The authors show that data on characteristics of drill mud penetration zone improve reliability of geoinformation obtained from borehole geology and geophysics. The developed procedure for the data interpretation takes into account the geomechanics and hydrodynamics of drilling. A part of the drill mud penetration zone is represented by mudcake which prevents direct measurement of porosity and permeability. The article describes the experimental studies on growth of the mudcake on the samples of low-permeable sandstone from the Jurassic reservoir rock mass using an original facility. The petrophysical nonuniformity of the mudcake was determined. The repeated measurements revealed the mudding zone.