Method for the characterization of organic carbon in a sample of a surface formation

Abstract

The invention relates to a method for characterizing the carbon of a sample of a surface formation. The method comprises heating the sample in question under an oxidizing atmosphere between an initial temperature of between 100 and 300°C and a final temperature of between 650 and 1000°C, during which quantities of CO and CO2 released are continuously measured. Then, from these quantities and a limit temperature of the heating sequence of between 540 and 600°C, preferably 580°C, the organic carbon present in said 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 the characterization of 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 the present invention relates to the field of characterization of carbon contained in surface formations, and in particular in soils. The present invention finds a particular application in the case of soils with low and very low organic carbon content.

[0002] In order to respond to ecological challenges, or to comply with certain environmental legislation / directives, stakeholders in soil science and environmental geosciences (research laboratories, design offices, environmental agencies, farmers) are increasingly required to implement protocols for monitoring the impacts of human activities on carbon stocks in soils and eco-agro-systems. These monitoring and impact studies require the ability to study large series of samples in relatively short timeframes compared to conventionally used methodologies. In addition, these methods are often accompanied by environmental and safety constraints that increase analytical time and costs, and often require the use of specialized service providers (e.g., analysis laboratories).

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

[0004] Soils with low organic carbon contents deserve particular attention, because 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 contents (hereinafter referred to as Corg) of soils in the presence of mineral carbon (hereinafter referred to as Cmin) is problematic and represents a real technical challenge.

[0006] Quantification of organic carbon for soils lacking mineral carbon is quite simple with standard analysis tools, such as an elemental analyzer. On the other hand, it is more complex in the case of soils in the presence of Cmin. Indeed, in this case, the determination of total Corg requires sample pretreatment (calcimetry or decarbonation). For analytical methods requiring a calcimetry step (measurement of 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), the quantification of carbon forms consists on the one hand of measuring the total carbon (hereinafter referred to as Ctotal) by dry combustion and on the other hand of determining the total Cmin by calcimetry. The Corg is then obtained as the difference between the Ctotal and the total Cmin. However, this method has limitations, as it is not suitable for soil samples rich in Cmin and with low Corg contents. These limitations are linked to the uncertainties inherent in the high values ​​of Ctotal and total Cmin which accumulate and lead to poor Corg measurements, or even negative contents.For analytical methods requiring a decarbonation step (analysis consisting of contacting a soil sample with a volume of dilute HCl, in order to eliminate the different forms of carbonates present, releasing carbon dioxide), the quantification of carbon forms consists on the one hand of measuring the total carbon by dry combustion and on the other hand of determining the total Corg after decarbonation of the sample. The total Cmin is then obtained by the difference between the Ctotal and the total Corg. This type of pretreatment often leads to altering the most labile organic fraction and, consequently, to underestimating the Corg contents and overestimating the Cmin contents. Prior art

[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] Concerning more specifically the characterization of organic carbon, we know the documents (Disnar et al., 2003; Sebag et al., 2006; Saenger et al., 2013; Plante et al., 2009) which describe thermal stability indicators determined from a thermal analysis of the organic matter of soils. More precisely, these documents describe the implementation of the "ROCK-EVAL® BULK ROCK" thermal analysis, initially developed for source rock samples (Behar et al., 2001), and which includes measurements of 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 then to a heating sequence in an oxidizing atmosphere.According to this method, the heating sequence in an inert atmosphere of this method is characterized by an initial temperature Tl of the pyrolysis furnace generally between 300°C and 350°C, a temperature which is maintained for a predetermined duration of a few minutes. Then, the pyrolysis temperature is gradually increased to a temperature T2, generally 650°C. The quantity of hydrocarbon compounds released during this thermal cracking phase is estimated by measuring the surface area of ​​a second peak, noted S2. In parallel, the quantities of CO and CO2 are also measured and represented in the form of curves.The CO (respectively CO2) measurement curve shows two peaks, a first peak classically noted S3CO (respectively S3CO2) and 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) and 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 of the sample resulting from heating in an inert atmosphere is subjected to heating in an oxidizing atmosphere: from a temperature between approximately 300°C and 400°C, and preferably 300°C, the temperature of the residue of the sample in question is raised according to a temperature gradient of between 20 and 40°C / minute, up to an end of oxidation temperature of between 750 and 950°C, and preferably 850°C. During this heating sequence in an atmosphere. oxidizing, the quantities of CO and CO2 released by the sample residue are measured and represented in the form of curves, leading to a peak classically noted S4CO (respectively S4CO2) which is considered to correspond to the quantity 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 corresponding to the HC released during the pyrolysis phase. More precisely, the method described in these documents is based on a deconvolution to subdivide the S2 peak (also noted 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 at less than 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 also know 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 precisely, the authors delimit on the S2 thermogram five surfaces, denoted Al, A2, A3, A4 and A5, each corresponding to temperature ranges. The Al and A2 surfaces, whose temperature ranges are respectively between 200-340°C, and 340-400°C, make it possible to consider the thermal classes of highly labile (Al) or labile (A2) organic carbon. The A3 surface, between 400 and 460°C, corresponds to a more resistant thermal class of organic carbon.And for A4 and A5 surfaces, whose temperature ranges are respectively between 460-520°C and 520-650°C, the carbon forms 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, the indicator I uses the surfaces Al and A2 to highlight the degree of transformation of the immature organic fraction, and is therefore representative of the lability of the organic carbon of a sample. The indicator R is based on the 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 of 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. In addition, on soils with low Corg contents, the 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 contents.

