Method for characterizing organic carbon in a sample of a surface formation

Abstract

The invention relates to a method for characterizing the carbon in a sample of a surface formation. The method comprises heating the sample under an oxidizing atmosphere from an initial temperature of 100 to 300 °C to a final temperature of 650 to 1000 °C, during which the quantities of CO and CO2 released are continuously measured. Then, based on these quantities and a limiting temperature of the heating sequence of 540 to 600 °C, preferably 580 °C, the organic carbon present in the sample is characterized by determining an indicator representative of the thermal stability of the organic carbon and / or a parameter representative of the thermal lability of the organic carbon. Figure 3 to be published.

Description

Title of the invention: Method for characterizing organic carbon in a sample of a surface formation. Technical field

[0001] The present invention relates to the field of soil science and environmental geosciences, more specifically to the characterization of carbon content in surface formations, and particularly in soils. The present invention finds particular application in the case of soils with low and very low organic carbon content.

[0002] In order to address environmental challenges or comply with certain environmental laws / directives, stakeholders in soil science and environmental geosciences (research laboratories, consulting firms, environmental agencies, farmers) are increasingly required to implement protocols for monitoring the impacts of human activities on soil carbon stocks and agro-ecosystems. These monitoring and impact studies require the ability to analyze large series of samples within relatively short timeframes compared to conventional methodologies. Furthermore, these methods are often accompanied by environmental and safety constraints that increase analytical time and costs, and frequently necessitate the use of specialized service providers (e.g., analytical laboratories).

[0003] Organic forms of carbon stored in surface formations, and in particular in soils, represent a major issue for agriculture and climate. They play a crucial role in the structural quality and fertilizing value of soils, but above all they are involved in the carbon cycle, representing the largest reservoir of organic carbon on the Earth's surface.

[0004] Soils with low organic carbon content deserve special attention, as their rehabilitation and restoration would allow the storage of significant quantities of organic carbon and thus contribute to efforts to mitigate anthropogenic greenhouse gas emissions, but also to improve soil quality and health.

[0005] However, quantifying and characterizing the low organic carbon content (hereafter referred to as Corg) of soils in the presence of mineral carbon (hereafter referred to as Cmin) is problematic and represents a real technical challenge.

[0006] Quantifying organic carbon in soils devoid of mineral carbon is fairly simple with standard analytical tools, such as an elemental analyzer. However, it is more complex in the case of soils containing Cmin. Indeed, In this scenario, determining total carbon (Corg) requires sample pretreatment (calcimetry or decarbonation). For analytical methods requiring a calcimetry step (measuring the volume of CO2 released by the action of hydrochloric acid (HCl) on the carbonate minerals in a sample, for example, according to the standardized method NF ISO 10693), quantifying carbon forms involves, firstly, measuring total carbon (hereafter denoted Ctotal) by dry combustion and, secondly, determining total minus carbon (Cmin) by calcimetry. Corg is then obtained as the difference between Ctotal and total minus carbon. However, this method has limitations, as it is not suitable for soil samples rich in minus carbon and low in Corg. These limitations are related to the uncertainties inherent in high values ​​of Ctotal and total minus carbon, which accumulate and lead to inaccurate Corg measurements, or even negative values.For analytical methods requiring a decarbonation step (analysis consisting of contacting a soil sample with a volume of dilute HCl to eliminate the various forms of carbonates present by releasing carbon dioxide), the quantification of carbon forms involves, firstly, measuring the total carbon by dry combustion and, secondly, determining the total organic carbon (Corg) after decarbonation of the sample. The total minimum carbon (Cmin) is then obtained by subtracting the total Corg from the total C. This type of pretreatment often leads to alteration of the most labile organic fraction and, consequently, to underestimation of the Corg content and overestimation of the Cmin content. Previous technique

[0007] The following documents will be cited during the description:

[0008] Behar, F., Beaumont, V., Penteado, HDB, 2001. Rock-Eval 6 technology: per formances and developments. Oil Gas Sci. Technol. 56, 111-134. https: / / doi.Org / 10.2516 / ogst:2001013.

[0009] Disnar, J.-R., Guillet, B., Kéravis, D., Di-Giovanni, C., Sebag, D., 2003. Soil organic matter (SOM) characterization by Rock-Eval pyrolysis: scope and limitations. Org. Geochem. 34, 327-343. https: / / doi.org / 10.1016 / 80146-6380(02)00239-5.

[0010] Plante, Alain F.; Fernandez, José M.; Leifeld, J. (2009) Application of thermal analysis techniques in soil science. In : Geoderma, vol. 153, n° 1-2, p. 1-10. DOI: 10.1016 / j.geoderma.2009.08.016.

[0011] Saenger, A., Cécillon, L., Sebag, D., Brun, J.-J., 2013. Soil organic carbon quantity, chemistry and thermal stability in a mountainous landscape: A Rock-Eval pyrolysis survey. Org. Geochem. 54, 101-114. https: / / doi.org / 10.1016 / j .orggeochem.2012.10.008

[0012] Sebag, D., Disnar, J.R., Guillet, B., Giovanni, C.D., Verrecchia, E.P., Durand, A., 2006. Monitoring organic matter dynamics in soil profiles by ‘Rock-Eval pyrolysis’: bulk characterization and quantification of dégradation. Eur. J. Soil Sci. 57, 344-355. https: / / doi.org / 10.! 111 / j. 1365-2389.2005.00745.

