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How do oil and natural gas move from source rocks into reservoir rocks?

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Multiple large offshore petroleum oil rigs with cranes and equipment stand in the ocean, connected by walkways above the water. — petroleum Liquid petroleum (crude oil)—which is refined into gasoline, diesel, heating fuels, asphalt, plastics, and other useful chemical compounds—is often extracted from the ground using a series of drills and pumps.

The movement of oil and natural gas from source rocks to reservoir rocks occurs in two primary stages driven by geological processes and physical properties.

Primary Migration

Hydrocarbons are generated within fine-grained source rocks, such as shales or clays, through the geothermal maturation of kerogen. Primary migration is the release of these hydrocarbons from the source rock's narrow pores and capillaries. This process is often initiated by the compaction of overlying sediments, which expels water along with the hydrocarbons in solution, suspension, or emulsion.12

Secondary Migration

Once expelled, hydrocarbons undergo secondary migration through coarser-grained, permeable reservoir rocks like sandstones or carbonates. Because these fluids are less dense than the water saturating subsurface formations, they naturally float upward over geologic time. This movement can be facilitated by active water flow, diffusion, or tectonic activity such as folding. Migration continues until the fluids are trapped by nonporous barriers, forming an accumulation, or reach the surface as a seep.312