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Organic Characterization of the “TÉGulines Clay Formation”, East Paris-Basin, France: Bulk and Molecular Investigations

Abstract : The Tégulines Clay formation (Paris Basin, France) is currently under study as a potential host for a sub-surface geological repository of low-level long-lived radioactive waste. For that purpose, Lerouge et al. (2018) characterized the chemical, mineral, petrological, and textural properties of this rock highlighting a potential reactivity of the organic matter (OM) due to weathering processes. Our objective is to fully characterize the quantity and the quality of the OM of this marine clay formation. Indeed, the refractory or reactive character of OM is an indispensable data for evaluating the overall geochemical reactivity of the rock formation and its long-term evolution. Several cores and trenches were performed between October and December 2017 in the “Brienne-le-Chateau area” (Aube department, France). Several techniques, ranging from bulk to molecular scales were used in order to get a global characterization of the OM fraction. Rock Eval (RE) analysis permit us to get several parameters such as the Total Organic Carbon (TOC) content, Hydrogen Index (HI : hydrogenated quality of total OM) or the Oxygen Index (OI : inherited oxygenated OM or oxidized character of total OM). The quantitative organic petrography (palynofacies) consists on the microscopic observation of isolated OM, permitting the identification and the quantification of the organic compounds present in a constant amount of sediments (1 gram) after the elimination of carbonate and silicate by hydrochloric and hydrofluoric treatments. Finally, in order to get information at molecular scale, the labile fraction (lipids) of sediments was extracted from dried sediments, fractionated into neutral, acidic and polar compounds, derivatized using BSTFA and analyzed by GCMS. The non-extractible kerogen fractions were also investigated using molecular investigations by on line flash-pyrolysis (Py-GCMS). The trench TPH1-1 showed, for the first 30 cm, an OM content characteristic of a soil litter, with preserved palynofacies. Surficial formations of the rest of the TPH1-1 trench (0.3-5m) showed really low OM content. Tégulines Clay samples of the AUB230 borehole (5-68 m) showed gradual oxidation along the profile (Figure 1). Below 55 m, the OM was preserved and revealed the initial conditions of the Tégulines Clay formation with palynofacies characteristics of marine source of OM. Between 55 and 10 m, the oxidation conditions were intermediate between the bottom and the top of the core with, at 25 m a pool of OM more preserved (higher HI and palynofacies characteristic of more preserved conditions). More oxidative conditions were observed between 10 and 6 m. The molecular data of the labile fraction was in accordance with the particulate and bulk investigation with a high Carbon Preference Index (CPI) at the surface (0-30 cm), revealing more preserved conditions. Between 1 and 6 m, the OM showed more oxidized conditions (CPI around 1). Between 10 and 66 m, the CPI is higher than 1 showing more preserved conditions. The Tégulines Clay formation crosscut in the AUB240 borehole isunder cover and can be considered as representative of the initial reduced conditions of the formation. In Tégulines Clay samples of the AUB240 borehole, more OM is well preserved with orange and gelified OM, similar to amorphous OM of the Kimmeridge clay formation. Tégulines Clay samples of the AUB1010 borehole, similar to those of the AUB230 borehole showed a gradient of oxidation. Between 2 and 6 m, the OM was oxidized and the rest of the core was more homogenous with better preservation. This study permitted to characterize the OM oxidation profile of the Tégulines Clay formation and to compare it with previous mineralogical, geological and petrophysical data. The organic investigation showed, for the Trench TPH1-1 oxidative conditions except for the surface sample. The AUB230 and AUB1010 borehole showed a gradual oxidation profile along the core and for the AUB230 core some preserved areas. The AUB240 borehole revealed more preserved conditions. All the samples were characterized by low content in OM. This low content was associated with more refractory conditions, supposing a limited influence of the OM on the transfer and reactivity of radionuclides. References Lerouge et al., 2018. A deep alteration and oxidation profile in a shallow clay aquitard: example of the Tégulines Clay, East Paris Basin, France. Geofluids.
Keywords : GEOF
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Submitted on : Wednesday, October 6, 2021 - 1:40:27 PM
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M. Le Meur, M. Boussafir, Claude Le Milbeau, M. Debure, C. Lerouge. Organic Characterization of the “TÉGulines Clay Formation”, East Paris-Basin, France: Bulk and Molecular Investigations. 29th International Meeting on Organic Geochemistry, Sep 2019, Gothenburg, Sweden. pp.1-2, ⟨10.3997/2214-4609.201902765⟩. ⟨insu-03367759⟩

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