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Why must a gas reservoir have an impervious cap rock?
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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.
A gas reservoir must be capped by an impervious rock layer to form an effective seal that prevents natural gas from escaping upward or laterally.1
Because natural gas migrates from source rocks into more porous and permeable reservoir rocks due to pressure differences, it requires a closed, container-like structure to accumulate. An impervious cap rock—typically composed of dense materials like shale or evaporites—acts as a barrier that traps the gas within the reservoir, ensuring it remains concentrated for future extraction. Without this seal, the gas would continue to migrate and dissipate, making it impossible to form a viable reservoir.12