[en] The differential gamma ray log is a geochemical tool making use of a gamma spectrometry technique, with a (Na I) - thallium activated detector, for quantitative determination of uranium, thorium, and potassium in formations encountered in bore holes. The geological applications of this log have been based on laboratory work carried out to define the geochemical cycle of the naturally radioactive elements in the sedimentary system. Apart from applications relative to radioactive elements exploration, the log may be used for lithological identification and facies reconstruction. Direct determination of clay content and quantitative estimation of mica in sand reservoirs is given