Traps, Seals, and the Petroleum System

Why Hydrocarbons Accumulate

Oil and gas are lighter than the water that saturates the subsurface, so they are buoyant and rise until something stops them. A trap is the configuration of rock that stops them, and a seal (or caprock) is the impermeable rock, usually shale or salt, that caps the trap so the hydrocarbons cannot leak upward. Without both a trap and a seal, any oil that forms simply migrates to the surface and is lost.

Structural and Stratigraphic Traps

Traps come in two broad families. Structural traps are made by deformation: an anticline (an arch where oil pools in the crest) or a fault that juxtaposes reservoir against seal. Stratigraphic traps are made by the rock body itself: a pinchout where a sand thins to nothing updip, or an unconformity where tilted beds are truncated and sealed by younger rock. The widget builds one of each. Pick a trap type and turn up the charge to watch oil migrate in and fill the structure.

Notice the spill point. Oil fills the trap from the crest downward, and once the oil-water contact reaches the lowest closing contour, any additional oil spills out and migrates on. The vertical distance from the crest to the spill point sets the maximum column height the trap can hold.

The Petroleum System

A trap is only useful if the other pieces are present. A working petroleum system needs a source rock rich in organic matter, buried and heated enough to generate hydrocarbons; a migration pathway from source to trap; a reservoir with porosity and permeability to hold and flow the fluids; a seal; and correct timing, since the trap must exist before the oil arrives. Reservoir modeling assumes the system worked and focuses on the reservoir and trap, but the other elements set what is there to model.