Abstract: In one embodiment the invention comprises a method for generating a log of a subsurface formation, in which data from well logging measurements are utilized to derive a relationship between porosity and irreducible water saturation for the formation and the derived relationship between porosity and irreducible water saturation is applied to a porosity log of the subsurface formation to generate a log of irreducible water saturation. In another embodiment the invention comprises a method for calculating a theoretical water saturation log for a subsurface formation which utilizes a relationship based on porosity, irreducible water saturation and height above the petroleum-water contact level.