SECULAR, EPISODIC CHANGES IN STABLE CARBON-ISOTOPE COMPOSITION OF CRUDE OILS

Stable carbon isotopic compositions of C15+ saturate and aromatic hydr ocarbon fractions from 514 crude oils from known source rocks were exa mined for secular, episodic variations. The oils have been correlated by means of biomarker analyses and/or geological inferences to their r espective source rocks and were further categorized into seven source rock types based on geochemical and petrologic indicators of depositio nal environments. Thus, these oil samples represent a wide range of so urce paleoenvironments from thirteen age divisions (Riphean to Neogene ). The stable carbon isotopic compositions of both fractions become en riched in C-13 With decreasing geologic age. Three major, abrupt shift s toward C-13 enrichment occurred at the Precambrian-Cambrian/Ordovici an, Triassic/Jurassic and Paleogene/Neogene boundaries. Analysis of sm aller scale isotopic shifts reveals four distinct groups of oils, gene rally independent of organic facies and secondary alteration processes . Comparison of evolutionary changes in the biosphere to episodic chan ges 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. (C) 1998 Elsevier Science B.V. All rights reserved.