
Crude oil is a highly variant natural resource. The quality ranges are similar to coal and depending on the maturation of the crude the quality can be high or low (younger crude's are of lower quality). One of the first indications of quality is color. The variations in oil color can be dramatic, and very indicative of the quality of that crude. Not all crude oil is black - higher quality oils can be golden or amber in color.

All the quality measures here are based on the ability to produce the desired products. In the U.S., about 50% of the oil is converted into gasoline. So an oil that produces a higher % of gasoline "cuts" is more desirable and have a higher quality oil. Take note, we have used much of the higher quality crude oil already! Now we need to use the lower quality oils too and the general trend is to use increasingly lower quality crudes. This quality reduction has an impact on how we refine the crude into the desired products.
Viscosity
Viscosity is the resistance to flow. Do not use the term "Thickness" which is a length measurement. The higher the viscosity the slower the liquid will flow and the lower the quality. We have many techniques for measuring viscosity, some of which are quite high-tech. Here is one of the simplest, utilizing one of the testing devices in one of our petroleum labs over in Hosler. Please watch the following (2:41) video.
Click for the transcript.
[Video opens with Dr. Mathews standing with a viscometer.] Dr. Mathews: This is a Saybolt Viscometer. There are an awful large number of ways to measure viscosity. This is perhaps one of the simplest. I am going to pour this into a heated reservoir. [Dr. Mathews pours a beaker of Pennsylvania Crude Oil into the machine.] Dr. Mathews: And do the same with the other crude oil. [Dr. Mathews pours a beaker of Gulf of Mexico crude oil into the machine.] Dr. Mathews: The reason the reservoir is heated is because the temperature is a factor that influences viscosity. And now I am going to do a very simple experiment. What I am going to do is yank these chains and we are going to see how long of a time difference it is between the Pennsylvania crude on the left and the Gulf of Mexico crude on the right, to see how much more viscous the Gulf of Mexico crude is. So here we go. [Dr. Mathews pulls two chains which allow the oil to flow through the machine and starts a timer.] Dr. Mathews: As you can see the Pennsylvania crude oil, the higher quality crude oil, the old deep crude oil, is flowing out very rapidly. There is the same quantity in each reservoir and they are at the same temperature. Whereas the Gulf of Mexico, the blacker of the crude oils, is taking much longer to come out. Again, a very easy determination of the quality of the crude oil is the viscosity. [The Pennsylvania crude oil has finished flowing out of the machine.] Forty-two seconds for the Pennsylvania crude, and we are going to be here for a while for the Gulf of Mexico. [The Jeopardy theme plays as we wait for the Gulf of Mexico crude to finish flowing out. Dr. Mathews shows the stopwatch every once in a while. It ends at three minutes and seventeen seconds.] [Video ends.]
The viscosity process is a measure of quality because the chemical structure of the crude influences its flowability. Longer chain molecules, for example, are harder to flow than short chains because of non-bonding interactions. If you have had any chemistry you will recall ionic (type of bonding in salt crystals) and covalent bonding (the type of bonding between 2 carbon atoms). Those are bonding interactions. There are several non-bonding interactions that occur which attract (and repel) molecules. It is the relative strength of these non-bonding interactions that influence the resistance to flow.
Elemental Composition
For coal, we used the correct terminology, which was ultimate analysis. For crude, that terminology we use is Elemental Analysis. Crude oil is complex, it contains C, H, N, S, O, and metals too. But the bulk of the composition is C and H, the rest being the N, S, O, and metals. S is a good indication of the quality of the crude because as the oil is heated underground the weak S-C bond can break, producing H2S (hydrogen sulfide gas). So, older crudes - higher quality - will have lower S content. Higher S crudes also cost more to process as S is a catalyst poison it has to be removed or the extensive catalysts used in the petrochemical industry would be damaged, as would your catalytic converter. The atomic H/C ratio is also an indicator of quality (why?)
Element | Percentage |
---|---|
Carbon | 84 - 87% |
Hydrogen | 11 - 14% |
Sulphur | 0 - 6% |
Nitrogen | 0 - 1% |
Oxygen | 0 - 2% |
Chemical Structures
Hydrocarbons are molecules that contain only the elements of carbon and hydrogen. These are the bulk of the crude oil. We find 4 types of chemical structures of hydrocarbon in crude oil:
- Straight-chain
- Paraffins
- Branched-chain paraffins
- Aromatics Naphthalenes
Please watch the following (:08) video. Paraffin - Straight & Branched
We have seen normal (for example n-heptane) and branched (2,2,4 iso-octane) examples of the paraffins. They all have the same formula: CnH2n+2 (n is the number of carbon atoms). For example, in the cetane molecule above, to determine the molecular weight (Mw) you can count the carbons (x 12 the amu of a carbon atom) and count the hydrogen atoms (x 1 amu) and add the numbers together to obtain the molecular weight. Or you can use the formula:
Cetane has 16 carbon atoms (but if we used decane you would know how many carbons it contained, right?) so C16H(2 x 16)+2 OR C16H34 and the Mw is = (12 x 16) + (1 x 34) = 226 amu (atomic mass units).
The paraffins are the desired contents of the crude oil. Long chains (> 60 carbon atoms are wax) used to be used extensively for the production of candles. Now we use the shorter chains produce gasoline, diesel and jet fuel (and many other products). Note that each molecule might have many structural isomers, for example, a molecule containing 10 carbon atoms has 75 structural isomers. If an isomer is an unfamiliar term to you, I'd suggest looking it up online.
Please watch the following (:09) video. Aromatics
Aromatics are found in both crude oil and coal. In crude oil they are now undesirable because of soot production during combustion.

