Investigations of natural material (using the example of Paleozoic deposits of the Dnieper-Donets depression and Mezozoic deposits of Northern Bulgaria), as well as test data have shown that the main factor determining the composition of relict hydrocarbons of oils (molecular mass distribution of n-alkanes, the values of the pristan/phytan ratio, tetracyclic/pentacyclic naphthenes) is the facial-genetic type of hydrocarbons. Paleodepth, paleotemperature, etc. do not modify their qualitative composition.