[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 comprises 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 faster, simpler characterization of all organic carbon, and this 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 one thermal gradient between 1°C / min and 50°C / min, and at least one quantity of CO and one quantity of CO 2 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 the 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 in the form of CO 2 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 limit temperature, said limit temperature being between 540°C and 600°C and preferably being 580°C.

[0027] According to an implementation of the invention, said content Ai of carbon released in the form of 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 implementation of the invention, said initial temperature (T0') may be 200°C.

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

[0032] According to an implementation of the invention, it is also 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 sequence of temperatures up to said limit temperature, and where S4'CO represents said quantity of CO released during said sequence of temperatures beyond said limit temperature.

[0035] According to an implementation of the invention, if said final temperature is greater than or equal to 850°C, it is also possible to determine at least one parameter representative of a mineral carbon content defined by a formula of the type:

[0036] oxyMinC = [55*^]

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

[0038] According to an implementation of the invention, said temperature sequence may comprise an isothermal plateau of a 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 3 minutes.

[0039] According to one implementation 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 an implementation of the invention, said limit temperature can be determined by searching for a minimum in a curve representing an evolution of the quantity of carbon QC released in the form of CO and CO2 during said sequence of temperatures 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 an implementation of the invention, it is possible to determine said minimum of said curve representing said evolution of said quantity of carbon QC released in the form of CO and CO2 during said sequence of temperatures in a graphical or numerical manner. List of figures [Fig 1]

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

[0045] [Fig.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] [Fig.3] (respectively [Fig.4]) illustrates an example of a curve representing the evolution of a quantity of carbon released in the form of CO2 during heating under an oxidizing atmosphere according to the invention for a sample with a high organic carbon content (respectively for a sample with a low organic carbon content). [Fig 5A] [Fig5B]

[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 method according to the invention for samples with a low organic carbon content (respectively with a high organic carbon content). [Fig 6]

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

[0049] [Fig.7] shows, for the samples with a 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 method 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 method according to the prior art in the form of gray circles. Description of the embodiments

[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, loose or secondarily consolidated, originating from the mechanical and / or chemical disintegration of pre-existing rocks, and formed at the lithosphere / biosphere / atmosphere interface. A distinction is made between (i) "allochthonous superficial formations" (such as colluvium, alluvium, loess, etc.) which have undergone or are still undergoing near or distant displacements, and no longer rest on their parent material, and "autochthonous superficial formations" (such as arenas, alterities, 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 having 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 whose orifices have a diameter of 2 mm, dried at a temperature below 40°C, then crushed until fragments having dimensions less than 200 μm are obtained. Thus, subsequently, a "representative sample of the surface formation" is a sample taken from the surface formation studied, and having optionally undergone a preparation comprising sieving, drying (not under an inert atmosphere) and crushing.

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

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

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

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

[0058] The steps of the method according to the invention are described below in a non-limiting manner for a soil sample. The steps of the method according to the invention can in fact just as easily be applied to a sample coming from another layer of a surface formation. 1. Heating sequence under oxidizing atmosphere

[0059] During this step, the sample is subjected to oxidation according to a predefined temperature sequence, the initial temperature (hereinafter referred to as T0') of which is between 100 and 300°C and is preferably 200°C, and the final temperature (hereinafter referred to as TF') is between 650 and 1000°C, and is preferably 850°C. According to the invention, the temperature sequence of this heating under an oxidizing atmosphere comprises at least one thermal gradient of between 1 and 50°C / min, preferably between 15° 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 in an inert atmosphere as described in the prior art. In other words, the method according to the invention is characterized, compared to the prior art, by a single heating sequence (in this case, under an oxidizing atmosphere). Or in other words, according to the invention, the sample is not subjected to heating in an inert atmosphere, prior to heating in an oxidizing atmosphere.

