Module 1 · The molecule and the rock

Source, reservoir, trap: why oil is where it is

Level 1 · Foundation 9 min read 3 questions

An oil field needs four things to have happened in the right order over tens of millions of years. Getting three of them is a dry hole. This is why oil is clustered rather than spread.

Oil is not distributed the way rainfall is. It is clustered into a relatively small number of sedimentary basins, and the reason is that a commercial accumulation requires a sequence of unlikely things to have happened in the right order. Geologists call the set of them a petroleum system.

What each element does to the economics

These are not just geological curiosities. Each element leaves a fingerprint on cost and on the character of the barrel:

Source richness and type

Marine algal source rocks tend to yield waxy, low-sulfur oil; source rocks deposited in restricted, oxygen-poor marine settings tend to give sulfur-rich oil. Sulfur content is set in the source rock, tens of millions of years before anyone drills.

Depth and temperature

Too shallow and the kerogen never cracked. Too deep and the oil cracked further into gas. The same basin often produces oil at one depth and gas at another.

Reservoir quality

Porosity is how much fluid the rock holds; permeability is how easily it flows. High permeability means fewer wells for the same output, which is the difference between a few dollars a barrel and tens of dollars a barrel in lifting cost.

Trap size

Sets how much is in one place, which decides whether a field can carry the fixed cost of a platform, a pipeline or an export terminal.

Why the Middle East

The material page for crude oil puts the geological case plainly: the ancient Tethys Sea laid down prodigious volumes of marine organic matter, the region then received thick carbonate reservoir rocks with excellent flow properties, and broad gentle structural traps formed at the right time. The result is a small number of very large fields in permeable carbonate, which is why a barrel there can be lifted for a fraction of what it costs almost anywhere else. The Saudi Arab Light journey follows one such barrel from reservoir to petrol and makes the point concretely: the same commodity, a fraction of the effort, because of what the molecule is and what rock it sits in.

When the system is incomplete

Sometimes the source rock cooked properly but there was no route out, or no trap to fill. The oil is then still sitting inside the source rock, dispersed through shale that has almost no permeability. That is tight oil, often called shale oil, and it is not a different substance - it is the same oil that never migrated. Sometimes migration reached the surface, the light fractions evaporated and bacteria degraded the rest, leaving heavy bitumen mixed into sand: the Alberta oil sands. Both are covered in lesson 6, and both require different extraction precisely because nature did not do the concentrating step.