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How do hydrocarbons move from source rocks into reservoir rocks?

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structures of common hydrocarbon compounds — hydrocarbon Structures assumed by hydrogen (H) and carbon (C) molecules in four common hydrocarbon compounds.

Hydrocarbons move from source rocks into reservoir rocks through a two-stage process of migration driven by geological forces and physical properties.

Primary Migration

Hydrocarbons are generated within fine-grained sedimentary rocks, such as shales or clays, through the geothermal transformation of kerogen. As sediments are buried, the pressure from overlying layers and the dehydration of clay minerals expel water, which carries the newly generated hydrocarbons out of the source rock's narrow pores and capillaries.1

Secondary Migration

Once expelled, hydrocarbons enter "carrier beds"—coarser-grained, more permeable rocks like sandstones or carbonates. This movement, known as secondary migration, is often facilitated by plate tectonics or rock folding. Because subsurface formations are water-saturated and hydrocarbons have a lower specific gravity than water, the oil and gas naturally float upward through the carrier beds. This migration continues until the fluids are trapped by nonporous barriers, forming an accumulation in a reservoir rock.21