The saturated compounds in a series of progressively higher boiling petroleum fractions, namely five vacuum gas oils (VGO) and the corresponding vacuum residue (VR) of an Arabian crude oil, were separated and then speciated by using field desorption time-of-flight mass spectrometry. Clear trends of a decreasing saturated compounds content with boiling point and a significant increase of naphthenic rings in the higher boiling fractions are described. Saturated compounds constitute nearly 60 wt % of the lightest VGO sample and only approximately 7.6 wt % in the VR. Saturated molecules have an average of 1-2 naphthenic rings in the light VGO, which increases to an average of 6 naphthenic rings in the VR, with a presence of compounds containing up to 12 naphthenic rings. Given the combination of many naphthenic rings and low number of carbon atoms, some of the polynaphthenic molecules are surprisingly compact, that is, with most (or even all) carbon atoms located in the saturated rings. The potential problem of entrained aromatic components in the saturated fraction is addressed through modeling the naphthenic ring distributions via probability density functions; here the gamma and normal distributions were adapted for this purpose. The modeled average number of naphthenic rings and the mass fraction of aromatic compounds obtained are overall validated by using carbon-13 nuclear magnetic resonance spectroscopy experiments. The mathematical description of the mass spectral data was extended to the carbon number distribution per naphthenic ring series, which allowed the exclusion of lighter boiling components contaminating one heavy VGO sample, and to close assignment gaps in lowly abundant series in the mass spectrometry data. The combination of fractionation, ionization by field desorption, identification of elemental formulas by using a time-of-flight mass spectrometry, and finally the mathematical description of the composition is suitable for the detailed characterization of heavy saturates in petroleum heavy ends.