[0013] Sebag, D., Verrecchia, E.P., Cécillon, L., Adatte, T., Albrecht, R., Aubert, M., Bureau, F., Cailleau, G., Copard, Y., Decaens, T., Disnar, J.-R., Hetényi, M., Nyilas, T., Trombino, L., 2016. Dynamics of soil organic matter based on new Rock-Eval indices. Geoderma284, 185-203. https: / / doi.Org / 10.1016 / j.geoderma.2016.08.025.

[0014] With regard to the characterization of organic carbon in particular, there are known documents (Disnar et al., 2003; Sebag et al., 2006; Saenger et al., 2013; Plante et al., 2009) that describe thermal stability indicators determined from a thermal analysis of soil organic matter. More specifically, these documents describe the implementation of the "ROCK-EVAL® BULK ROCK" thermal analysis, initially developed for source rock samples (Behar et al., 2001), which includes measurements of the quantities of hydrocarbon compounds (HC), carbon monoxide (CO), and / or carbon dioxide (CO2) released over time by a sample subjected to a heating sequence in an inert atmosphere followed by a heating sequence in an oxidizing atmosphere.According to this method, the heating sequence in an inert atmosphere is characterized by an initial temperature T1 of the pyrolysis furnace, generally between 300°C and 350°C, which is maintained for a predetermined duration of a few minutes. The pyrolysis temperature is then gradually increased to a temperature T2, typically 650°C. The quantity of hydrocarbon compounds released during this thermal cracking phase is estimated by measuring the area of ​​a second peak, denoted S2. Simultaneously, the quantities of CO and CO2 are measured and also plotted as curves.The CO (respectively CO2) measurement curve shows two peaks, a first peak classically noted S3CO (respectively S3CO2) which is considered to correspond to the CO (respectively CO2) generated by the cracking of the organic matter of the sample during heating under an inert atmosphere, and a second peak classically noted S3'CO (respectively S3'CO2) which is considered to correspond to the CO (respectively CO2) generated by the thermal decomposition of carbonate forms (notably calcite) during heating under an inert atmosphere.Then the residue from the sample obtained by heating in an inert atmosphere is subjected to heating in an oxidizing atmosphere: starting from a temperature between approximately 300°C and 400°C, and preferably 300°C, the temperature of the residue is raised according to a temperature gradient of between 20 and 40°C / minute, up to an oxidation termination temperature between 750 and 950°C, and preferably 850°C. During this heating sequence in an atmosphere... During the oxidative phase, the amounts of CO and CO2 released from the sample residue are measured and plotted as curves, leading to a peak classically denoted S4CO (respectively S4CO2), which is considered to correspond to the amount of CO (respectively CO2) generated by the combustion of organic matter during the oxidation cycle. However, the method described in the documents (Disnar et al., 2003; Sebag et al., 2006) only uses the S2 thermogram, which corresponds to the HC released during the pyrolysis phase. More precisely, the method described in these documents relies on deconvolution to subdivide the S2 peak (also denoted HCpyr) into areas reflecting different degrees of thermal stability of the soil organic matter.These documents lead to the distinction between "biopolymers," which are organic compounds pyrolyzed below 400°C and dominant in fresh biological samples, and "geopolymers," which correspond to various classes of compounds pyrolyzed above 400°C and dominant in non-biological samples, i.e., soils and sedimentary rocks.

[0015] We are also familiar with the document (Sebag et al., 2016) which describes two indicators also established from a thermal analysis as described above, comprising a two-phase heating sequence: a pyrolysis phase and an oxidation phase. These two indicators, denoted I and R, are also calculated from the S2 thermogram, which provides information on the total quantity of hydrocarbon compounds released during the pyrolysis phase. More specifically, the authors delineate five surfaces on the S2 thermogram, denoted Al, A2, A3, A4, and A5, each corresponding to a temperature range. The Al and A2 surfaces, whose temperature ranges are between 200–340°C and 340–400°C, respectively, allow us to consider the thermal classes of highly labile (Al) or labile (A2) organic carbon. Surface A3, between 400 and 460°C, corresponds to a more resistant thermal class of organic carbon.And for surfaces A4 and A5, whose temperature ranges are respectively between 460-520°C and 520-650°C, the forms of carbon are considered respectively as refractory (A4) and highly refractory (A5). Based on these surfaces, this document defines the following I and R indicators: .

[0016] I = l0gl(|(A^)(l)

[0017] R = ((A3 + A4 + A5) / 100)(2)

[0018] Thus, indicator I uses surfaces A1 and A2 to highlight the degree of transformation of the immature organic fraction, and is therefore representative of the lability of the organic carbon in a sample. Indicator R is based on surfaces A3, A4, and A5, and represents the proportion of thermally stable carbon compounds, and is therefore representative of the thermal stability of the organic carbon in a sample. By Elsewhere, this document also defines the I=f(R) diagram, which allows us to understand the dynamics of the most reactive fraction (rich in HC bonds) of soil organic matter, representing less than 20% of the total organic matter. As will be demonstrated below in the comparative application example, these indicators are not always representative of all forms of organic carbon. Furthermore, in soils with low Corg content, S2 thermograms are very noisy and poorly reproducible, which directly impacts the reliability of the I and R indicators calculated for samples with very low Corg levels.

