Why do oil and natural gas collect above water inside a petroleum trap?

In the context of petroleum engineering, oil and natural gas collect above water within a petroleum trap primarily due to differences in specific gravity and density.123

Because subsurface sedimentary formations are typically saturated with water, the migration of hydrocarbons occurs in an aqueous environment. Since oil and natural gas have a lower specific gravity than the surrounding formation water, they are buoyant and naturally rise through the porous and permeable reservoir rock over geologic time.213

When these fluids encounter a trap—a closed, inverted container sealed by an impermeable cap rock—they accumulate at the highest point. Within this structure, the fluids stratify based on their relative densities:14
* Natural gas, being the lightest, occupies the top of the trap.12
* Oil settles beneath the gas.12
* Water remains at the bottom, underlying the oil and gas.12

This vertical arrangement is maintained unless hydrodynamic pressure from moving water displaces the pool.1