What’s the difference between heavy and light crude oils? And why do American refineries need both

There are hundreds of varieties of crude oil around the world. Different types of oil require different refining processes to make the fuels we need in the quantities we need. Many American refineries need heavier crudes than what is largely produced in the United States.

• Crude oils have different viscosities or "gravities." "Heavy" crude oil is more viscous, while "light" crude oil is thinner.
• Crude oils also have different sulfur content. Low-sulfur crude is called "sweet" and high-sulfur crude is called "sour."
• Refineries run on a mix of crude oils in order to run efficiently and maximize outputs. More than 70% of U.S. refining capacity runs most efficiently with heavier crude. That is why 90% of crude oil imports into the United States are heavier than U.S.-produced shale crude.
• Because heavier crude is more difficult to process and requires refineries to make significant up-front investment costs, it tends to trade at a discount to light crude.

Why do U.S. refineries run on heavier crude oils that we need to import?

Long before the U.S. shale boom, when global production of light sweet crude oil was declining, we made significant investments in our refineries to process heavier, high-sulfur crude oils that were more widely available in the global market. These investments were made to ensure U.S. refineries would have access to the feedstocks needed to produce gasoline, diesel and jet fuel. Heavier crude is now an essential feedstock for many U.S. refineries. Substituting it for U.S. light sweet crude oil would make these facilities less efficient and competitive, leading to a decline in fuel production and higher costs for consumers.

The ability to import AND export crude oil and refined products is good for consumers AND safeguards U.S. energy security. Read more.