[0019] The present invention aims to overcome the drawbacks of the prior art. In particular, the present invention defines a new thermal analysis protocol, comprising a single heating sequence, in this case under an oxidizing atmosphere. The method according to the invention further includes the definition of new indicators, adapted to the new protocol, to allow the characterization of organic carbon in the case of samples with high and low organic carbon content. Thus, the present invention allows for faster, simpler, and complete characterization of organic carbon for samples characterized by a wide range of organic carbon contents. Summary of the invention

[0020] The invention relates to a method for characterizing the organic carbon present in a surface formation, from a representative sample of said surface formation, said sample not having been heated in an inert atmosphere beforehand.

[0021] According to the invention, the method comprises at least the following steps:

[0022] A) said sample is heated in an oxidizing atmosphere according to a predefined temperature sequence of which an initial temperature (T0') is between 100 and 300 °C, and a final temperature (TF') is between 650 and 1000 °C, said temperature sequence comprising at least a thermal gradient between 1 °C / min and 50 °C / min, and at least a quantity of CO and a quantity of CO2 released during said heating sequence in an oxidizing atmosphere are continuously measured;

[0023] B) The organic carbon present in said sample is characterized by determining, from said continuous measurements of said quantities of CO and CO2 released during said heating sequence in an oxidizing atmosphere, at least one indicator representative of the thermal stability of organic carbon defined by a formula of the type:

[0024] S = ((A3 + A4 + A5) / 100)

[0025] and / or a parameter representative of the thermal lability of organic carbon defined by a formula of the type: L = logl0(^)

[0026] where A1, A2, A3, A4, and A5 represent the carbon content released as CO2 and CO by said sample during said sequence of temperatures in a temperature range between said initial temperature (T0') and 340°C, between 340 and 400°C, between 400 and 460°C, between 460 and 520°C, and between 520 and a limit temperature, said limit temperature being between 540°C and 600°C and preferably 580°C.

[0027] According to one embodiment of the invention, said carbon content Ai released as CO2 and CO, i varying between 1 and 5, can be determined according to a formula of the type:

[0028] rTAi-max rTAi-maxQÇfyz*^. ~ TAi-mm « + TAi-min m

[0029] where QCO2 and QCO represent the evolution over time of said quantity respectively of CO2 and CO released during said sequence of temperatures between a minimum temperature TAi - min and a maximum temperature TAi - max of said temperature range of said content Ai and m is the mass of said sample.

[0030] According to one embodiment of the invention, said initial temperature (T0') may be 200°C.

[0031] According to one embodiment of the invention, said final temperature (TF') can be 850°C.

[0032] According to one embodiment of the invention, it is further possible to determine at least one parameter representative of an organic carbon content defined by a formula of the type:

[0033] oxyTOC(%) = [S4CO2] + [(S4CO + S4CO)^]

[0034] where S4CO2 and S4CO represent said quantities respectively of CO2 and CO released during said temperature sequence up to said limit temperature, and where S4'CO represents said quantity of CO released during said temperature sequence beyond said limit temperature.

[0035] According to one embodiment of the invention, if said final temperature is greater than or equal to 850°C, at least one parameter representative of a mineral carbon content defined by a formula of the type can be further determined:

[0036] oxyMinC = [55*^]

[0037] where S5 represents said quantity of CO2 released during said sequence of temperatures beyond said limit temperature.

[0038] According to one embodiment of the invention, said temperature sequence may include an isothermal plateau of predetermined duration at said initial temperature (T'0) of said temperature sequence, said predetermined duration of said isothermal plateau at said initial temperature (T'0) of said temperature sequence being between 1 and 5 minutes, and preferably worth 3 minutes.

[0039] According to one embodiment of the invention, said at least one thermal gradient of said temperature sequence may be between 15°C / min and 40°C / min, and very preferably be 25°C / min.

[0040] According to one embodiment of the invention, said limiting temperature can be determined by finding a minimum in a curve representing an evolution of the quantity of carbon QC released as CO and CO2 during said temperature sequence according to a formula of the type:

[0041] QC = QCO + QCO2

[0042] where QCO2 and QCO represent said evolution over time of said quantity respectively of CO2 and CO released during said sequence of temperatures.

[0043] According to one embodiment of the invention, the minimum of the curve representing the evolution of the quantity of carbon QC released as CO and CO2 during the temperature sequence can be determined graphically or numerically. List of figures [Fig 1]

[0044] Figure 1 schematically illustrates an example of implementing the temperature sequence under an oxidizing atmosphere of the process according to the invention. [Fig. 2]

[0045] Figure 2 illustrates an example of a thermogram obtained by means of an implementation of the temperature sequence under an oxidizing atmosphere according to the invention. [Fig. 3] [Fig. 4]

[0046] Figure 3 (or Figure 4) illustrates an example of a curve representing the evolution of the amount of carbon released as CO2 during heating under an oxidizing atmosphere according to the invention for a sample with a high organic carbon content (or for a sample with a low organic carbon content). [Fig. 5A] [Fig. 5B]

[0047] Fig. 5A (respectively Fig. 5B) shows in the form of black squares the distribution of the values ​​of a lability indicator as a function of the values ​​of a thermal stability indicator determined by the process according to the invention for samples with low organic carbon content (respectively with high organic carbon content). [Fig 6]

[0048] Figure 6 shows, for the samples with low organic carbon content in Figure 5A, the distribution of the lability indicator values ​​as a function of the thermal stability indicator values ​​determined by the process according to the invention under the in the form of black squares, as well as the distribution of the values ​​of a lability indicator as a function of the values ​​of a thermal stability indicator determined by a prior art process in the form of grey circles. [Fig 7]

[0049] Fig. 7 shows, for the samples with high organic carbon content of Fig. 5B, the distribution of the values ​​of the lability indicator as a function of the values ​​of the thermal stability indicator determined by the process according to the invention in the form of black squares, as well as the distribution of the values ​​of a lability indicator as a function of the values ​​of a thermal stability indicator determined by a process according to the prior art in the form of grey circles. Description of the implementation methods

[0050] The invention relates to a method for characterizing the organic carbon present in a surface formation.