[0061] Generally speaking, 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 start of oxidation.

[0062] Generally speaking, the temperature range for the final temperature TF' of the temperature sequence under an oxidizing atmosphere makes it possible to ensure total decomposition of all forms of organic carbon, even those which are very refractory. If the final temperature TF' is chosen to be greater than 850°C, the temperature sequence under an oxidizing atmosphere also makes it possible to ensure total decomposition of all forms of inorganic carbon, in particular carbonate minerals. In addition, as will be discussed below, this makes it possible to obtain a thermogram showing a trough, and thus to be able to determine the limiting temperature of the process according to the invention.

[0063] According to one implementation of the invention, the sequence of temperatures under an oxidizing atmosphere may further comprise 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 very preferably 1 minute. Such an isothermal plateau makes it possible to release the labile and / or volatile compounds absorbed on the surface of the particles constituting the mineral matrix of the soil.

[0064] [Fig.l] schematically illustrates an example of implementation of the temperature sequence under an oxidizing atmosphere of the method 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 linked by a thermal gradient.

[0065] According to the invention, quantities of CO and CO2 released during this heating sequence are continuously measured. According to one implementation of the invention, this measurement can be carried out using an infrared (IR) type detector. Note that such a sensor measures a flow of CO and / or CO2, and provides values ​​measured in millivolts (mV). Conventionally, a quantity of CO and a quantity of CO2 are determined by determining an area under the curves of CO and CO2 respectively measured (possibly between intermediate temperatures as will be defined below) by this sensor, and the area is divided by the mass in mg of the sample. Alternatively, other means of measuring the quantity of CO and / or CO2 may be used.

[0066] [Fig. 2] illustrates an example of a thermogram obtained by means of the heating sequence under an oxidizing atmosphere according to the invention. More precisely, the curve T' represents the evolution as a function of time t of the temperature T of the oxidation furnace during this step, 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, the portion of the CO2 (respectively CO) curve to the left of an oxidation limit temperature TLO, here substantially equal to 600 °C, is called S4CO2 (respectively S4CO).Conventionally, the portion of the CO2 (respectively CO) curve to the right of the oxidation limit temperature TLO is called S5 (respectively S4'CO). 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 the carbonate minerals in the sample during heating in an oxidizing atmosphere. When the values ​​of the S4'CO portion are non-zero, the S5 portion (respectively the S4'CO portion) of the CO2 (respectively CO) curve is considered to correspond to the CO2 (respectively CO) generated by combustion of the heat-resistant organic compounds present in the sample during heating in an oxidizing atmosphere.

[0067] Advantageously, the temperature sequence of the heating under an oxidizing atmosphere may further comprise one or more intermediate isothermal stages, at a temperature between the initial and final temperatures of the temperature sequence of the heating under an oxidizing atmosphere. According to one implementation of the invention, the temperature sequence of the heating under an oxidizing atmosphere may comprise 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 levels make it possible to better separate, in a curve representing the evolution as a function of temperature of the quantity of CO and CO2 released during heating in an oxidizing atmosphere, the different classes. thermal classes of organic carbon, more precisely respectively, a class corresponding to thermally very 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 in particular help to improve the result of step 2) of the method according to the invention described below because it allows the complete combustion of one class to be achieved before starting the combustion of the next. The intermediate isothermal stage(s) may be of a predetermined non-zero duration (for example greater than half a minute), preferably between 1 and 5 minutes, and very preferably 3 minutes. Such durations are sufficient to release all of the carbon associated with a given thermal class of carbon, particularly at the end of the oxidation cycle.According to one implementation of the invention, the temperature sequence of the heating under an oxidizing atmosphere may comprise a number of thermal gradients NG defined by NG = NII+1 where NII is the number of intermediate isothermal stages of the temperature sequence. Thus, the intermediate isothermal stage(s) are connected to each other by thermal gradients, and the intermediate isothermal stage at the lowest (respectively the highest) temperature is also connected by a thermal gradient to the initial temperature (respectively the 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 very preferably be 25°C / min.

