Determination of porosity and fluid saturation of underground formations

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:

• obtaining measurements, from within the borehole, of 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 response equations for each of the measurements and simultaneously solving the response equations for determining the porosity and fluid saturation of the volume of the formation from the determined volumes and salinity.