Carbonates thermal decomposition kinetics and their implications in using Rock-Eval ® analysis for carbonates identification and quantification

International audienceIn 2014, Pillot et al. [Identification and quantification of carbonate species using Rock-Eval pyrolysis,Oil Gas Sci. Technol. – Rev. IFP 69, 341–349. https://doi.org/10.2516/ogst/2012036] proposed to use the Rock-Eval® method as a reliable tool to identify and quantify carbonates in solid samples from the CO2 flux emittedby their progressive thermal decomposition during programmed heating under oxidant atmosphere. Nevertheless,several phenomena associated with the thermal decomposition of carbonates were not explained by theseauthors. This paper attempts to explain these phenomena by adding 5 new carbonate species to the 9 studiedby Pillot et al. https://doi.org/10.2516/ogst/2012036 and by developing a kinetic approach to the thermaldecomposition of carbonates. It appears that the kinetics of thermal decomposition of most carbonates is notof order 1 but varies according to carbonate species. Consequently, the thermal decomposition temperature varieswith both the sample weight and the temperature rate applied. The thermal stability of simple carbonates isexplained by the electronegativity of the cations associated with the carbonate anion. Our study provides furtherinsights into the use of Rock-Eval for the identification and quantification of different carbonate species

Specifications for carbonate content quantification in recent marine sediments using Rock-Eval pyrolysis.

11 pagesInternational audienceThe amount of CaCO3 in sediments and/or sedimentary rocks is usually measured by calcimetry while the nature of the carbonates is determined by X-ray diffraction. Recently, a carbonate recognition method based on the results of Rock-Eval pyrolysis was proposed in 2014 by Pillot et al. [1]. Rock-Eval pyrolysis is also widely used for the characterization of recent sediments. However, later in 2015 Baudin et al. [2] noticed that some of the characteristics of recent sediments tended to produce different results from those of more classical Rock-Eval analyses, causing bias in interpretations.In this study, the thermal stability of fossil and recent marine carbonated sediments was analyzed to identify differences in carbonate decomposition and to underline the importance of accounting for them in Rock-Eval analyses. The state of calcite preservation in recent marine sediments and sedimentary rocks at temperatures between 400 °C and 600 °C was characterized using different techniques (calcimetry, XRD, SEM imaging, etc.) for better interpretation of data obtained with Rock-Eval.Our results highlight a clear difference in the range of calcite decomposition temperatures during Rock-Eval analysis: between 550 °C and 775 °C for bulk clayey hemipelagic sediments versus 650 °C–840 °C after the same sediments were rinsed to get rid of the salt. During heating, water and hydroxide anions are released from clay minerals and react with salt crystals to form acid vapor. This acid vapor reacts with carbonates to produce CO2. The chemical decomposition of carbonate starts at temperatures that are lower than the typical range of decomposition temperatures, leading to overestimation of mineral carbon content (overestimation of the S5 peak) and to underestimation of organic carbon content (underestimation of the S4CO2 peak) with the Rock-Eval method. In the absence of clay minerals, such as in recent marine pure carbonate oozes, there is no evidence for this effect. It is therefore essential to prepare and rinse recent clay-rich carbonated sediment samples before Rock-Eval analysis to avoid misinterpretation

Reproducibility of Rock-Eval® Thermal Analysis for Soil Organic Matter Characterization

International audienceRock-Eval® (RE) is a thermal analysis technique increasingly used to characterise soil organic matter. To interpret the results, particularly when investigating differences between samples, it is necessary to know the expected ranges of analytical error associated with the RE measurements. Moreover, the RE analyzer is now at its seventh version (RE7) while most literature results were produced using the previous version (RE6). Thus, a characterization of the reproducibility of RE measurements is necessary. We measured the reproducibility of RE measurements using fifteen samples from French croplands and forests that were analysed on five different RE instruments, located in different laboratories and belonging to both generations RE6 and RE7. From each RE analysis, we extracted RE parameters commonly used for soil organic matter characterization and we performed the prediction of the active and stable soil organic carbon fractions using a machine learning model (PartySOC) that uses RE parameters. We obtained a measure of the expected relative errors in RE parameters and PartySOC predictions per instrument, across instruments of the same generation and across generations. We found that the parameters total organic carbon (TOC), mineral carbon (MinC) and R-index are well reproducible, even across the RE6 and RE7 generations. Instead, the hydrogen index (HI) and oxygen index (OI) are more sensitive to signal variations, even within the same generation, especially when TOC is low. The PartySOC predictions were well reproducible across RE6 instruments but not across RE generations. In the future, the results of this study will help discriminate relevant differences between soil samples characterised using RE thermal analysis