How was oil formed?
Oil was formed from the remains of animals and plants (diatoms) that lived millions of years ago in a marine (water) environment before the dinosaurs. Over millions of years, the remains of these animals and plants were covered by layers of sand and silt. Heat and pressure from these layers helped the remains turn into what we today call crude oil. The word petroleum means rock oil or oil from the earth.
![Three images, about Petroleum & Natural Gas Formation. Adapted from the National Energy Education Development Project.
The first image is about the Ocean 300 to 400 million years ago. Tiny sea plants and animals died and were buried on the ocean floor. Over time, they were covered by layers of sand and silt.
The second image is about the Ocean 50 to 100 million years ago. Over millions of years, the remains were buried deeper and deeper. The enormous heat and pressure turned them into oil and gas.
The third image is about Oil & Gas Deposits. Today, we drill down through layers of sand, silt, and rock to reach the rock formations that contain oil and gas deposits.](https://webarchive.library.unt.edu/dentonfracking/20141023092211im_/http://www.eia.gov/energyexplained/images/OILGASFORMATION.gif)
Products made from a barrel of crude oil
Did you know crude oil can be sweet or sour?
Crude oil is called sweet when it contains only a small amount of sulfur and sour if it contains a lot of sulfur. Crude oil is also classified by the weight of its molecules. Light crude oil flows freely like water, while heavy crude oil is thick like tar.
After crude oil is removed from the ground, it is sent to a refinery by pipeline, ship, barge, or rail. At a refinery, different parts of the crude oil are separated into useable petroleum products. Crude oil is measured in barrels.
A 42 U.S. gallon barrel of crude oil yields about 45 gallons of petroleum products. This gain from processing the crude oil is similar to what happens to popcorn, which gets bigger after it is popped.
![This graphic illustration of a barrel
shows the percentage of products that are made from 44 gallons of crude oil for 2011: 19% gasoline; 2% liquefied
petroleum gas (LPG); 1% heavy fuel oil (residual); 7% other products; 4% jet fuel; 1% other distillates
(heating oil); and 11% diesel fuel.](https://webarchive.library.unt.edu/dentonfracking/20141023092211im_/http://www.eia.gov/energyexplained/images/charts/products_from_barrel_crude_oil-small.jpg)
Source: U.S. Energy Information Administration,
Petroleum Navigator, Refinery Yield, 2013 preliminary data
Note: A 42 U.S. gallon barrel of crude oil yields about 45 gallons of petroleum products.