[0051] By "superficial deposit" is meant a continental or coastal formation, unconsolidated or secondarily consolidated, resulting from the mechanical and / or chemical weathering of pre-existing rocks, and formed at the lithosphere / biosphere / atmosphere interface. A distinction is made between (i) "allochthonous superficial deposits" (such as colluvium, alluvium, loess, etc.) which have undergone or are still undergoing short or long-distance displacements, and no longer rest on their parent material, and "autochthonous superficial deposits" (such as sandy soils, weathered material, flint clays, etc.) which have evolved in situ from a parent material which still constitutes their substrate.

[0052] By soil, we mean all the external layers of surface formations, the properties of which are directly controlled by the mutual actions of water, air and living and dead organisms, or even human activities for the most recent periods.

[0053] The method according to the invention requires at least one representative sample of the surface formation: this sample may have been taken manually from a pit or by coring using an auger. Advantageously, the sample as taken is sieved using a sieve with 2 mm diameter orifices, dried at a temperature below 40°C, and then ground to obtain fragments with dimensions less than 200 µm. Thus, a "representative sample of the surface formation" is hereafter referred to as a sample taken from the surface formation under study and having optionally undergone preparation including sieving, drying (not under an inert atmosphere), and grinding.

[0054] The method according to the invention can advantageously, but not exclusively, be put into implemented using the ROCK-EVAL® device (IFP Energies nouvelles, France), as described in patents FR 2227797 (US 3953171) and FR 2472754 (US 4352673). The ROCK-EVAL® device includes at least: • an oxidation furnace in an oxidizing atmosphere, • means of measuring carbon monoxide (CO) and carbon dioxide (CO2).

[0055] The process can alternatively be implemented using any furnace allowing heating in an oxidizing atmosphere, cooperating with one or more devices for measuring carbon monoxide and / or carbon dioxide.

[0056] Thus, unlike prior art processes, the process according to the invention does not require a heating sequence under an inert atmosphere (pyrolysis). Indeed, the Applicant observed, through analyses carried out on numerous samples with low organic carbon content, that the curves representing hydrocarbon compounds, CO, and CO2 released during the pyrolysis phase exhibit low reproducibility, introducing random noise and generating quantifications and characterizations of organic carbon with a possible measurement error. The Applicant then observed that, based on the shape of soil thermograms, it is possible to establish indicators to characterize soil organic matter from oxidation signals, in particular from a combined signal corresponding to the sum of CO and CO2 emitted during the single oxidation phase.

[0057] Since the sample to be analyzed has not been preheated in an inert atmosphere (in other words, the process is applied directly to a representative sample of the surface formation as described above), the process according to the invention comprises at least the following steps: 1. Heating sequence under an oxidizing atmosphere 2. Characterization of the organic carbon present in the sample

[0058] The steps of the process according to the invention are described below in a non-limiting manner for a soil sample. The steps of the process according to the invention can in fact be applied equally well to a sample taken from another layer of a surface formation. 1. Heating sequence under an oxidizing atmosphere

[0059] During this step, the sample is subjected to oxidation according to a predefined temperature sequence, the initial temperature of which (hereafter denoted T0') is between 100 and 300 °C and preferably 200 °C, and the final temperature (hereafter denoted TF') is between 650 and 1000 °C, and preferably 850 °C. According to the invention, the temperature sequence of this heating under an oxidizing atmosphere includes at least a thermal gradient of between 1 and 50 °C / min, preferably between 15 °C and 40 °C / min, and preferably 25 °C / min.

[0060] Thus, during this step, heating under an oxidizing atmosphere is applied to the sample itself, and not to the residue of the sample resulting from heating under an inert atmosphere as described in the prior art. In other words, the process according to the invention is characterized, compared to the prior art, by a single heating sequence (in this case, under an oxidizing atmosphere). Or, to put it another way, according to the invention, the sample is not subjected to heating under an inert atmosphere prior to heating under an oxidizing atmosphere.

[0061] In general, the temperature range for the initial temperature T0' of the temperature sequence under an oxidizing atmosphere makes it possible to avoid episodes of instantaneous combustion of the sample residue at the beginning of oxidation.

[0062] Generally, the temperature range for the final temperature TF' of the temperature sequence under an oxidizing atmosphere ensures complete decomposition of all forms of organic carbon, even highly refractory ones. If the final temperature TF' is chosen to be above 850°C, the temperature sequence under an oxidizing atmosphere also ensures complete decomposition of all forms of inorganic carbon, including carbonate minerals. Furthermore, as will be discussed below, this allows for obtaining a thermogram with a dip, thus enabling the determination of the limiting temperature of the process according to the invention.

