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The ability to use lead isotopes as a means of dating sedimentary,stratiform ore deposits suggests that lead isotope analysesof 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 analyseswere performed on suites of kerogens, crude oils, and unleadedgasolines to assess lead isotopic relationships during catagenesisof kerogen and refining of crude oil. Alaskan North Slope kerogen–crude oil samples from Cretaceoussource rocks, as well as unleaded gasoline refined from NorthSlope crude oil feedstock, exhibit overlapping or concordant206Pb/207Pb and 206Pb/204Pb ratios (1.215–1.225 and 18.9–19.2,respectively). This indicates that lead isotopic ratios areconservative, being passed on from kerogen to crude oil duringcatagenesis and from crude oil to gasoline during refining.Lead isotope ratios of North Slope hydrocarbons are consistentwith those expected of Cretaceous crustal rocks in the region,indicating that lead isotope ratios of kerogens and crude oilsserve as age-sensitive genetic markers during petroleum generation. A second suite of crude oils, with known source rock ages (Devonianto Tertiary), was also analyzed. Lead isotopic ratios of crudeoils of different geologic age vary systematically, increasingwith decreasing geologic age, being consistent with averagecrustal lead growth models. Results indicate that crude oillead isotopic evolution follows the same systematics as crustalrocks, increasing through the Phanerozoic because of productionof 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 arein good agreement with known source rock ages, with age resolutionpotentially approaching that of individual geologic periods. The observed lead isotopic variations of crude oils explainwhy lead isotopic ratios of unleaded gasolines produced by differentmanufacturers differ. Unleaded gasoline 206Pb/207Pb and 206Pb/204Pbratios range from approximately 1.19 and 18.40 to approximately1.24 and 19.5, respectively, and are consistent with lead isotopicranges of Cretaceous-/Tertiary-age crude oils, which are blendedin variable proportions with North Slope crude oil to producemost U.S. unleaded gasolines. However, each of the six manufacturer'sunleaded gasoline lead isotope ratios are distinct, indicatingthat lead isotopes can be used to discriminate among sourcesof unleaded gasoline accidentally released into the environment. |
