Crude oil is a fossil fuel and, as the name implies, it is derived from fossils. Over time the remains of vegetation and various organisms, such as bacteria and algae, are covered by an increasing amount of soil before they are converted through heat and pressure into matter composed largely of hydrocarbons. This material, known as crude oil or petroleum, is the product of eons of this process.
Scientists believe most of the crude oil extracted today is generated from marine organisms, such as zooplankton and algae. They suspect larger animals may also be part of the mix.
Once these remains begin to change as a result of heat and pressure, they become what geologists refer to as kerogen, a waxy substance. With more heat, kerogen becomes liquid and gas hydrocarbons.
Crude oil hydrocarbons largely consist of cycloalkanes and alkanes, as well as oxygen, nitrogen, sulfur and copper, iron and nickel.
Geologists know where to locate oil reservoirs based on rock formation types. Such formations are characterized by deeply buried source rock containing hydrocarbon material, a permeable reservoir rock for it to gather in and a rock seal to keep the material from reaching the surface before it is extracted with drills and pumps.