The mild thermolysis of resins and asphaltenes from petroleum of different genotypes was performed. It was found that the same C14-C16 polyalkyl-substituted bicyclo[4.4.0]decanes were formed as a result of the thermolysis of resins and asphaltenes. The relative concentration of the thermodynamically more stable isomer 2,3,3,7,7-pentamethylbicylo[4.4.0]decane in the thermolysis products of resins was higher than that in the thermolysis products of asphaltenes and close to equilibrium concentrations. Simultaneously, the amount of the thermodynamically less stable isomer 1,2,3,7,7-pentamethylbicylo[4.4.0]decane generated in the thermolysis products of asphaltenes was greater than that in the thermolysis products of resins, and its relative concentration was far from equilibrium concentrations. The degree of the maturity of organic matter, which was evaluated based on the relative thermodynamic stability of C15 sesquiterpanes in the thermolysis products of resins and asphaltenes, increased in the order asphaltenes-resins-petroleum.