boiling point. Geochemistry Asphaltenes are today widely recognised as dispersed, chemically altered fragments of kerogen, which migrated out of the source rock for the oil, during oil catagenesis. Asphaltenes had been thought to be held in solution in oil by resins (similar structure and chemistry, but smaller), but recent data shows that this is incorrect. Indeed, it has recently been suggested that asphaltenes are nanocolloidally suspended in crude oil and in toluene solutions of sufficient concentrations. In any event, for low surface tension liquids, such as alkanes and toluene, surfactants are not necessary to maintain nanocolloidal suspensions of asphaltenes. The nickel to vanadium ratio of asphaltenes reflect the pH and Eh conditions of the paleo-depositional environment of the source rock for oil (Lewan, 1980;1984), and this ratio is, therefore, in use in the petroleum industry for oil-oil correlation and for identification of potential source rocks for oil (oil exploration). Occurrence Heavy oils, oil sands, bitumen and biodegraded oils (as bacteria cannot assimilate asphaltenes, but readily consume saturated hydrocarbons and certain aromatic hydrocarbon isomers – enzymatically controlled) contain much higher proportions of asphaltenes than do medium-API oils or light oils. Condensates are virtually devoid of asphaltenes. Measurement Because the ratio of electron spins per gram is constant for a particular species of asphaltene then the quantity of asphaltene in an oil can be determined by measuring its paramagnetic signature (EPR). Measuring the EPR signature of the oil at the wellhead as the oil is produced then gives a direct indication of whether the amount of asphaltene is changing (e.g. because of precipitation or sloughing in the tubing below). In addition, asphaltene aggregation, precipitation or deposition can sometimes be predicted by modeling or machine learning methods and can be measured in the laboratory using imaging methods or filtration. Production problems Asphaltenes impart high viscosity to crude oils, negatively impacting production. Furthermore, the variable asphaltene concentration in crude oils within individual reservoirs creates a myriad of production problems. Heat exchanger fouling Asphaltenes are known to be one of the largest causes of fouling in the heat exchangers of the crude oil distillation preheat train. They are present within micelles in crude oil, which can be broken down by reaction with paraffins under high temperature. Once the protective micelle has been removed polar asphaltenes agglomerate and are transported to the tube walls, where they can stick and form a foulant layer. Asphaltene removal Chemical