The terms “petroleum” and “oil” are often used interchangeably, but they have distinct meanings in the energy industry. Petroleum is the scientific term for the raw, naturally occurring material extracted from the earth. “Oil” is a much broader descriptor that refers to this raw material (crude oil) and the multitude of refined products derived from it. The primary difference lies in the level of processing: petroleum is the original complex mixture, while many ‘oils’ are specialized, refined substances.
Petroleum: The Geological Source Material
Petroleum, meaning “rock oil,” is the general name for a naturally occurring, yellowish-black liquid mixture found beneath the Earth’s surface. Classified as a fossil fuel, it formed over millions of years from the anaerobic decay of ancient organic matter, primarily tiny marine organisms. This raw, unprocessed substance is most accurately called crude oil. Crude oil is a highly complex blend of compounds that is not directly usable in its extracted form.
Chemically, crude oil is composed predominantly of hydrocarbons, which are molecules consisting only of hydrogen and carbon atoms. These hydrocarbons exist in a vast range of sizes, from light, volatile gases like methane to heavy, thick, semi-solid compounds. The elemental composition of petroleum generally ranges from 83% to 87% carbon and 10% to 14% hydrogen by weight, along with smaller amounts of nitrogen, oxygen, and sulfur. The exact mix of these compounds—including paraffins, naphthenes, and aromatics—varies significantly depending on the geological source, which determines the crude oil’s specific properties.
Oil: A Category of Refined Products and Liquids
The term “oil” is a generic descriptor for any viscous, nonpolar liquid that does not readily mix with water, a definition that includes vegetable and animal fats. In the energy industry, “oil” can refer to the raw material (crude oil) or any of the hundreds of commercially viable products created from it. The U.S. Energy Information Administration defines the broader category of “petroleum” as including both the raw crude oil and the finished petroleum products.
Once crude oil is refined, it yields a spectrum of products commonly referred to as oils. These include lubricating oil used in machinery, fuel oils like kerosene and diesel, and heavy bunker oils used for marine shipping. The use of the word “oil” for these refined substances often causes confusion, as it conflates the original raw material with its specialized derivatives. These refined oils possess specific physical and chemical characteristics, such as different viscosities and volatility, engineered for their intended uses.
From Crude to Consumer: The Refining Divide
The transformation of crude oil (raw petroleum) into usable refined oils and fuels occurs through fractional distillation. This is the first step in an oil refinery, designed to separate the complex mixture of hydrocarbons based on their boiling points. Crude oil is heated to a high temperature, typically around 600 degrees Celsius, which vaporizes most of the hydrocarbon components.
The resulting vapor is piped into the bottom of a tall, vertical distillation column, which is hottest at the bottom and cooler toward the top. As the vapor rises, the different hydrocarbons cool and condense back into liquid form at various levels, or “fractions,” based on their boiling points. Lighter, smaller molecules, such as those used for gasoline, have lower boiling points and condense near the top of the column.
Middle-weight fractions, including kerosene and diesel fuel, condense lower down in the middle sections of the column. The heaviest, longest-chain hydrocarbons, which have the highest boiling points, remain as liquids at the bottom, yielding products like lubricating oils, asphalt, and heavy fuel oils. This separation process highlights the operational difference: petroleum is the input, and the wide range of commercial ‘oils’ and fuels are the distinct outputs derived from this thermal separation.