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Why are strong cap rocks especially important for natural gas accumulations?

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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.

Strong cap rocks are essential for natural gas accumulations because they serve as an impermeable seal that prevents the gas from escaping.12

Because natural gas is highly dispersible and possesses a low initial concentration, it has a strong tendency to migrate upward or laterally through permeable rock formations. To form a viable gas reservoir, the gas must be trapped within a porous and permeable rock layer—such as sandstone or fissured limestone—that is capped by a denser, relatively impermeable stratum, such as shale or evaporites.314

Without this effective barrier, the gas would continue to migrate until it dissipated, failing to collect in the concentrated, exploitable quantities necessary for extraction. Essentially, the cap rock creates the "closed inverted container" required to hold the gas in place within a petroleum trap.234