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How does natural gas migrate from source rocks into reservoir rocks?

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natural gas The Troll A natural gas production platform in the North Sea, 80 km (50 miles) northwest of Bergen, Norway. Troll A, the largest movable structure ever built, rests on the seafloor some 300 metres (990 feet) below the surface and rises more than 100 metres (330 feet) above the sea. The platform regulates the recovery of gas from 40 wells located on the seafloor.

Natural gas migrates from fine-grained source rocks—such as shales, clays, or clastic carbonates—into porous and permeable reservoir rocks through a two-stage process driven by geological forces and fluid properties.12

Primary Migration

After natural gas is generated from kerogen via geothermal action, it must first exit the narrow pores and capillaries of the source rock. This primary migration is often initiated by the compaction of overlying sediments, which squeezes fluids out of the rock. As clays dehydrate under increasing temperature and pressure, hydrocarbons are expelled in solution, suspension, or emulsion alongside water.12

Secondary Migration

Once expelled, the gas enters "carrier beds," which are coarser-grained, more permeable sedimentary rocks like sandstones or carbonates. Because the specific gravity of natural gas is significantly lower than that of the water saturating subsurface formations, the gas floats upward through these carrier beds over geologic time. This movement continues until the gas is trapped by a nonporous barrier, forming an accumulation.312