Oil is often called a fossil fuel, and the name is literal: it forms from the fossilized remains of ancient life. But it is not dinosaurs — the real source is far smaller and far older. Crude oil begins as the bodies of microscopic marine plankton and algae that lived and died in ancient seas.
Oil forms from buried marine plankton and algae preserved as kerogen in source rock. Heat and pressure over millions of years mature that kerogen into oil and gas, mostly within the "oil window" of about 60–120 °C.
From plankton to source rock
When marine plankton and algae die, they sink to the seafloor. In oxygen-poor settings, this organic matter is buried by sediment before it can fully decay. As more layers pile on top over millions of years, the organic-rich mud is compacted and turns into source rock, with the organic material preserved as a waxy substance called kerogen.
The solid, waxy organic material trapped in source rock. It is the precursor to oil and gas — heat and pressure over geologic time convert kerogen into liquid and gaseous hydrocarbons.
The oil window
Burial is only the beginning. As the source rock sinks deeper, it gets hotter and is squeezed under enormous pressure. This slow "cooking" — known as maturation — gradually breaks the kerogen down into hydrocarbons. The sweet spot is the oil window, a temperature range of roughly 60 to 120 °C where kerogen generates liquid oil most efficiently.
- Too shallow / too cool — the kerogen stays immature and little oil forms.
- The oil window (~60–120 °C) — kerogen matures into crude oil.
- Too deep / too hot — the molecules break down further into natural gas.
Migration and trapping
Newly formed oil and gas are lighter than the surrounding water, so they migrate upward through porous rock until they are stopped by an impermeable layer. There they accumulate in a reservoir — a trap of porous rock sealed by a cap rock. This combination of source rock, migration path, reservoir and seal is what creates an oil and gas field worth drilling.
The whole process takes millions of years, which is why oil is a finite, non-renewable resource: it forms far more slowly than we extract it.
Built by the team behind OpsFlo — field service & billing software for oilfield service companies. Capture tickets at the wellsite and bill in days, not weeks.
Frequently asked
No. Crude oil forms mainly from microscopic marine plankton and algae, not dinosaurs. That organic matter is preserved as kerogen and matured into oil by heat and pressure over millions of years.
The oil window is the temperature range of roughly 60–120 °C where source rock generates liquid oil most efficiently. Cooler and the kerogen stays immature; hotter and it breaks down into natural gas.
Oil takes millions of years to form from buried organic matter, far slower than the rate at which it is produced and consumed. That is why it is considered a finite, non-renewable resource.