How Oil and Gas Form: From Plankton to Trap (Infographic)

Oil is not something the earth has; it is something the earth cooked. Nearly every barrel ever produced began as microscopic sea life that died, sank into oxygen-poor water, was buried before it could rot, and was then heated for millions of years until it turned into hydrocarbons that floated upward and got caught under a seal. This infographic draws those eight steps in order; the notes underneath link every term to its full definition.

Key takeaways

• Oil and gas come from plankton and algae, not dinosaurs. The organic matter has to be buried faster than it can decay, which is why still, oxygen-poor seas and lakes are where source rocks form.
• The buried organic matter becomes kerogen, a waxy solid locked in black shale. Nothing happens until heat does: in the oil window kerogen cracks into oil, deeper and hotter into gas.
• Hydrocarbons are lighter than water, so they leave the source rock and float upward. Migration is a journey through porous rock, not a flow through caves.
• A reservoir is rock with connected pores: porosity is the space, permeability is whether the pores connect. There is no underground lake.
• The story ends where an impermeable cap rock and a fold or fault make a trap, with gas above oil above water. Miss any one of source, migration path, reservoir, seal or trap and the well is dry.

The five things that all have to be true

Geologists talk about a petroleum system: a source rock that generated hydrocarbons, a migration path they could travel, a reservoir that could hold them, a seal that could stop them, and a trap that was already shaped when the oil arrived. Exploration is the search for the place where all five line up, and most dry holes are the discovery that one of them did not. Read the eight steps below with that checklist in mind and the diagram becomes a map of the five.

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The eight steps

1. Life sinks

Plankton and algae live and die in enormous numbers, and their remains drift to the bottom. In most of the ocean they are eaten or oxidised before they reach it. In a still basin whose bottom water holds little or no oxygen, they survive the fall and pile up as an organic-rich ooze. That single condition, a lack of oxygen at the sea or lake floor, is the reason source rocks are rare and valuable.

2. Burial

Rivers keep delivering mud and sand, and each new layer buries the last. Buried quickly enough, the organic layer is sealed away from oxygen and from the bacteria that need it, so instead of rotting it is preserved and compacted. Over millions of years the water is squeezed out and the mud hardens into shale, with the organic matter trapped inside it.

3. Source rock and kerogen

Pressure and mild heat convert the preserved organic matter into kerogen, a waxy, insoluble solid dispersed through the rock. A shale rich enough in kerogen to generate worthwhile hydrocarbons is a source rock. Its quality is measured as total organic carbon and its character as kerogen type: algal and marine kerogens tend toward oil, woody land-plant kerogen toward gas and coal.

4. The oil window

Burial brings heat, and heat is what turns kerogen into oil. Through a band of temperature and depth called the oil window the long kerogen molecules crack into liquid hydrocarbons. Buried deeper and hotter, the oil itself cracks into gas, first wet gas with condensate and then dry methane. Hotter still and only a carbon residue remains. Whether a basin holds oil or gas is largely a question of how deep its source rock went.

5. Migration

Oil and gas are lighter than the water that fills every pore in the rock, so once generated they are expelled from the source rock and pushed upward by buoyancy. Migration runs along permeable carrier beds, faults and fractures, always toward lower pressure and shallower depth. Some hydrocarbons make it all the way to the surface and are lost as seeps; the ones that matter are stopped on the way.

6. Reservoir rock

A reservoir is not a cavern. It is rock with pore space between its grains, typically sandstone or limestone, and a good one feels like a hard rock in the hand. Porosity is the fraction of the rock that is open space and sets how much fluid it can hold; permeability is how well those spaces connect and sets how fast fluid can move through them. Both are measured on core and estimated from logs.

7. Seal and trap

Rising hydrocarbons stop when they meet a rock they cannot pass, an impermeable cap rock of shale, salt or tight limestone. A seal alone is not enough; the geometry beneath it has to form a container. An anticline, a fault block or a pinched-out sand body makes a trap, either a structural trap made by folding and faulting or a stratigraphic trap made by the rock layers themselves. Inside, the fluids sort by density: gas at the top, oil beneath it, water at the base.

