Analysis of the composition of crude oils can be complex, but the simple analysis scheme is to divide an oil into its saturate, aromatic, resin, and asphaltene (SARA ) fractions.
A method for characterization of heavy oils based on fractionation, whereby a heavy oil sample is separated into smaller quantities or fractions, with each fraction having a different composition.
Fractionation is based on the solubility of hydrocarbon components in various solvents used in this test. Each fraction consists of a solubility class containing a range of different molecular-weight species.
In this method, the crude oil is fractionated to 4 solubility classes, referred to collectively as SARA: saturates, aromatics, resins, and asphaltenes.
Saturates are generally iso- and cyclo-paraffins, while aromatics, resins, and asphaltenes form a continuum of molecules with increasing molecular weight, aromaticity, and heteroatom contents.
Asphaltenes may also contain metals such as nickel and vanadium. This method is sometimes referred to as Asphaltene/Wax/Hydrate Deposition analysis.