[0063] According to one embodiment of the invention, the temperature sequence under an oxidizing atmosphere may further include an isothermal plateau at the initial temperature T0' of a predetermined non-zero duration (for example, greater than half a minute), preferably between 1 and 3 minutes, and most preferably 1 minute. Such an isothermal plateau allows the release of labile and / or volatile compounds absorbed onto the surface of the particles constituting the mineral matrix of the soil.

[0064] Fig. 1 schematically illustrates an example of implementation of the temperature sequence under an oxidizing atmosphere of the process according to the invention, in which the temperature varies between an initial temperature T0' maintained for a non-zero predetermined duration and a final temperature TF', the two temperatures being related by a thermal gradient.

[0065] According to the invention, quantities of CO and CO2 released during this heating sequence are continuously measured. In one embodiment of the invention, this measurement can be performed using an infrared (IR) detector. It should be noted that such a sensor measures a flux of CO and / or CO2 and provides measured values ​​in millivolts (mV). Conventionally, the quantity of CO and the quantity of CO2 are determined by calculating the area under the curves of CO and CO2 respectively measured (possibly between intermediate temperatures as defined below) by this sensor, and dividing the area by the mass in mg of the sample. Alternatively, other means of measuring the quantity of CO and / or CO2 can be used.

[0066] Figure 2 illustrates an example of a thermogram obtained using the heating sequence under an oxidizing atmosphere according to the invention. More specifically, the curve T' represents the evolution of the temperature T of the oxidation furnace as a function of time t during this step, and the CO2 curve (respectively the CO curve) represents the evolution of the intensity QCO2 in mV (respectively QCO) of the signal from an IR sensor, from which the quantity of CO2 (respectively CO) released over time during the heating sequence under an oxidizing atmosphere T' is determined. Conventionally, S4CO2 (respectively S4CO) is the portion of the CO2 (respectively CO) curve to the left of a limiting oxidation temperature TLO, here approximately equal to 600 °C, and which is conventionally considered to correspond to the CO2 (respectively CO) generated by the cracking of the organic matter in the sample during heating under an oxidizing atmosphere.Conventionally, the portion of the CO2 (or CO) curve to the right of the oxidation-limiting temperature (OLT) is denoted S5 (or S4'CO, respectively). When the values ​​of the S4'CO portion are zero or close to zero, the S5 portion of the CO2 curve is considered to correspond to the CO2 generated by the decomposition of carbonate minerals in the sample during heating under an oxidizing atmosphere. When the values ​​of the S4'CO portion are non-zero, the S5 portion (or S4'CO portion) of the CO2 (or CO) curve is considered to correspond to the CO2 generated by the combustion of heat-resistant organic compounds present in the sample during heating under an oxidizing atmosphere.

[0067] Advantageously, the temperature sequence of the heating under an oxidizing atmosphere may further include one or more intermediate isothermal stages, at a temperature between the initial and final temperatures of the heating sequence under an oxidizing atmosphere. According to one embodiment of the invention, the temperature sequence of the heating under an oxidizing atmosphere may include a first intermediate isothermal stage at a temperature between 200 and 360 °C, and preferably 340 °C, and / or a second intermediate isothermal stage at a temperature between 380 and 420 °C, and preferably 400 °C, and / or a third intermediate stage at a temperature between 440 and 480 °C, and preferably 460 °C, and / or a fourth intermediate stage at a temperature between 520 and 600 °C, and preferably 580 °C.These isothermal stages allow for better separation of the different classes in a curve representing the evolution as a function of temperature of the quantity of CO and CO2 released during heating under an oxidizing atmosphere. thermal classes of organic carbon, more specifically, respectively, a class corresponding to highly thermally labile organic carbon, a class corresponding to thermally labile organic carbon, a class corresponding to thermally resistant organic carbon, and a class corresponding to thermally refractory organic carbon. This can notably contribute to improving the result of step 2) of the process according to the invention described below because it allows for the complete combustion of one class before initiating the combustion of the next. The intermediate isothermal rest period(s) can be of a predetermined non-zero duration (for example, greater than half a minute), preferably between 1 and 5 minutes, and most preferably 3 minutes. Such durations are sufficient to release all the carbon associated with a given thermal class of carbon, particularly at the end of the oxidation cycle.According to one embodiment of the invention, the temperature sequence for heating under an oxidizing atmosphere may include a number of thermal gradients NG defined by NG = NII+1 where NII is the number of intermediate isothermal plateaus in the temperature sequence. Thus, the intermediate isothermal plateau(s) are linked together by thermal gradients, and the intermediate isothermal plateau at the lowest (respectively highest) temperature is also linked by a thermal gradient to the initial (respectively final) temperature of the temperature sequence. According to this embodiment, the thermal gradient(s) may be between 1°C and 50°C / min, preferably between 15 and 40°C / min, and most preferably 25°C / min.

[0068] 2) Characterization of the organic carbon present in the sample

[0069] During this step, the aim is to characterize the organic carbon present in the soil sample, from the quantities of CO and CO2 measured continuously during step 1) described above.

