A method of determining the porosity and fluid saturation for a given volume of an underground formation through which a borehole is drilled, the method comprising: measuring, from within the borehole, parameters relating to characteristics of the underground formation; determining volumetric values for resistivity, hydrogen index, density and neutron capture cross-section for the given volume from the measured parameters, each volumetric value including a contribution from formation matrix, water, oil and gas; determining, from the volumetric values, the volumes of rock matrix, pore water, oil and gas, and the salinity of the pore water; and determining the porosity and fluid saturation of the volume of the formation from the determined volumes and salinity.