The formation of petroleum reservoirs occurs over millions of years as oil and gas accumulations develop in underground traps formed by structural or stratigraphic geological features. These accumulations usually occur in the more porous and permeable sedimentary rock, where the petroleum molecules seep into the small inter-granular spaces, or in joints and fractures of the rock. The reservoir is the portion of the formation containing oil and gas that is hydraulically connected.
Conventional reservoirs need a source rock, a migration path, a reservoir rock and a cap rock.
The source rock is a rock that is rich in organic matter. These form as algae or other plant and animal life die and are buried in the sand. Over millions of years, these rock formations are covered by other rock formations and the pressure and temperature of the rock increases. The organic matter converts to oil and gas as it is heated.
As the oil and gas forms in the source rock, it begins to migrate through the inter-granular spaces, the joints and the fractures of the rock. The oil and gas molecules are less dense than the rock and water and naturally migrate slowly upward towards the surface.
The molecules continue migrating until they hit cap rock. Cap rock is typically a layer of impermeable rock that lacks the inter-granular spaces the molecules need to continue traveling. It overlays the reservoir rock and forms a trap. At this point, with no where for the molecules to go, and more molecules continuing to migrate out of the source rock, the molecules begin to accumulate in the rock directly below the cap rock – the reservoir rock.
If there is no cap rock, the molecules travel all the way to the surface, resulting in a natural oil seep.
In recent years, unconventional reservoirs have been developed. These unconventional reservoirs are unique in that the source rock is the reservoir rock. No migration path or cap rock is needed, as the oil and gas are produced directly from the source rock where it was generated.
Terry, Ronald E., J. Brandon. Rogers, and B. C. Craft. Applied Petroleum Reservoir Engineering. Third ed. Massachusetts: Prentice Hall, 2014. Print.