Lead isotopes as tracers of crude oil migration within deep crustal fluid systems

• Published in: Earth and Planetary Science Letters
• Main Authors: Fetter, Nadège, Blichert-Toft, Janne, Ludden, John, Lepland, Aivo, Sánchez Borque, Jorge, Greenhalgh, Erica, Garcia, Bruno, Edwards, Dianne, Telouk, Philippe, Albarède, Francis
• Other Authors: Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS), British Geological Survey Edinburgh, British Geological Survey (BGS), Laboratoire de Sciences de la Terre (LST), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Geological Survey of Norway (NGU), Norwegian Petroleum Directorate, IFP Energies nouvelles (IFPEN), Geoscience Australia
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2019
• Subjects: Northern Europe; oil migration; Pb isotopes; porous-media convection; crude oil; black shales; [SDE]Environmental Sciences; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Barents Sea
• Online Access: https://ifp.hal.science/hal-02335910; https://ifp.hal.science/hal-02335910/document; https://ifp.hal.science/hal-02335910/file/1-s2.0-S0012821X1930439X-main.pdf; https://doi.org/10.1016/j.epsl.2019.115747

International audience Although Pb, U, and Th may be fractionated between crude oil and formation waters, Pb isotopes are not. This unique property makes Pb isotopes a particularly useful marker of hydrocarbon generation and migration. Here we show that Pb isotopes offer a new vision of long-range (secondary) oil migration relevant to the formation of oil fields. North Sea oils are largely generated from Jurassic black shales, yet their Pb isotopes are mixtures of Cenozoic to Proterozoic end-members. The same observation is made for crude oils from the Paris Basin, the Barents Sea, Libya, Kuwait, Kazakhstan, and Australia. Bulk Pb in crude oil therefore, for the most part, is foreign to its source rock(s). Our high-precision Pb isotope data on 195 crude oils worldwide, the first such data set in the published literature, and 17 Northern European black shales indicate that deep-seated Pb components originating beneath the source rocks are ubiquitous in crude oil. This implies that oil fields are embedded in basinal convective systems of hydrous fluids heated from below. Plumes of hot fluids rise from the lower thermal boundary layer, which Pb isotopes require douse the basement, into the core of the porous-flow convective cell where they dissolve the newly formed hydrocarbons sequestered in the source rocks. The fluids finally unload unmixed formation waters and crude oil at the base of the upper (conductive) boundary layer where they can be trapped in favorable sites. Based on these new insights we argue that Pb isotopes in crude oil constitute a good tracer of oil migration.