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 four 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.