Petroleum, commonly known as crude oil, is a naturally occurring mixture of hydrocarbons that exists as a liquid beneath the Earth’s surface. As the world’s most significant source of energy and a foundational raw material for countless products, its classification often causes confusion. The raw material, crude oil, is overwhelmingly classified as a natural resource. This classification is based on the geological processes that form it entirely without human intervention over vast spans of time.
Defining Natural and Synthetic Materials
Natural materials are substances found in the environment, formed through geological, biological, or atmospheric processes. These resources are either used in their found state or undergo minimal physical processing, such as cutting or simple refining, that does not chemically alter their molecular structure.
Synthetic materials are substances created or significantly altered by human technology, typically through chemical processes in a laboratory or industrial setting. The defining characteristic of a synthetic material is that its starting components are chemically changed to produce a new substance with different properties. This process, called chemical synthesis, regroups atoms and molecules to create a structure that does not exist in the same form in nature.
The Biogenic Origin of Crude Oil
The classification of crude oil as a natural resource is supported by the widely accepted biogenic theory of its formation. This theory posits that petroleum originates from the decomposition of ancient biological matter, primarily marine organisms like plankton and algae. Millions of years ago, these organisms died and drifted to the seafloor, where they were buried under layers of sediment.
As sediment accumulated, the organic matter was subjected to immense pressure and elevated temperatures over geological time scales. The initial organic matter slowly transformed into a waxy substance called kerogen, a precursor to petroleum. This kerogen-rich sedimentary rock is known as the source rock.
As the source rock was buried deeper, it entered the “oil window,” where temperatures typically fall between 60°C and 150°C. Within this window, the heat and pressure drove a process called catagenesis. This cracked the complex kerogen molecules into the simpler hydrocarbon chains that constitute crude oil and natural gas. The resulting liquid petroleum then migrated out of the source rock and accumulated in porous reservoir rocks, completing the natural formation process.
Distinguishing Crude Oil from Synthetic Derivatives
While crude oil is natural, confusion about its classification stems from the fact that it serves as the raw material for countless synthetic products. Once extracted, the raw mixture of hydrocarbons must be processed in refineries to become useful. The initial step, called fractionation, separates the crude oil into different components, or fractions, based on their boiling points, yielding products like gasoline, diesel, and kerosene.
A significant portion of petroleum is further transformed through intensive chemical processing, which crosses the line into synthetic creation. Processes like cracking and polymerization break down or rebuild the hydrocarbon molecules into new substances, such as plastics, synthetic rubbers, and various chemical feedstocks. Synthetic motor oils, for example, are made by chemically modifying petroleum components, creating uniform molecules that do not exist naturally in crude oil. These end products are correctly classified as synthetic materials, even though their ultimate origin is the natural resource of crude oil.