1. A method for determining spatial distribution and concentration of clay in a core sample, the method comprising: selecting a water-soluble salt of a metal with a high atomic weight as an X-ray contrast substance providing a selective ion-exchange reaction with the clay, the salt having a general formula R + M − , where R + is selected from a group consisting of Ba 2+ ; Sr 2+ ; Tl + ; Rb +. .. , M − is selected from a group consisting of Cl n ; NO n ; OHn; CH3COO, SO 4 ;. .. in accordance with a standard table of inorganic substances' water solubility, injecting the X-ray contrast substance into a core sample, upon completion of the selective ion exchange reaction injecting a non-contrasting displacing agent into the core sample, scanning the sample by computer X-ray microtomography, obtaining a computer tomography image of the core sample, selecting an area of interest and a reference cross-section at the obtained computer tomography image of the core sample, obtaining grayscale histograms in cross-sections of the core sample and determining spatial distribution and concentration of the clay in the core sample by means of histogram analysis starting from the reference cross-section histogram.