[0070] More specifically, according to the invention, it is a matter of determining at least one indicator representative of the thermal stability of the organic carbon of the sample considered, hereafter denoted S, and / or an indicator representative of the lability of the organic carbon of the sample considered, hereafter denoted L, according to the following formulas: [0071 ] S = ((A3 + A4 + A5) / 100) (3)

[0072] L = iogio( (4)

[0073] where A1, A2, A3, A4, and A5 correspond to the carbon content released as CO2 and CO during the heating sequence within a temperature range respectively between the initial temperature of the heating sequence (i.e., T0') and 340°C, between 340 and 400°C, between 400 and 460°C, between 460 and 520°C, and between 520°C and a predefined limit temperature. According to the invention, the temperature The limiting temperature is between 540°C and 600°C and is preferably 580°C. The limiting temperature according to the invention corresponds to the temperature at which the release of organic carbon ends (for temperatures below the limiting temperature) and at which the release of mineral carbon contained in a sample begins (for temperatures above the limiting temperature).

[0074] Thus, the temperature range used to determine the term A5 described above has, compared to the prior art, been lowered from 650°C to 600°C. Indeed, the Applicant was able to observe, after analyses carried out on numerous samples with varying carbon contents, that the limiting temperature beyond which mineral carbon is released and no longer organic carbon is 600 °C when the sample undergoes a single heating in an oxidizing atmosphere, contrary to the protocol according to the prior art where the limit between CO2 of organic origin and CO2 of mineral origin in the oxidation phase after a pyrolysis phase is established at 650 °C (Behar et al. 2001).

[0075] Figure 3 (or Figure 4) illustrates an example of a curve representing the evolution of the amount of QC-CO2 carbon (in mgC g1) released as CO2 during heating under an oxidizing atmosphere according to the invention for a sample with a high organic carbon content (or for a sample with a low organic carbon content), as well as an alternation of gray and white areas, labeled A1 to A5, delimiting the portions of the CO2 curve involved in calculating the terms A1 to A5 defined above by their minimum and maximum temperatures. A dip in the curves can be observed in both Figure 3 and Figure 4 at a temperature of approximately 600°C. This illustrates the fact that, regardless of the carbon content of the sample, the limiting temperature beyond which the sample releases mineral carbon and no longer organic carbon is 600 °C when the sample undergoes a single heating in an oxidizing atmosphere.

[0076] According to one embodiment of the invention, the limiting temperature according to the invention can be determined by finding a minimum in a curve representing the evolution of the quantity of carbon QC released as CO and CO2 during heating under an oxidizing atmosphere, expressed according to a formula of the type:

[0077] QC = QCO*^ + QCO2*ÿ (5)

[0078] where QCO and QCO2 represent the evolution over time (and therefore as a function of temperature) of the quantity of CO and CO2 released respectively (measured in mV) during the temperature sequence, and the coefficients 12 and 12 correspond to the 28 44 molar mass of carbon divided by the molar mass of CO and CO2, respectively. This determination can be carried out graphically or, preferably, numerically (using a minimum search function). It is quite clear that The minimum sought is between 540 and 600°C. Advantageously for this implementation, the final temperature of the temperature sequence according to the invention is greater than or equal to 850°C, so that a trough can be clearly distinguished in the curve, and thus facilitate the determination of the minimum of the curve, whether graphically or numerically.

[0079] According to one embodiment of the invention, the carbon content released as CO2 and CO associated with a term Ai according to the invention, i varying between 1 and 5, can be determined in the following manner:

[0080] cTAi-maxQco2*£ (6) TAi-min m TAi-min m

[0081] where QCO and QCO2 represent the evolution over time (and therefore as a function of temperature) of the quantity of CO and CO2 released respectively (measured in mV) during the temperature sequence between the minimum temperature T Ai - min and the maximum temperature T Ai - max associated with the term Ai according to the invention, and m is the mass of the sample (in g). In other words, the area under the continuous measurement curve of CO and under the measurement curve of CO2 between the minimum temperature T Ai - min and the maximum temperature T Ai - max associated with each of the terms Ai according to the invention defined in equations (3) and (4) is determined, weighted by the mass of the sample, and by the coefficients 12 and 12 corresponding to the molar mass of carbon divided by the mass 28 44 molar mass of CO and CO2 respectively

[0082] Furthermore, according to the invention, at least one indicator representative of the quality of organic carbon is determined by taking into account the quantity of CO and CO2 measured continuously. Indeed, even if the major part (more than 90%) of the total organic carbon is emitted in the form of CO2, the quantity of CO emitted is not negligible. The present invention thus makes it possible to take into account all of the organic carbon. This also means that, compared to the prior art, the indicators according to the invention take into account the entire organic fraction of the soil, and not only the hydrocarbon compounds released by pyrolysis of soil organic matter.

[0083] Conventionally, the terms Al and A2, associated with temperatures between 200 and 340°C and between 340 and 400°C respectively, allow for the consideration of classes (or "pools") of highly labile (Al) or labile (A2) carbon compounds. The term A3, associated with temperatures between 400 and 460°C, corresponds to classes of more resistant carbon compounds. And for the terms A4 and A5, associated with temperatures between 460 and 520°C and between 520 and a limiting temperature between 540 and 600°C respectively, the forms of carbon are considered refractory (A4) and highly refractory (A5).

[0084] According to a preferred embodiment of the invention, it is further possible to determine at minus an organic carbon content from the quantities of CO and CO2 measured continuously during step 1).

[0085] According to this preferred embodiment of the invention, a parameter representative of the organic carbon content released by the sample can be determined according to a formula of the type:

[0086] oxyTOC = [S4CO2] + [(S4CO + S4CO)]

[0087] where S4CO2 and S4CO represent the quantities of CO2 and CO released respectively during the heating sequence in an oxidizing atmosphere up to the limit temperature according to the invention, which is between 540 and 600°C, and preferably 580°C, and S4'CO represents the quantity of CO released during the heating sequence in an oxidizing atmosphere beyond the limit temperature according to the invention.

