Porosity of the Bazhenov Formation rocks is concentrated in two main components: Mineral matrix and solid organic matter (kerogen). Since organic matter may occupy up to 50% of the rock volume and the majority of pores are open (connected), this type of porosity can significantly contribute to the total porosity saturated with hydrocarbons. The aim of the study is to investigate the porosity evolution in organic matter (kerogen) of Bazhenov Formation rocks during physical modeling of thermal maturation. Experimental studies of isothermal treatment of the Bazhenov Formation rocks have shown development of new pores in type II kerogen. At a temperature of 390°C single pores with a size of 50-70 nm were observed. Higher temperatures (400-420 ° C) resulted in bigger pores with a size of more than 1 micron. These results expand the existing knowledge of the Bazhenov Formation porosity in two aspects. Firstly, they show that the kerogen's porosity can significantly exceed the intercrystalline porosity in case of high thermal maturation. Secondly, thermal treatment leads not only to kerogen conversion but also to the formation of new pores in organic matter.