About 100 C-13 NMR signals of the main n-alkyl- and isoalkyl fragments of molecules (monomethyl-substituted and isoprenane moieties containing up to 15 carbon atoms) in petroleum fractions of saturated hydrocarbons of different compositions were assigned, thus allowing for the detailed characterization of the quantitative composition of these fractions including the ratio between the diastereomers of isoprenane fragments. Two groups of compounds with dimethyl-substituted alkyl fragments (each of them can make up similar to 1% of the total saturated hydrocarbons) were found for the first time in crude oils. A nonequilibrium diastereomer ratio for the isoprenane fragments was found in a sample of hydrothermal oil from the Uzon volcano caldera: the configuration of the chiral centers of dominant isomers is the same as that of the C-7 and C-11 atoms of natural phytol. It was assumed that the analysis of the diastereomer ratio of the 2,6,10Me(3)-fragment in specimens at relatively early stages of catagenesis (black shale hydrocarbons, hydrothermal oils) can be a convenient means of assessing their maturity. A method is described for taking into account the influence of the recording temperature and the analyte concentration in a solution on the position of the signals in the C-13 NMR spectra with an accuracy sufficient for the assignment of the signals at any composition of the saturated hydrocarbon fraction.