The conducted studies of statistical distribution of secondary mineral contents in sandy rock have revealed the following regularity – the forms of distribution of these contents are polymodal. Aim. To prove the following hypothesis – at unification of polymodal statistical distribution of intensity of the established geochemical process, each sample of intensity values, within a separate mode, will be characterized by a predictable principle of realization, interpreted by a certain chemical reaction. Methods. Intensities of geochemical reactions caused by superimposed epigenesis were determined using the method of statistical-correlation interpretation of the materials of geophysical studies of wells. Statistical intensities of se-condary processes of kaolinitization, carbonatization and pelitization of sandy sediments were calculated on the basis of regularities of geochemical dominance of covariance influence on the registered parameters of geophysical well surveys. As a result, we obtained the dependence of the intensity of rock transformation on the product of statistical correlation parameters of geophysical data: correlation coefficient and interval parameter. Using the universal equation of modes of unified polymodal probability density distributions for transformation of any open systems, the author unified the modes of intensities of secondary processes and determined the principles of states of transformation processes within each calculated mode. The author studied the secondary transformations of sandy sediments of the Tambay oil and gas fields and investigated the correlations between the sampled intensities of the considered processes within each manifested mode. Conclusions. It was confirmed that each mode of intensity of the considered geochemical process is caused by the manifestation of a certain geochemical reaction. The carried out comparison of the descriptions of the principles of the state of processes with the studied states of geochemical transformations prove the identity of their dynamic modes of transformation. Knowing the intensity of the secondary process as an attribute of the calculated mode, it is possible to determine the mode of the geochemical state of the open reservoir system.