The Petroleum System: Source, Reservoir, Seal, Trap & Migration

A petroleum system is the geological framework that allows hydrocarbons to be generated, migrate, and accumulate in economically viable quantities. For a commercial accumulation to exist, five distinct elements must be present and must overlap in both time and space:

1. Source Rock: A rock containing organic matter (kerogen) that, when heated, generates oil or gas.
2. Reservoir Rock: A porous and permeable rock (like sandstone) that can hold the hydrocarbons and allow them to flow.
3. Seal (Caprock): An impermeable rock (like shale) overlying the reservoir that prevents hydrocarbons from escaping upward.
4. Trap: The geometric configuration of the reservoir and seal that stops hydrocarbons from migrating laterally, forcing them to pool.
5. Migration: The movement of hydrocarbons from the source rock into the reservoir trap.

The reservoir must possess two critical properties: porosity (the void space holding the oil) and permeability (the connectivity allowing fluid flow). A rock can be highly porous but impermeable (like pumice), making it a poor reservoir. We will quantify these properties in Module 3.

The seal acts as a lid. Finally, the trap creates the closure. Traps are broadly classified as structural (caused by tectonic deformation, like a dome or a fault) or stratigraphic (caused by changes in rock type or layering, like a pinch-out). We will detail traps in Module 2.

Once generated, hydrocarbons must move. Primary migration (or expulsion) is the movement of oil and gas out of the low-permeability source rock into more permeable carrier beds. Secondary migration is the subsequent movement through the carrier beds into the reservoir trap. This secondary migration is driven primarily by buoyancy-because oil and gas are less dense than water, they rise upward until stopped by a seal.