For oil and hydro-carbon withdrawal from the underground, we see here typical case of reservoir.
We may have an aquifer contact with the oil and then the oil in contact with the free gas. We have to define a few terms for the pressure, which means capillary pressure for the oil-water system, then capillary pressure for the oil-gas system and capillary pressure for the water-gas system.
Usually we have cup above the reservoir, which is an impermeable layer.