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

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

[0070] More precisely, according to the invention, it is a question of determining at least one indicator representative of the thermal stability of the organic carbon of the sample considered, noted S hereinafter, and / or an indicator representative of the lability of the organic carbon of the sample considered, noted L hereinafter, 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 in the form of CO2 and CO during the heating sequence in 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 limit temperature is between 540°C and 600°C and is preferably 580°C. The limit temperature according to the invention corresponds to the temperature at which the release of organic carbon ends (for temperatures below the limit temperature) and at which the release of mineral carbon contained in a sample begins (for temperatures above the limit 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 limit 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, unlike 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] [Fig.3] (respectively [Fig.4]) illustrates an example of a curve representing the evolution of the quantity of carbon QC-CO2 (in mgC g1) released in the form of CO2 during heating under an oxidizing atmosphere according to the invention for a sample with a high organic carbon content (respectively for a sample with a low organic carbon content), as well as an alternation of gray and white zones, noted Al to A5, delimiting the portions of the CO2 curve involved in the calculation of the terms Al to A5 defined above by their minimum and maximum temperatures. It is possible to observe in both [Fig.3] and [Fig.4] a dip in the curves 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 an implementation of the invention, the limit temperature according to the invention can be determined by searching for a minimum in a curve representing the evolution of the quantity of carbon QC released in the form of 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 respectively of CO and CO2 released (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 (by a minimum search function). It is 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 an implementation of the invention, the carbon content released in the form of 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 respectively released (measured in mV) during the temperature sequence between the minimum temperatures T Ai - min and maximum 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 CO measurement curve and under the CO2 measurement curve between the minimum temperatures T Ai - min and maximum 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 the coefficients 12 and 12 corresponding to the molar mass of carbon divided by the mass 28 44 molar 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 majority (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 the organic matter in the soil.

[0083] Conventionally, the terms Al and A2, associated with temperatures between 200 and 340°C, and between 340 and 400°C, respectively, make it possible to consider classes (or "pools" in English) 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 limit temperature between 540 and 600°C, respectively, the carbon forms are considered refractory (A4) and highly refractory (A5).

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

[0085] According to this preferred implementation 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 respectively released during the heating sequence in an oxidizing atmosphere up to the limit temperature according to the invention of 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 implementation of the invention, if the final temperature TF' of the sequence of temperatures under an oxidizing atmosphere is greater than or equal to 850°C, it is also possible to determine a mineral carbon content of the sample from the quantities of CO and CO2 measured continuously during step 1).

[0089] According to this preferred implementation of the invention, a parameter representative of the content of mineral carbon 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 limit temperature according to the invention. Examples

[0092] The characteristics and advantages of the method according to the invention will appear more clearly on reading the application example below.

[0093] About fifty soil samples with low and high Corg contents were analyzed. More specifically, for the soils with low Corg content, about twenty soil samples were selected from their Corg contents 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 the soils with high Corg contents, about thirty soil samples were selected, whose Corg contents varied between 0.8 and 14.5%, with an average content of 3%. Prior to their analysis, the samples were finely ground (dimension less than 200 μm) previously dried at 40 °C until their weight stabilized and sieved with a 2 mm sieve.

[0094] Each sample was analyzed with:

[0095] - the method described in the document (Sebag et al., 2016), to determine the in indicators R and I 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 including, upstream of an oxidation phase, a pyrolysis phase whose final temperature is 650°C. Furthermore, the maximum temperature associated with the determination of term A5 in equation (2) is defined at 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 the lability of organic carbon, and whose formulas are given in equations (3) and (4) above.

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

[0098] [Fig. 6] shows, for samples with low organic carbon content, in the form of black squares, the distribution of the values ​​of the indicator L as a function of the values ​​of the indicator S determined by the method according to the invention, as well as, in the form of gray circles, the distribution of the values ​​of the indicator I as a function of the values ​​of the indicator R determined by the method 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. On the Corg stability indicators, the opposite is observed, that is to say that the indicator R according to the prior art is higher than the indicator S according to the invention. Thus, [Fig.6] highlights thermal stability signatures of the organic matter of the soils that differ between the signature measured with the indicators I and R according to the prior art and that measured with the indicators L and S according to the invention. Furthermore, the lability indicators according to the prior art and according to the invention (respectively I and L) are very little positively linked, with a coefficient of determination R2 = 0.3, as are the stability indicators according to the prior art and according to the invention (respectively R and S), 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 has also been determined that the indicator I according to the prior art is . anti-correlated with the indicator S according to the invention (r = -0.29) and the indicator R according to the prior art is anti-correlated with the indicator L according to the invention (r = -0.53). This can be explained by the different construction of the lability and stability indicators.

