Аннотация:The new theoretical estimation criterion of organic matter transformation degree (xi) considering the nature thereof has been proposed based on the mathematical description of methane formation process in the kerogene substratum. Formulae for calculation of this criterion has been obtained. Activation energy distribution functions have been calculated for organic matter of humus and sapropel types taking into account the comparison of theoretical isotope composition calculations of the produced methane with Shtahl's regression dependencies. These functions are used to calculate the xi-criterion. Comparison of the xi-criterion has been undertaken with the temperature-time index being the most applicable today for estimation of degree of organic matter transformation. The. isotope bond numbers method has been used to calculate the temperature dependence of kinetic isotope effects of methane formation.