Carbon Isotope Composition of Marine Crude Oils

A histogram of δ13C 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 δ13C values, in conjunction with pristane/phytane ratios and sulfur contents. Oils with δ13C values of –32.0 to –28.0‰ are mainly marine shale oils older than Oligocene; oils with δ13C values of –28.0 to –23.5‰ are mainly deltaic oils of varying geologic age or Mesozoic carbonate oils; and oils with δ13C values heavier than –23.5‰ are mainly marine shale oils of Miocene age. The δ13C variation among oil groups is explained primarily by different factors that control the fractionation of carbon isotopes during primary production of organic carbon. Miocene and younger marine shale oils are isotopically heavier than older marine shale oils because of decreased atmospheric CO2 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 δ13C values reflect the time-invariant average δ13C value of land plant carbon at about –25‰. 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 δ13C values of oils. As a result, δ13C values 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.