Sometimes when I read articles and blogs about oil and the products made from oil, I get the impression that there is a lack of understanding about refineries and how common products like gasoline are made. I’m going to take a stab at explaining it in a two-part series.

This first post will explain crude oil and the second will explain refineries and how the two fit together (or not). For those familiar with crude oil and refining, please pardon my simplifications. Simplification is how I have been able to grasp some of these concepts. The second part of this series will discuss refineries and how crude oil is processed into valuable products that consumers and other manufacturers use every day.

To understand oil and all of the products made from oil, it is important to understand what a hydrocarbon is and how it represents the many different chemical components of oil. A hydrocarbon is a molecule made up of atoms. The two most important atoms are carbon and hydrogen, hence the name “hydrocarbon.” I listed carbon first because that is the key atom that defines a hydrocarbon. Hydrocarbons must have carbon atoms. Some of the hydrocarbon molecules look like straight chains, some are chains with branches and others appear as geometric shapes, like a hexagon or pentagon.

Here are some straight-chain hydrocarbons that come out of the oil well as a mixture:

Now let’s compare straight-chain hydrocarbons with branched-chain hydrocarbons:

And finally, let’s look at hydrocarbons that look like geometric patterns. They can appear with the carbons and hydrogens shown or they can appear as the simple geometric pattern:

Now that we know what a hydrocarbon is, let’s turn our attention to crude oil and look at the different types and the language used to describe them. First, crude oil is a naturally occurring mixture of different hydrocarbons, such as straight- and branched-chain like pentane, hexane and octane, as well as those having geometric patterns, like cyclopentane and cyclohexane. A fairly small percentage of crude oil is made up of molecules related to benzene. These are often removed during the refining process, due to strict air regulations, and used to make plastics, medicine and other valuable products.

I break crude oil into two different types, although there are many variations along the way. You may hear the terms “heavy crude” and “light crude.” Heavy crude is very thick because it contains bigger molecules than light crude and more often than not contain sulfur, nitrogen and oxygen within those big molecules. Light crude, on the other hand, is not as thick because it mostly contains smaller molecules that are represented by chains, branched chains and simple geometric patterns, even though it can contain some of the larger molecules. It also doesn’t have as much of the sulfur, nitrogen and oxygen content as the heavy crudes.

The other terms you hear associated with crude are “sour” and “sweet.” These terms do not refer to taste; rather, they refer to the amount of sulfur in the oil. Sour crude has a high sulfur content and often contains a particularly stinky chemical called hydrogen sulfide, which can make it smell like rotten eggs. Sweet crude has a lower sulfur content. Because of strict air regulations, sulfur must be removed from the crude oil during processing at a refinery.

The second part of this series will discuss refineries and how the hydrocarbons in crude oil are processed into valuable products that consumers and other manufacturers use every day.

Jim Cooper

Posted by Jim Cooper

Jim Cooper is the Senior Petrochemical Advisor for AFPM. To learn more about AFPM, visit