The determination of the quantity of resin is important in estimating the potential damage created by asphaltenes.

Asphaltene precipitates as a result of pressure drop, shear (turbulent flow), acids, solution carbon dioxide (CO₂), injected condensate, mixing of incompatible crude oils or other conditions or materials that break the stability of the asphaltic dispersion. For example, in matrix acidizing, iron ions in solution favour the precipitation of asphaltene deposits.