[2] In an oil or gas field, hydrocarbons migrate into rocks and can be trapped if there is a permeability barrier to prevent upward escape.
Capillary action can obscure the true water contact in permeable media like sandstone.
Capillary pressure prevents the hydrocarbons from expelling all of the water in the pores, which creates a transition zone between the fully saturated hydrocarbon levels and the fully saturated water levels.
In poorly porous intervals, the oil-water, gas-water or gas-oil contacts can similarly be obscured, which makes estimation of hydrocarbon reserves difficult.
Descriptions of the well's petrophysics will then often further qualify to delineate a gas-down-to, oil-up-to, oil-down-to and water-up-to line, clearly showing the uncertainties involved.