Where does crude oil come from?

Dead critters, plenty of pressure, a lot of heat, and hundreds of thousands of years in time. Crude oil is formed from the remains of dead organisms (diatoms) such as algae and zooplankton that existed millions of years ago in a marine environment.

As they lived these organisms absorbed energy from the sun and stored it as carbon molecules within their bodies. Once they died their remains sank to the bottom of the oceans or riverbeds and were buried in layers of sand, mud and rock.

Over millions of years, the remains were buried deeper and deeper under more sediment and organic materials.

The enormous pressure, high temperatures, and lack of oxygen transformed the organic matter into a waxy substance called kerogen. With even more heat, pressure, and time the kerogen undergoes a process called catagenesis which transforms the kerogen into hydrocarbons. Different combinations of pressure, heat, and the original composition of organic material will determine the type of hydrocarbon formed. In this case, the hydrocarbons form crude oil.

Some oil might make it all the way to the surface where it pools, in other cases the oil will get trapped under impermeable layers of rock or clay where it will form underground reservoirs.