In the paper, factor analysis as a geostatistical method used for the interpretation of borehole geophysical data is presented. The result of the procedure is a factor log, which contains valuable information about the lithology of the sequence of strata. We found a strong connection between one special factor and shale content. Analogously to gamma ray index, the so-called factor index was defined, which proved to be a good shale indicator. The method gives consistent results providing reliable information in water-bearing sedimentary structures.