What is oil?

Oil and natural gas together make petroleum. Petroleum, which is Latin for “rock oil,” is a fossil fuel, meaning it was made naturally from decaying prehistoric plant and animal remains. It is a mixture of hundreds of different hydrocarbons—molecules containing hydrogen and carbon—that exist sometimes as a liquid (crude oil) and sometimes as a vapor (natural gas). Hydrocarbons are typically made from the remains of dinosaurs, pre-historic sea creatures and vegetation that have been buried in the earth for millions of years.

Hydrocarbons

Hydrocarbons are chemical compounds that contain only hydrogen and carbon. They are most often used as fuels that produce energy in the form of heat. Hydrocarbons are combustible and are the main components of fossil fuels such as petroleum, coal, and natural gas. Hydrocarbons come in many lengths and structures, from straight chains to branching chains to rings. Hydrocarbons contain a backbone of carbon atoms, called a carbon skeleton. The hydrogen atoms attach to the backbone. Refineries and petrochemical plants separate hydrocarbons into different chemical groups and chemically cross-linking hydrocarbon chains to make many substances such as synthetic rubber, nylon, and plastic containers.

Crude Oil

Crude oil comes in many forms. It varies in color, from clear to tar black, and in viscosity, from liquid to almost solid. Usually it is black, but green, red, or brown oils are not uncommon. The liquid crude oil comes straight from the ground. It is dark and sticky and will condensate, or evaporate easily, if it is clear and volatile. The hydrocarbons in crude oil have either ring- or chain-shaped molecules.

Light and Heavy Oil

Thin and volatile oils, or crude oils that readily evaporate, are called “light oils.” These oils float easily on water. Heavy oil is any oil that does not flow easily. Some heavy oils are so dense that they sink, rather than float, in water (except sea water, which has a higher density than fresh water).

Bitumen/Tar/Asphalt

Sometimes underground oil seeps up to the surface. Exposed to the air, its most volatile components evaporate, leaving a black ooze or lump. When it is solid and sticky like toffee, it is called asphalt. When it is like syrup or molasses, it is called bitumen. These forms are also referred to as pitch or tar.

Oil Mixture

Oil is primarily 84% carbon and 14% hydrogen combined as chemical compounds called hydrocarbons. The three main types of oil hydrocarbons are:

• Alkanes
• Aromatics
• Naphthenes

The general formula for alkanes is CnH2n+2. The simplest hydrocarbon/alkane is methane, CH4.

Formulas of well-known alkanes are :

Different sources of oil throughout the world have different levels of these hydrocarbons. For example, Saudi heavy crude oil is much higher in alkanes than many other crude oils.

Natural Gas

Natural gas is a combustible mixture of hydrocarbon gases. When burned, it provides a great deal of energy. Unlike other fossil fuels, natural gas is clean burning and emits lower levels of potentially harmful byproducts into the air. While natural gas is formed primarily of methane, it can also include ethane, propane, butane, and pentane. It can be found alone or with oil deposits. Oil contains some compounds that are so volatile that they easily evaporate, forming natural gas, and some oil deposits have such a high proportion that they are virtually all gas.

Want to learn more?

What Is Oil? Alberta Energy
Gives a text description of crude oil, with links to statistics and frequently asked questions about oil

Kindergarten through 12th grade Learning Resources Canadian Centre for Energy
Provides teaching guides targeted toward specific school grade levels

Energy Kids’ Page US Energy Information Administration
Provides text and graphic illustrations with information on various energy resources, including how each resource is formed and used

HowStuffWorks.com
Explains the chemical makeup of crude oil and hydrocarbons

Naturalgas.org
Provides a text overview of natural gas including its chemical composition and details on how it is formed

What Is Oil? San Joaquin Geological Society

Defines oil-related terms and provides energy information

What are petroleum hydrocarbons? Colorado State University
Illustrates classes of hydrocarbons through charts and illustrations of their chemical makeup