How Is Crude Oil Formed?

It is generally believed that crude oil was formed from the remains of animals and plants (called biomass) that lived millions of years ago. Over eons the biomass was covered by layers of mud, silt, and sand that formed into sedimentary rock. Geologic heat and the pressure of the overlying rock turned the biomass into a hydrocarbon-rich liquid that we call crude oil, and eventually forced it into porous rock strata called reservoirs. There are also formations or deposits of hydrocarbon-saturated sands and shale where geologic conditions have not been sufficient to turn the hydrocarbons into liquid.  

Wells are drilled into oil reservoirs to extract the crude oil. "Natural lift" production methods that rely on the natural reservoir pressure to force the oil to the surface are usually sufficient for a while after reservoirs are first tapped. In some reservoirs, such as in the Middle East , the natural pressure is sufficient over a long time. The natural pressure in many reservoirs, however, eventually dissipates. Then the oil must be pumped out using “artificial lift” created by mechanical pumps powered by gas or electricity.  Over time, these "primary" methods become less effective and "secondary" production methods may be used. A common secondary method is “waterflood” or injection of water into the reservoir to increase pressure and force the oil to the drilled shaft or "wellbore." Eventually "tertiary" or "enhanced" oil recovery methods may be used to increase the oil's flow characteristics by injecting steam, carbon dioxide and other gases or chemicals into the reservoir. In the United States, primary production methods account for less than 40% of the oil produced on a daily basis, secondary methods account for about half, and tertiary recovery the remaining 10%. Extracting oil (or “bitumen”) from oil/tar sand and oil shale deposits requires mining the sand or shale and heating it in a vessel or retort, or using “in-situ” methods of injecting heated liquids into the deposit and then pumping out the oil-saturated liquid.

Where does my gasoline come from? 

The United States consumes about 21 million barrels (882 million gallons) of petroleum products each day, almost half in the form of gasoline used in over 210 million motor vehicles traveling over 7 billion miles per day.

Most gasoline is made from crude oil, formed from the remains of plants and animals that lived hundreds of millions of years ago. These remains were covered with layers of sediment over time. With extreme pressure and high temperatures over millions of years, these remains became the mix of liquid hydrocarbons (an organic chemical compound of hydrogen and carbon) that we call crude oil. Refineries break down these hydrocarbons into different products. These “refined products” include gasoline, diesel fuel, heating oil, jet fuel, liquefied petroleum gases, residual fuel oil, and many other products.

The most basic refining process separates crude oil into its various components. Crude oil is heated and put into a distillation column where different hydrocarbon components are boiled off and recovered as they condense at different temperatures.

The molecular structure of the input is further changed in processes using heat and pressure as well as catalysts that increase the rate of reactions without being consumed themselves.

The characteristics of the gasoline produced depend on the type of crude oil that is used and the setup of the refinery where it is produced. Gasoline characteristics are also affected by other ingredients that may be blended into it, such as ethanol. Most of the fuel ethanol added to gasoline is made from corn grown in the United States. The gasoline performance must meet industry standards and environmental regulations that vary by location.