How does an impermeable cap rock keep oil and natural gas from escaping?

In petroleum engineering, an impermeable cap rock acts as a physical barrier that prevents the upward or lateral migration of petroleum and natural gas. Because oil and gas are less dense than the water typically present in subsurface sedimentary rock formations, they naturally migrate upward through porous and permeable reservoir rocks over geologic time.123

The cap rock, often composed of dense materials like shale or evaporites, seals the top of a petroleum trap. By creating a closed, container-like structure, the cap rock stops these hydrocarbons from escaping to the surface, where they would otherwise be lost to oxidation and bacterial degradation. In cases such as fault traps, the movement of rock along a fault line can also bring an impermeable stratum into contact with a permeable reservoir, effectively blocking further migration.213