Figure 23. Stable carbon-13 isotope ratios for saturated vs. aromatic hydrocarbons from Covenant field oil and Cretaceous and Permian oils. Units on both axes of the graph depict the carbon isotopes measured in the oil versus the Pee Dee Belemnite (PDB) standard in parts per thousand; a negative value implies the oil sample is depleted in the heavy isotope relative to the standard. Data sources: 1=Sprinkel et al., 1997; 2=Lillis et al., 2003; 3=Baseline DGSI, 2005. CV= Canonical value (Sofer, 1984).