This is all about crude oil composition and our inability to measure it and describe in unambiguous terms. Crude oil is an immensely complex, continuous mixture of organic compounds spanning a wide molecular weight range. Components of this complex mixture depend upon each other for their solubility. It is a delicate balance.

The strategy to unravel crude oil composition has always been “divide and conquer”. However, over the years much more progress was made dividing than conquering!

Early attempts to unravel crude oil composition involved crude oil distillation to separate it into fractions such as naphtha, kerosene, atmospheric and vacuum gas oil, and residue. This concept is still the basis of modern crude oil evaluation for refining purposes.

The material that could not be further distilled was referred to as residue or bitumen. Early work to unravel the composition of this “non-distillable” material, at the time used primarily to manufacture asphalts, involved separation based on solubility. It is often forgotten that early research on asphaltenes was driven by asphalt manufacturing, where asphaltenes are not only a very desirable component but the component that determines to a great extent asphalt properties.

Originally, petroleum ether was used to separate bitumen into insoluble asphaltenes and soluble maltenes. The latter were further separated by adsorption on an active earth to produce a fraction of oils, which was extracted with petroleum ether and a fraction of resins, which was adsorbed by the active earth. Many modifications of this original approach were published over the years. Worthwhile mentioning is the so-called SARA (Saturates, Aromatics, Resins, Asphaltenes) method. Even SARA method exists in many different versions, making comparison of the results difficult or impossible. Numerous separation and characterization schemes were developed over the years, resulting in proliferation of vague terminology e.g., asphaltenes, maltenes, resins,etc. Similar terms have been used to describe dissimilar fractions, leading to further confusion.

It is important to bear in mind that a separation of a complex mixture such as crude oil does not produce fractions with defined molecular composition. The separation step is typically needed to simplify sample matrix and make it amenable to a measurement method. It is thus the separation and measurement together that are to be standardized to provide unambiguous information. A good example would be a distillation step to produce a naphtha fraction followed by DHA method (detailed hydrocarbon analysis).

Let’s get back to the “asphaltenes” and the need for a standardized method. Do we know what we want to separate and measure? Do we have separation and measurement methods which are capable of providing unambiguous results? If the answers to both questions are yes, I think we may want to proceed with the standardization efforts.

M. M. ("Mietek") Boduszynski, PhD
Consulting Scientist
ChevronTexaco, Energy Technology Company
Richmond, CA