Crude oil is concentrated ancient sunlight: marine plankton that bloomed hundreds of millions of years ago, buried in seafloor mud and slow-cooked into hydrocarbons. Not dinosaurs — plankton. Here's the five-step recipe, and why it only worked in certain places.
| 1 · Deposition | Marine plankton and algae die and settle into oxygen-poor seafloor mud, where they escape decay — the raw material of every oil field. |
| 2 · Burial | Layer upon layer of sediment buries the organic mud over millions of years, turning it to shale and squeezing out water. |
| 3 · The oil window | At roughly 2–4 km depth and 60–120°C, kerogen in the shale "cracks" into liquid oil. Hotter/deeper (120–200°C) yields natural gas instead. |
| 4 · Migration | Oil, lighter than water, creeps upward through porous rock — sometimes for millions of years — until it escapes at the surface (tar seeps) or is caught. |
| 5 · The trap | An impermeable cap over a fold or fault stops the migration and concentrates oil into a reservoir — the thing explorers actually hunt for. |
Related reading: what is fracking (producing oil that never migrated) · what oil is used for · how much is left · glossary.
From the remains of ancient marine microorganisms — mostly plankton and algae — that sank into oxygen-poor seafloor mud, were buried under sediment, and were then cooked at 60–120°C and high pressure for millions of years. The organic matter first becomes kerogen, then breaks down into liquid hydrocarbons within the "oil window" of burial depth (roughly 2–4 km).
No — that's a durable myth (helped along by old gasoline marketing). The overwhelming source is microscopic marine life: plankton, algae and bacteria. Dinosaurs were land animals whose remains rarely reached the oxygen-free marine environments needed for preservation, and their total biomass was tiny compared with the oceans' microorganisms.
Millions of years — typically tens of millions between deposition and a mature oil accumulation. Most oil produced today formed from organic matter deposited 50–300 million years ago (the Jurassic and Cretaceous were especially prolific). This is why oil is "non-renewable" on any human timescale: we consume in a year what nature took roughly a million years to form.
Five things must coincide: a source rock rich in organic matter, enough burial to cook it, a porous reservoir rock (sandstone, limestone) to hold it, an impermeable seal to stop it escaping, and a trap geometry that concentrates it — formed before the oil migrated. Miss any one and there's no field, which is why the Middle East (ideal ancient shallow seas plus huge gentle traps) holds so much.
Conventional oil migrated out of its source rock into a reservoir where it flows readily to a well. Shale ("tight") oil never left the source rock — it sits locked in impermeable stone and needs horizontal drilling plus hydraulic fracturing to flow. See the fracking explainer for how that works.