The petroleum system is the geologist's framework for explaining why oil and gas exist in one place and not another. It describes the chain of events — from buried organic matter to a trapped, sealed accumulation — that must all happen, and happen in the right order, to create a producible field.
All five elements — source rock, migration, reservoir rock, trap, and seal — must be present and correctly timed. A reservoir formed after the oil already migrated through, for example, will be empty.
The five elements
An organic-rich sedimentary rock (usually marine shale) whose kerogen is cooked by heat and burial into hydrocarbons.
The movement of hydrocarbons out of the source rock and, driven by buoyancy, upward through carrier beds until they are trapped.
A porous, permeable rock (sandstone or carbonate) that stores the hydrocarbons and lets them flow to a well.
A geometry — an anticline, fault, or stratigraphic feature — that halts buoyant migration and concentrates the hydrocarbons.
An impermeable cap rock, such as shale or salt, that prevents the trapped hydrocarbons from escaping upward.
Why timing is everything
Having all five elements is not enough; they must align in time. The source rock must reach maturity and expel hydrocarbons after the reservoir, trap, and seal are already in place. If the trap forms after migration has finished, the oil has already escaped or dispersed and there is nothing to capture. Geologists call this the critical moment — the time at which the system either works or fails.
This is why exploration is as much about reconstructing geologic history as it is about mapping rocks. Each of the five elements has its own dedicated guide:
- Source rock — where hydrocarbons are generated.
- Migration — how they move.
- Reservoir rock — where they accumulate.
- Traps and seals — what holds them in place.
Together these explain the conventional accumulations that fuel much of global production. Where hydrocarbons never migrate out of the source rock, you instead have an unconventional resource.
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Frequently asked
A petroleum system requires a source rock, migration, a reservoir rock, a trap, and a seal. All five must be present and correctly timed for oil or gas to accumulate in a producible field.
The source rock must generate and expel hydrocarbons after the reservoir, trap, and seal are already in place. If the trap forms too late, the migrating oil escapes before it can be captured, and the system fails despite having all the parts.
A trap is the geometry — such as an anticline or fault — that halts buoyant migration. A seal is the impermeable cap rock, such as shale or salt, that prevents the trapped hydrocarbons from leaking upward out of the trap.