[0088] According to another preferred embodiment of the invention, if the final temperature TF' of the temperature sequence under an oxidizing atmosphere is greater than or equal to 850°C, a mineral carbon content of the sample can also be determined from the quantities of CO and CO2 measured continuously during step 1).

[0089] According to this preferred embodiment of the invention, a parameter representative of the mineral carbon content released by the sample can be determined according to a formula of the type:

[0090] oxyMinC = [ S5 ]

[0091] where S5 represents the quantity of CO2 released during the sequence of temperatures in an oxidizing atmosphere beyond the limiting temperature according to the invention. Examples

[0092] The characteristics and advantages of the method according to the invention will become clearer upon reading the application example below.

[0093] Approximately fifty soil samples with low and high Corg content were analyzed. More specifically, for soils with low Corg content, about twenty soil samples were selected based on their Corg content, which was classified into classes (Corg content less than < 0.01%, less than 0.02%, between 0.03 and 0.04%, and between 0.05 and 0.09%). For soils with high Corg content, about thirty soil samples were selected, with Corg contents ranging from 0.8% to 14.5%, with an average content of 3%. Prior to analysis, the samples were finely ground (particle size less than 200 µm), dried at 40°C until their weight stabilized, and then sieved through a 2 mm sieve.

[0094] Each sample was analyzed with:

[0095] - the process described in the document (Sebag et al., 2016), for determining the in R and I indicators, representing respectively the thermal stability of organic carbon and the lability of organic carbon, and whose formulas are given in equations (1) and (2) above. As a reminder, the process described in (Sebag et al., 2016) is based on a sample heating protocol comprising, upstream of an oxidation phase, a pyrolysis phase with a final temperature of 650°C. Furthermore, the maximum temperature associated with the determination of the A5 term in equation (2) is defined as 650°C in (Sebag et al., 2016).

[0096] - the method according to the invention, for determining the representative indicators S and L respectively of the thermal stability of organic carbon and of the lability of organic carbon, and whose formulas are given in equations (3) and (4) above.

[0097] Figure 5A (and Figure 5B, respectively) shows, in the form of black squares, the distribution of the values ​​of indicator L as a function of the values ​​of indicator S determined by the process according to the invention for samples with low organic carbon content (and high organic carbon content, respectively). It can be observed that indicator L (labile soil Corg) varies between 0.34 and 0.77 with an average value of 0.53, while indicator S (stable soil Corg) varies between 0.29 and 0.67 with an average value of 0.45 in soils with low Corg content (Fig. 5A). Furthermore, we can observe that the L indicator varies between -0.07 and 0.69 with an average value of 0.36, while the S indicator varies between 0.30 and 0.87 with an average value of 0.48 in soils with high Corg content ([Fig.5B]).

[0098] Figure 6 shows, for samples with low organic carbon content, in the form of black squares, the distribution of the values ​​of indicator L as a function of the values ​​of indicator S determined by the process according to the invention, and, in the form of gray circles, the distribution of the values ​​of indicator I as a function of the values ​​of indicator R determined by the process according to the prior art. It can be observed that, for the same sample, the lability indicator L according to the invention is higher than the lability indicator I according to the prior art. For the stability indicators of Corg, the opposite is observed, that is to say, the indicator R according to the prior art is higher than the indicator S according to the invention. Thus, Figure 6 shows the distribution of the values ​​of indicator I according to the invention.[6] highlights different thermal stability signatures of soil organic matter between the signature measured with indicators I and R according to the prior art and that measured with indicators L and S according to the invention. Furthermore, the lability indicators according to the prior art and according to the invention (I and L respectively) are very weakly positively correlated, with a coefficient of determination R² = 0.3, as are the stability indicators according to the prior art and according to the invention (R and S respectively), with a coefficient of determination R² = 0.1. In other words, the lability indicator according to the prior art I contributes 30% to the indicator according to the invention L, and the indicator according to the prior art R contributes only 10% to the indicator according to the invention S. It was also determined that the indicator I according to the prior art is . anti-correlated with the S indicator according to the invention (r = -0.29), and the R indicator according to the prior art is anti-correlated with the L indicator according to the invention (r = -0.53). This can be explained by the different construction of the lability and stability indicators.

[0099] Figure 7 shows, for samples with high organic carbon content, under the The distribution of indicator L values ​​as a function of indicator S values ​​determined by the method according to the invention is shown in the form of black squares, and the distribution of indicator I values ​​as a function of indicator R values ​​determined by the prior art method is shown in the form of gray circles. It can be observed that, for the same sample, the lability indicator L according to the invention is higher than the lability indicator I according to the prior art. The opposite is observed for the Corg stability indicators; that is, the R indicator according to the prior art is higher than the S indicator according to the invention. Thus, [Fig. 7] highlights different thermal stability signatures of soil organic matter between the signature measured with indicators I and R according to the prior art and that measured with indicators L and S according to the invention.Furthermore, the lability indicators according to the prior art I and according to the invention L are only very weakly negatively correlated, with a coefficient of determination R² = 0.2, as are the stability indicators according to the prior art R and according to the invention S, with a coefficient of determination R² = 0.1. This means that as the lability indicator according to the invention L increases, the lability indicator according to the prior art I decreases, and as the thermal stability indicator according to the invention S increases, the thermal stability indicator according to the prior art R decreases. Moreover, it has been observed that indicators I and R, and indicators L and S, are divergent and only weakly correlated. Indeed, the Pearson correlation coefficients are -0.41 between indicator L and indicator I, and -0.26 between indicator S and indicator R.The divergence between these four indicators is also illustrated by positive correlations between indicator L and indicator R (r = 0.45) and between indicator S and indicator I (r = 0.32). This can be explained by the fact that indicators I and R are obtained from a different analytical principle (pyrolysis phase versus oxidation phase) on a small fraction of organic carbon, whereas indicators L and S are calculated from a different signal (S4CO and S4CO2 peaks versus peak S2) obtained by a single oxidation phase and which represents all of the organic carbon.

