[en] A relationship between neutron moderation properties of water-, oil- and gas-bearing reservoirs and depth factors was studied. Presented are calculations of the spatial distribution and moderation length of epithermal neutrons from the point source under theoretical modelling of geological media with regard for thermodynamic conditions of their occurrence, porosity, mineralogical density as well as physical and chemical properties and the composition of fluids which fill pores. The possibility of principle of the detecting and evaluating the porosity of reservoirs at a great depth using data of the neutron-neutron method with epithermal neutrons is shown