It is well-known that under suitable borehole conditions there is a relationship between porosity and the deflection of a neutron log curve. This relationship finds practical use, calibration of the neutron deflections being made by reference to porosities measured on cores. It is shown that theoretically core analysis is not mandatory for neutron log calibration and that the calibration may be achieved by direct plotting of suitable neutron and electric log data. For this purpose the concept of the neutron deflection corresponding to a formation factor of unity is introduced. It is shown that this concept makes it possible to use a combination of electrical and radioactivity log data to locate zones of oil saturation in carbonate rock formations and even to estimate the oil saturation. Application of the method to sandstones is also considered. In oil-base mud logging, neutron-log data may be conveniently combined with resistivities read off induction logs to give information bearing on the location of saturated zones and the estimation of connate water salinities. Finally a combination of electrical and radioactivity log data is theoretically capable of contributing to the quantitative elucidation of the "dirty sand" problem.