A histogram of (isotope){13}C values of 621 post-Ordovician marine oils shows a trimodal distribution. Within this distribution, four groups of oils can be recognized on the basis of (isotope){13}C values, in conjunction with pristane/phytane ratios and sulfur contents. Oils with (isotope){13}C values of -32.0 to -28.0 o/oo are mainly marine shale oils older than Oligocene; oils with (isotope){13}C values of -28.0 to -23.5 o/oo are mainly deltaic oils of varying geologic age or Mesozoic carbonate oils; and oils with (isotope){13}C values heavier than -23.5 o/oo are mainly marine shale oils of Miocene age. The (isotope){13}C variation among oil groups is explained primarily by different factors that control the fractionation of carbon isotopes during primary production of rganic carbon. Miocene and younger marine shale oils are isotopically heavier than older marine shale oils because of decreased atmospheric CO[2] concentration since 25 Ma, which has resulted in decreased isotope fractionation by marine plankton during photosynthesis. Deltaic oils are mostly derived from terrestrial organic matter, and their (isotope){13}C values reflect the time-invariant average (isotope){13}C value of land plant carbon at about -25 o/oo. Mesozoic carbonate oils are isotopically heavy due mainly to salinity and temperature effects during primary production in restricted environments from which terrestrial organic matter is excluded. Processes that occur after oil generation generally have smaller effects on (isotope){13}C values of oils. As a result, (isotope){13}C val es of oils are useful for determining oil-oil and oil-source rock relationships and, in conjunction with other geochemical properties, can indicate the possible age and depositional environment of source rocks.