How do nonporous cap rocks trap petroleum underground?

In the context of petroleum production, nonporous cap rocks act as a physical barrier that prevents the upward or lateral migration of petroleum.

Because petroleum is less dense than the surrounding water, it naturally migrates upward through porous and permeable reservoir rocks, such as sandstone or limestone. When this migrating petroleum encounters an impermeable layer—typically composed of nonporous rock like shale or dense limestone—it becomes trapped.12

This containment creates a petroleum trap, allowing the oil and associated natural gas to accumulate in a reservoir. The specific geometry of these traps, often influenced by geological features like folding or faulting, determines the size and shape of the deposit.12