The carbon isotopic composition of the C (sub 15+) saturate and aromatic hydrocarbon fractions and the gas chromatographs of the C (sub 15+) saturate fraction of 339 oils were geochemically and statistically evaluated. The average isotopic compositions of the C (sub 15+) aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical. The average isotopic composition of the C (sub 15+) saturate fraction of oils derived from terrigenous organic sources is slightly more negative (0.9 per mil) than the average for marine oils, but this difference is insufficient to be used as a reliable source indicator. The isotopic differences between oils sourced from terrigenous and marine organic matter manifest themselves in the isotopic relationship between the saturate and aromatic hydrocarbon fractions. The isotopic relationship is the following: Oils of terrigenous organic source: delta 13 C aro = 1.12delta 13 C sat + 5.45; Oils of marine organic source: delta 13 C aro = 1.10 delta 13 C sat + 3.75. The difference between the two equations was evaluated statistically and a statistical parameter, CV (the canonical variable), CV = -2.53delta 13 C sat + 2.22 delta 13 C aro -11.65, was used to distinguish between oils. CV values larger than 0.47 indicate predominantly a terrigenous organic source for the oil, whereas CV values smaller than 0.47 indicate mostly a marine organic source.--Modified journal abstract.