[0099] [Fig.7] shows, for samples with high organic carbon content, under the in the form of black squares, the distribution of the values ​​of the indicator L as a function of the values ​​of the indicator S determined by the method according to the invention, as well as, in the form of gray circles, the distribution of the values ​​of the indicator I as a function of the values ​​of the indicator R determined by the method 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. On the Corg stability indicators, the opposite is observed, that is to say that the indicator R according to the prior art is higher than the indicator S according to the invention. Thus, [Fig.7] highlights thermal stability signatures of the organic matter of the soils which differ between the signature measured with the indicators I and R according to the prior art and that measured with the 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 very little negatively linked, 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 the more the lability indicator according to the invention L increases, the more the lability indicator according to the prior art I decreases, and the more the thermal stability indicator according to the invention S increases, the more the thermal stability indicator according to the prior art R decreases. Furthermore, it has been observed that the indicators I and R and the indicators L and S are divergent and very little linked. 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 the L indicator and the R indicator (r = 0.45) and between the S indicator and the I indicator (r = 0.32). This can be explained by the fact that the I and R indicators are obtained from a different analytical principle (pyrolysis phase versus oxidation phase) on a small fraction of organic carbon while the L and S indicators are calculated from a different signal (S4CO and S4CO2 peaks versus S2 peak) obtained by a single oxidation phase and which represents the totality of the organic carbon.

[0100] Thus, the present invention allows the characterization of the organic carbon present in a sample of a surface formation in a simple, rapid and reliable manner. The method according to the invention is in particular simpler, faster and therefore more economical than the methods according to the prior art because it does not require a pyrolysis heating sequence (saving a pyrolysis furnace or an oxidation furnace allowing pyrolysis, HC detectors, energy for pyrolysis heating in particular, duration of approximately 40 minutes instead of 1h30 according to the prior art). The method according to the invention also allows a more reliable characterization of organic carbon than according to the prior art. Indeed, the relevance of these new descriptors lies in their capacity to characterize the thermal stability of the entire organic part of the organic matter of the soil, and not only the organic part linked to the quantities of hydrocarbon compounds of the organic matter of the soil like the descriptors according to the prior art.

Claims

Claims

1. 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 heated in an inert atmosphere beforehand, 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 one thermal gradient between 1°C / min and 50°C / min, and at least one quantity of CO and one 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 the organic carbon defined by a formula of the type: S = ((A3 +A4 + A5) / 100) and / or a parameter representative of the thermal lability of the organic carbon defined by a formula of the type: L^loglOp”2) where Al, A2, A3, A4, and A5 represent the content of carbon released in the form of 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 limit temperature, said limit temperature being between 540°C and 600°C and preferably 580°C.;

2. Method according to claim 1, in which said content Ai of carbon released in the form of CO2 and CO is determined, i varying between 1 and 5, according to a formula of the type: . _ fTAhnaxQCO*ÿ frAi-maxQCO2^ 1 TAi-min m ' TAi-min m 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 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. Method according to one of the preceding claims, in which said initial temperature (T0') is 200°C.

4. Method according to one of the preceding claims, in which said final temperature (TF') is 850°C.

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

6. Method according to one of the preceding claims, in which, 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 sequence of temperatures beyond said limit temperature is further determined.

7. Method according to one of the preceding claims, in which said temperature sequence comprises an isothermal stage of a predetermined duration at said initial temperature (T'O) of said temperature sequence, said predetermined duration of said isothermal stage at said initial temperature (T'O) of said temperature sequence being between 1 and 5 minutes, and preferably being 3 minutes.

8. Method according to one of the preceding claims, in which said at least one thermal gradient of said temperature sequence is between 15°C / min and 40°C / min, and is very preferably 25°C / min.

9. Method according to one of the preceding claims, in which said limit temperature is determined by searching for a minimum in a curve representing an evolution of the quantity of carbon QC released in the form of CO and CO2 during said sequence of tem- temperatures according to a formula of the type: QC = eCO*^ + gCO2*ÿ where QCO2 and QCO represent the evolution over time of the said quantity of CO2 and CO respectively released during the said sequence of temperatures.

10. Method according to claim 9, in which said minimum of said curve representing said evolution of said quantity of carbon QC released in the form of CO and CO2 during said sequence of temperatures is determined graphically or numerically.

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