Stable Carbon Isotope Compositions of Crude Oils: Application to Source Depositional Environments and Petroleum Alteration

Abstract

The carbon isotopic composition of the C15+ saturate and aromatic hydrocarbon fractions and the gas chromatographs of the C15+ saturate fraction of 339 oils were geochemically and statistically evaluated. Results of this study show that the average isotopic compositions of the C15+ aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical. The average isotopic composition of the C15+ saturate fraction of oils derived from terrigenous organic sources is slightly more negative (0.9^pmil) than the average for marine oils, but this difference is insufficient to be used as a reliable source indicator. Reports in the literature stating that marine oils are isotopically more positive than te rigenous oils, and that the difference can be utilized to distinguish between the two are, therefore, not supported by this study. The isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions. The isotopic relationship is the following: Oils of terrigenous organic source: ^dgr13Caro = 1.12^dgr13Csat + 5.45; Oils of marine organic source: ^dgr13Caro = 1.10^dgr13Csat + 3.75. The difference between the two equations was evaluated statistically and a statistical parameter, CV (the canonical variable), CV = -2.53^dgr13Csat + 2.22^dgr13Caro - 11.65, was used to distinguish between marine and terrigenous 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. Thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2^pmil variations in the isotopic composition of one family of oils. Finally, stable carbon isotopes can, in some situations, help correlate biodegraded oils to their nondegraded counterparts.