Petroleum samples from 17 fields and 9 reservoirs could be grouped into 3 families on the basis of their V-Ni ratios and S contents. The V-Ni ratio represents a genetic indicator while the S content is a measure of bacterial influence. Other elements detd. (i.e., by neutron activation anal. and wavelength-dispersive x-ray fluorescence) were Cr, Fe, Co, Zn, Se, Br, I, and Ag.