The fraction of the pore volume occupied by water in a reservoir at maximum hydrocarbon [[saturation]]. In [[water wet|water-wet]] rock. it represents the layer of adsorbed water coating solid surfaces and the [[pendular saturation|pendular]] water around grain contacts and at pore throats. Irreducible water saturation is an equilibrium situation. It differs from "residual water saturation," measured by [[core analysis]] because of filtrate invasion and the gas expansion that occurs when the core is removed from the bottom of the hole and brought to the surface.