[0100] Thus, the present invention allows for the simple, rapid, and reliable characterization of organic carbon present in a sample of a surface formation. The method according to the invention is, in particular, simpler, faster, and therefore more economical than prior art methods because it does not require a pyrolysis heating sequence (saving the need for a pyrolysis furnace or an oxidation furnace capable of pyrolysis, HC detectors, and energy for pyrolysis heating). Specifically, the duration is approximately 40 minutes instead of 1 hour 30 minutes according to the prior art. Furthermore, the method according to the invention allows for a more reliable characterization of organic carbon than according to the prior art. Indeed, the relevance of these new descriptors lies in their ability to characterize the thermal stability of the entire organic component of the soil organic matter, and not only the organic component related to the quantities of hydrocarbon compounds in the soil organic matter, as with the descriptors according to the prior art.

Claims

Demands

1. A method for characterizing the organic carbon present in a surface formation, from a representative sample of said surface formation, characterized in that, said sample not having been pre-heated in an inert atmosphere, it comprises at least the following steps: A) said sample is heated in an oxidizing atmosphere according to a predefined temperature sequence of which an initial temperature (TO') is between 100 and 300 °C, and a final temperature (TF') is between 650 and 1000 °C, said temperature sequence comprising at least a thermal gradient between 1 °C / min and 50 °C / min, and at least a quantity of CO and a quantity of CO2 released during said heating sequence in an oxidizing atmosphere are continuously measured;B) The organic carbon present in said sample is characterized by determining, from said continuous measurements of said quantities of CO and CO2 released during said heating sequence in an oxidizing atmosphere, at least one indicator representative of the thermal stability of organic carbon defined by a formula of the type: S = ((A3 + A4 + A5) / 100) and / or a parameter representative of the thermal lability of organic carbon defined by a formula of the type: L^log lOp”2) where A1, A2, A3, A4, and A5 represent the carbon content released as CO2 and CO by said sample during said temperature sequence in a temperature range respectively between said initial temperature (T0') and 340°C, between 340 and 400°C, between 400 and 460°C, between 460 and 520°C, and between 520 and a limiting temperature, said limiting temperature being between 540°C and 600°C and preferably equal to 580°C.;

2. A method according to claim 1, wherein said carbon content Ai released as CO2 and CO, i varying between 1 and 5, is determined according to a formula of the type: . _ fTAhnaxQCO*ÿ frAi-maxQCO2^ 1 TAi-min m ' TAi-min m where QCO2 and QCO represent the time evolution of said quantity of CO2 and CO respectively released during said sequence of temperatures between a minimum temperature T Ai - min and a maximum temperature T Ai - max of said temperature range of said content Ai and m is the mass of said sample.

3. A method according to any one of the preceding claims, wherein said initial temperature (T0') is 200°C.

4. A method according to any one of the preceding claims, wherein said final temperature (TF') is equal to 850°C.

5. A method according to any one of the preceding claims, wherein, in addition, at least one parameter representative of an organic carbon content defined by a formula of the type is determined: oxyTOC(%} = [S4CO2] + [(54CO + ] where S4CO2 and S4CO represent said quantities respectively of CO2 and CO released during said temperature sequence up to said limit temperature, and where S4'CO represents said quantity of CO released during said temperature sequence beyond said limit temperature.

6. A method according to any one of the preceding claims, wherein, if said final temperature is greater than or equal to 850°C, at least one parameter representative of a mineral carbon content defined by a formula of the type: oxyMinC - [55*^ ] where S5 represents said quantity of CO2 released during said temperature sequence beyond said limit temperature.

7. A method according to any one of the preceding claims, wherein said temperature sequence includes an isothermal plateau of predetermined duration at said initial temperature (T'O) of said temperature sequence, said predetermined duration of said isothermal plateau at said initial temperature (T'O) of said temperature sequence being between 1 and 5 minutes, and preferably 3 minutes.

8. A method according to any one of the preceding claims, wherein said at least one thermal gradient of said temperature sequence is between 15°C / min and 40°C / min, and is most preferably 25°C / min.

9. A method according to any one of the preceding claims, wherein said limiting temperature is determined by finding a minimum in a curve representing an evolution of the amount of carbon QC released as CO and CO2 during said temperature sequence. temperatures according to a formula of the type: QC = eCO*^ + gCO2*ÿ where QCO2 and QCO represent the evolution over time of said quantity respectively of CO2 and CO released during said sequence of temperatures.

10. A method according to claim 9, wherein said minimum of said curve representing said evolution of said quantity of QC carbon released as CO and CO2 during said temperature sequence is determined graphically or numerically.

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