Secular, episodic changes in stable carbon isotope composition of crude oils

Stable carbon isotopic compositions of C15+ saturate and aromatic hydrocarbon fractions from 514 crude oils from known source rocks were examined for secular, episodic variations. The oils have been correlated by means of biomarker analyses and/or geological inferences to their respective source rocks and were further categorized into seven source rock types based on geochemical and petrologic indicators of depositional environments. Thus, these oil samples represent a wide range of source paleoenvironments from thirteen age divisions (Riphean to Neogene). The stable carbon isotopic compositions of both fractions become enriched in 13C with decreasing geologic age. Three major, abrupt shifts toward 13C enrichment occurred at the Precambrian–Cambrian/Ordovician, Triassic/Jurassic and Paleogene/Neogene boundaries. Analysis of smaller scale isotopic shifts reveals four distinct groups of oils, generally independent of organic facies and secondary alteration processes. Comparison of evolutionary changes in the biosphere to episodic changes in stable carbon isotopic compositions throughout the Phanerozoic indicates that isotopic shifts of crude oils may generally be related to the diversity of preserved phytoplankton.