8. Finding it

None of this is visible from the surface, so explorers image it. A seismic survey sends sound into the ground and records the echoes from each layer, revealing the folds and faults where traps may lie; a direct hydrocarbon indicator on the seismic data can hint at the fluid itself. A region with the right ingredients is a play; a specific drillable target within it is a prospect; and the only way to be sure is to drill it, which is where How an Oil Well Works picks up the story.

A worked example: a marine basin

Picture a shallow sea two hundred million years ago whose deep, quiet floor collects plankton faster than the sluggish bottom water can consume it. Deltas bury the ooze under kilometres of sand and mud. The organic layer becomes a black shale rich in marine kerogen. Then the basin is stretched, faulted and tilted, sandstones are deposited above the shale and a thick mudstone caps them, and the whole stack is buried deeper still. The shale enters the oil window and expels oil; the oil rises through the sandstone and is caught against the mudstone in tilted fault blocks that were already there. That sequence, with a source rock maturing after the traps formed, is the story behind many of the world’s great oil provinces, and every part of it is a step on the diagram.

Where the story fails

• Immature source. Plenty of kerogen, never buried deep enough. The rock is rich and the basin is empty.
• Overcooked source. Buried too deep for too long; whatever it made was gas long ago, and much of that has escaped.
• Bad timing. The source matured before the traps formed, so the oil rose straight past where the trap would later be.
• No seal. Excellent reservoir, nothing to stop the oil, and a field of surface seeps instead of a discovery.
• Breached trap. Uplift and erosion, or a later fault, opened the seal. The gas is gone and what oil remains is heavy and biodegraded.
• Tight reservoir. The oil is there but the pores barely connect. This is the world of shale plays and hydraulic fracturing, where the source rock is drilled as its own reservoir.

Variants

• Biogenic gas. Bacteria make methane from organic matter at shallow depth and low temperature, with no oil window involved. Many shallow gas fields are biogenic.
• Coal and coal-bed methane. Woody, land-plant kerogen tends to make coal rather than oil, and gas held in coal seams is produced as coal-bed methane.
• Sweet and sour. Sulfur compounds from the source rock or from later bacterial action make crude and gas sour; without them they are sweet. The difference decides how the fluids are processed and priced.
• Unconventional plays. In a shale play the source rock and reservoir are the same rock, and the oil window maps directly onto the map of the play: oil in the shallower, cooler part, gas in the deeper, hotter part.

Frequently asked questions

Does oil come from dinosaurs?

No. The overwhelming majority of oil and gas formed from microscopic marine plankton and algae, with land plants contributing mainly to coal and gas. Dinosaurs were far too few, and lived in the wrong places, to matter.

How long does it take?

Millions of years at least, and tens of millions is typical: time to bury the organic matter deep enough, time to cook it, and time for it to migrate and be trapped. The process is still going on today, but far more slowly than oil is being produced.

How deep does oil form?

The oil window is commonly placed at roughly 60 to 120 degrees Celsius, which with a typical geothermal gradient means burial of about two to four kilometres. It varies by basin, and gas forms deeper and hotter.

Is there an underground lake of oil?

No. Oil sits in pore spaces between rock grains, often the size of sand grains or smaller. A reservoir is a rock, not a cavity, which is why porosity and permeability matter so much.

What is the difference between oil shale and shale oil?

Oil shale is a kerogen-rich rock that never reached the oil window; hydrocarbons have to be made from it artificially by heating. Shale oil is real crude that a mature source rock generated but never released, produced by drilling the shale itself. See kerogen.

Why does oil rise?

Because it is less dense than the water that fills the rock around it, so buoyancy pushes it upward through any connected pore space until a seal stops it. That is why traps are found at the tops of folds, not the bottoms.

Where does the next step, drilling, come in?

Once a prospect is mapped, a well is drilled into the trap. The guide How an Oil Well Works takes the story from there, from spud to first oil.

Part of the OilfieldTerms.com guides. Every term above links to its definition in the glossary.