How are natural gas, crude oil, and bitumen arranged when they occur in the same petroleum deposit?

In a typical petroleum reservoir, natural gas, crude oil, and water often arrange themselves based on their relative densities within a porous reservoir rock. Because of their lower density, natural gas and crude oil migrate upward until they are trapped by an impermeable cap rock.12

Within such a trap, the fluids generally stratify according to gravity:
* Natural gas typically occupies the highest portion of the reservoir.
* Crude oil accumulates beneath the gas.
* Water (residual formation water) occupies the lowest portion of the pore space, as it is denser than the hydrocarbons.324

Bitumen, however, is a highly viscous, extra-heavy form of petroleum that does not behave like conventional liquid oil. It is often found as an impregnation filling the pore spaces of sediments, such as in oil sands, rather than as a mobile fluid that stratifies in the same manner as gas and light oil. Bitumen is frequently the result of the degradation of oil that has seeped toward the surface, where the loss of volatiles and biological processes have increased its density and viscosity.451