Christopher Johnson, BURN Digital Producer

Americans use a LOT of oil. About 20 million barrels a day. Much of that oil – before it’s turned into products we use in our cars, our homes, and our plastic water bottles – exists in 2 basic forms: light (sweet) crude, and heavy crude.

heavy vs light crude

Heavy (left) vs light crude oil.

Light crude is the most desirable because it’s low in sulfur – an element that must be processed out of oil. That means companies have to spend less time and energy turning light crude into useful products.

That low sulfur content is also why it’s sometimes called “sweet.” Early prospectors used to taste the oil, to know what they had on their hands.

Light crude is found largely in the US Gulf states and Nigeria. Canada also has large sweet crude deposits.

In Saudi Arabia, Mexico, Venezuela, and Iraq, heavy crude is most common. “Heavy” because it has a lot of impurities – including wax – that have to be removed through costly processing.


Liquid petroleum pumped up out of the ground needs refining. Once the sulfur and other impurities are removed, it goes through a distillation process that Michael Poehl – who teaches about oil refining at The University of Texas at Austin – says isn’t unlike making your own liquor.

Knowing what you’ve got on the other side of the refining process takes some chemistry. Here are a few basics.

hydrocarbon chains

Hydrocarbon chains of varying lengths lead to different petroleum products. (

Petroleum is made up of compounds that include hydrogen atoms and carbon atoms. They bind to form hydrocarbon molecules.

Each of those hydrocarbons is like a chain link. When just a few hydrocarbons are linked together, you get a very light petroleum product – like methane and propane gases. Several dozen hydrocarbon links locked together means heavier stuff – lubricating or heavy fuel oil.

So, a company pours a drum of crude oil into a refinery. The refining process breaks up the oil’s hydrocarbon chains into various lengths, so that out comes a bunch of different products.

The most valuable is gasoline. About 40% of the crude that goes into a refinery comes out as gasoline.

Refinery components

Components and outputs of a typical refinery.

There are different kinds of refineries. Those built mainly to process light crude are focused on creating gasoline. Heavy crude refineries are designed to first get the impurities out, and then process the oil. Poehl says that the key to refinery economics is to optimize whichever crude strain is being processed. That means finding the cheapest, fastest, most efficient ways to get as much product as possible out of the crude that goes in.


Another oil source is tar sands – loose sand or sandstone saturated by a kind of thick, goopy petroleum called bitumen. Tar sands are especially plentiful in Northwestern Canada.

Tar sand hands

They are tough to work with because the petroleum is so heavy. In these deposits, the natural oil/sand mixture has to be heated in order to extract the petroleum. Refiners may have to process a ton of sand to get a barrel of oil, Poehl explains. Still, a lot of oil people are betting on tar.

One major tar sands project is the proposed 1,700- mile Keystone XL Pipeline, which would run from western Canada through the central United States and down to the Gulf of Mexico.


The plan has sparked a lot of debate, and opposition from farmers and environmental groups that say the tar sands are especially corrosive and thick, and therefore prone to leaking from the pipeline and onto farm land.