[3] Migration or the origin of conventional reservoirs

Conventional oil reservoirs: how they are formed?

Under very high pressures from overlying rocks, the light liquid oil is squeezed out of its source rock in which it was formed.

Subsequently, the oil penetrates into  surrounding rocks through a system of interconnected fractures and pores. Oil always tends to move upwards due to its low density differential pressure decreasing towards towards the surface. Going up and upper through strata overlying its source rock, crude oil often takes a winding path as it seeks a place wherein an equilibrated pressure will stop to push it upward. It seeks the surface of the ground.

In the absence of any impediments, the oil will eventually seep out to the surface to form, upon volatilization of its lighter components, extremely dense deposits of wax and native asphalt. This, however, occurs on an exceptional basis.

Rocks overlying the source of oil are thousands of meters extremely diverse. Some of them are impervious. In the absence of interconnected pores or fractures, rocks act like a cork that prevents any continued migration of oil. Moreover, if the rock structure prevents lateral movement the oil is trapped.

The so-called anticlines are the structures that typically form oil traps. Anticlines are rock strata that have been bent by the lateral stress with their central parts moved upward to form a saddle-like structure.


złoże ropy naftowej - schemat


 Conventional oil reservoir in the centre of an anticlinal structure

If a bed of impervious rocks overlies the anticline, then the migrating oil that reaches its top central section is trapped therein without any way out. Moreover, the water (which is heavier than oil) accumulates below the oil reservoir and closes the trap at the bottom.

Trapped oil is waiting for a chance to find a way out. A well drilled to the trap becomes its route of escape. Crude oil flows into the well by natural lift and then takes a different route: to the oil refinery, in order to become the world's most precious commodity.

The origin of unconventional oil accumulations, including shale oil, is explained in the following section: "Lost in action: unconventional shale oil accumulations".

author: Katarzyna Dybkowska

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