I took the soot picture above with a scanning electron microscope so we can see the very small (>1 micron) spherical soot particles. These spheres join together to form chains of spheres. To give you some idea of the scale: 80 microns is about the width of human hair. Take note that the aromatics have a much lower H/C ratio than the paraffins. The benzene ring contains double bonds (not shown). Aromatics can exist in complex structures containing many rings. The non-bonding interaction between these rings is strong and so pure compounds of 3 rings are solid at room temperature. The equivalent normal paraffin is a viscous liquid under the same conditions.
Please watch the following (:09) video. Naphthenes
These are cyclo-paraffins and the example of cyclohexane above looks like a benzene molecule. There are no double bonds within the ring and so every carbon (in this example) has 2 hydrogen atoms bonded to it. Cyclohexane has an interesting boat or chair configuration. Can you see the differences?
Classification
In a similar manner to coal, as the source rock is buried deeper, the temperature increases with increasing depth. Thus, looking at quality indicators allows for a classification system similar to that of coal rank.
Because "old deep" oil provides the highest quantity of gasoline, it is the higher quality crude oil.
Most graphs you are used to seeing or plotting have just 2 axes. This works fine if you're just comparing 2 components, but as you see below, we're comparing 3 general classifications for crude oil compound types. It is the ratio of these compound types (aromatics, paraffins, and naphthenes) that impacts the quality of the crude (in addition to S content, especially when the S is within the aromatic portion, which makes it much harder to remove during refining). So, to plot 3 items on a single graph we use ternary diagrams like the one you see above. At the three apexes, the composition would either be pure (100%) aromatics, pure naphthenes, or pure paraffins (clockwise from top). Along any of the borderlines of this triangle, you're looking at a mixture of just 2 of these components (aromatics – naphthenes or naphthenenes, paraffins or paraffins - aromatics). At any point within the triangle, the crude contains all three components, in varying degrees.

Take the example of 50% aromatics to begin with. To plot this point on the graph, you'd create a drop a horizontal line about halfway between the apex (100 %) and the base of the triangle opposite of that apex (0%), representing 50%. You repeat this process to locate the other %'s of the compound types on the graph, and the point you're after is the convergence of those three lines. Thus, the center of the triangle is: 33%, 33%, and 33% of aromatics, naphthalenes, and paraffins– crude oil that would generally fall into the "old shallow" classification. Here is a dynamic example.