In a hydrocarbon zone the rest is oil or gas, provided the pores are not occupied by something else. It is computed from porosity and resistivity with a saturation equation, or measured more directly on core or with dielectric and magnetic-resonance tools.
The computed value includes bound water. A high water saturation does not by itself mean the well will produce water. Irreducible water saturation is the part that stays when oil or gas has displaced everything it can. A saturation from the transition zone lies between irreducible and the fully water-bearing sand below the contact.