- ©2002. AAPG/DEG
The ability to use lead isotopes as a means of dating sedimentary, stratiform ore deposits suggests that lead isotope analyses of sedimentary kerogen and crude oil may also provide age-sensitive, genetic information that is pertinent to petroleum generation. In order to evaluate this premise further, lead isotopic analyses were performed on suites of kerogens, crude oils, and unleaded gasolines to assess lead isotopic relationships during catagenesis of kerogen and refining of crude oil.
Alaskan North Slope kerogen–crude oil samples from Cretaceous source rocks, as well as unleaded gasoline refined from North Slope crude oil feedstock, exhibit overlapping or concordant 206Pb/207Pb and 206Pb/204Pb ratios (1.215–1.225 and 18.9–19.2, respectively). This indicates that lead isotopic ratios are conservative, being passed on from kerogen to crude oil during catagenesis and from crude oil to gasoline during refining. Lead isotope ratios of North Slope hydrocarbons are consistent with those expected of Cretaceous crustal rocks in the region, indicating that lead isotope ratios of kerogens and crude oils serve as age-sensitive genetic markers during petroleum generation.
A second suite of crude oils, with known source rock ages (Devonian to Tertiary), was also analyzed. Lead isotopic ratios of crude oils of different geologic age vary systematically, increasing with decreasing geologic age, being consistent with average crustal lead growth models. Results indicate that crude oil lead isotopic evolution follows the same systematics as crustal rocks, increasing through the Phanerozoic because of production of radiogenic lead via radioactive decay of uranium isotopes. As observed with model lead ages of stratiform ore minerals, model lead ages based on crude oil lead isotopic ratios are in good agreement with known source rock ages, with age resolution potentially approaching that of individual geologic periods.
The observed lead isotopic variations of crude oils explain why lead isotopic ratios of unleaded gasolines produced by different manufacturers differ. Unleaded gasoline 206Pb/207Pb and 206Pb/204Pb ratios range from approximately 1.19 and 18.40 to approximately 1.24 and 19.5, respectively, and are consistent with lead isotopic ranges of Cretaceous-/Tertiary-age crude oils, which are blended in variable proportions with North Slope crude oil to produce most U.S. unleaded gasolines. However, each of the six manufacturer's unleaded gasoline lead isotope ratios are distinct, indicating that lead isotopes can be used to discriminate among sources of unleaded gasoline accidentally released into the environment.
Richard W. Hurst received his Ph.D. from the University of California, Los Angeles in 1975. He is a Professor of Biogeochemistry at California State University, Los Angeles and has been a consultant in environmental forensic geochemistry since 1978; he has served as an adjunct professor at the University of Maryland, Eastern Shore and is currently serving as a coadvisor to undergraduates in environmental geochemistry at the Massachusetts Institute of Technology. His research interests include the integration of isotope geochemistry with hydrogeological and statistical data to resolve problems associated with environmental remediation and petroleum exploration.