How do oil and natural gas migrate from source rocks to reservoir rocks?

In petroleum engineering, the movement of hydrocarbons from their origin to a reservoir occurs in two distinct phases based on rock type and pore size.

Primary Migration

Hydrocarbons are generated from kerogen within fine-grained sedimentary rocks, such as shales or clays. Primary migration is the process by which these hydrocarbons leave the narrow pores and capillaries of the source rock. This is often initiated by the pressure of overlying sediments during compaction and the dehydration of clay minerals, which expels water and allows hydrocarbons to move in solution, suspension, or emulsion.1

Secondary Migration

Once expelled, hydrocarbons enter coarser-grained, more permeable carrier beds, such as sandstones or carbonates. This secondary migration can be driven by plate tectonics, folding, or active water movement. Because oil and gas have a lower specific gravity than the water saturating subsurface formations, they float upward through these permeable rocks until they are trapped by nonporous barriers, forming an accumulation in reservoir rocks.21