Method for characterizing and quantifying the carbon in a sample of a superficial deposit

Abstract

The invention relates to a method for characterizing and quantifying the carbon in a sample of a superficial deposit. The method comprises heating, under an oxidizing atmosphere, the sample in question to between an initial temperature of 100 to 300°C and a final temperature of 650 to 1000° C, during which quantities of released CO and CO2 are measured continuously. Next, on the basis of these quantities and of an intermediate temperature of the heating sequence of between 540 and 600° C, at least parameters representative of the organic carbon and / or of the mineral carbon in the sample are determined.

Description

[0001] METHOD FOR THE CHARACTERIZATION AND QUANTIFICATION OF CARBON IN A SAMPLE OF A SURFACE FORMATION

[0002] Technical field

[0003] The present invention relates to the field of soil science and environmental geosciences, more specifically the present invention relates to the field of characterization and quantification of carbon contained in surface formations, and in particular in soils. The present invention finds particular application in the case of soils with a very low organic carbon content.

[0004] In order to address ecological issues, 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 soil carbon stocks and eco-agrosystems. 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., analytical laboratories).

[0005] Organic forms of carbon stored in surface formations, 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 primarily involved in the carbon cycle, representing the largest reservoir of organic carbon on the Earth's surface.

[0006] Soils with low organic carbon contents 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. In the current context where soil carbon is at the heart of several international agendas, the quantification of the different forms of carbon is crucial, for example, for MRV activities (Measuring, Reporting and Verification) and for global simulation models on the carbon cycle. However, quantifying with high precision 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.

[0007] The quantification of organic carbon for soils lacking mineral carbon is quite simple with standard analytical tools, such as an elemental analyzer. However, 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 of 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.This method, however, has limitations (calculation of negative Corg contents), 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. 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 alteration of the most labile organic fraction and, consequently, to underestimating Corg contents.

[0008] Prior art

[0009] The following documents will be cited during the description:

[0010] Behar F., Beaumont V., De B., Penteado HL (2001) Rock-Eval 6 Technology: Performances and Developments, Oil & Gas Science and Technology 56, 111-134.

[0011] Methods for thermal analysis of soil organic matter are known, based on 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. These methods were initially developed in the petroleum industry, for the purpose of characterizing the organic fraction of sedimentary rocks.The ROCK-EVAL® device (IFP Energies nouvelles, France) is thus known, developed by the applicant and described in particular in patents FR 2227797 (US 3953171) and FR 2472754 (US 4352673), which comprises a pyrolysis furnace separate from an oxidation furnace, a flame ionization type detector (FID) for detecting hydrocarbon compounds (HC) and an infrared type detector (IR) for detecting carbon monoxide (CO) and / or carbon dioxide (CO2). Methods developed for specific applications in the field of the petroleum industry are also known, each having its own sequence of heating temperatures in pyrolysis and / or heating in an oxidizing atmosphere. In particular, the "ROCK-EVAL® BULK ROCK" method is known, dedicated more particularly to conventional source rock samples (Behar et al., 2001).The heating sequence in an inert atmosphere of this method is characterized by an initial temperature T1 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 measured and also 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 considered 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 oxidizing atmosphere, the quantities of CO and CO2 released by the residue of the sample 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 the organic matter during the oxidation cycle.From these measurements, this method defines a certain number of standard parameters including the following: the parameter noted TOC (for "Total Organic Carbon" in English) which corresponds to the organic carbon content of the sample, determined from the total quantity of HC released by the sample and the quantities of CO and CO2 released below limit temperatures during the pyrolysis phase and the oxidation phase, the parameter noted MinC (for "Minerai Carbon" in English) which corresponds to the mineral carbon content of the sample, determined from the quantities of CO and CO2 released by the sample above limit temperatures during the pyrolysis phase and the oxidation phase.

[0012] One of the main limitations of the protocol described above is the difficulty of separating the organic and mineral parts of soil carbon during the pyrolysis phase. The consequence of this limitation is an underestimation of Corg and an overestimation of Cmin.

[0013] We also know the patent application W02022 / 200093 A1 which defines a statistical correction to be made to the measurements of the standard parameters TOC and MinC carried out according to a protocol close to the "ROCK-EVAL® BULK ROCK" method. More precisely, in this process, the mineral carbon Cmin and organic carbon Corg contents are determined according to the following formulas:

[0014] Cmin = MinC — k. TOC ;

[0015] Corg = TOC + k. TOC, where k is a correction factor ranging from 0.04 to 0.12. However, as will be demonstrated in the application example below, this method is not sufficiently accurate in the case of soils with very low organic carbon content.

[0016] The present invention aims to overcome the drawbacks of the prior art. In particular, the present invention defines a new simplified 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 TOC and MinC parameters, adapted to the new protocol, to enable the determination of organic carbon and mineral carbon contents of a sample. The method according to the invention has a precision at least equivalent to the methods according to the prior art, or even more precise in the case of samples with a very low organic carbon content. Thus, the present invention allows a faster, simpler and at least as reliable characterization of the carbon forms present in a sample, and this for samples characterized by a wide range of organic carbon contents. Summary of the invention

[0017] The present invention relates to a method for characterizing and quantifying the carbon present in a surface formation, from a representative sample of said surface formation. Said method is characterized in that, said sample not having been heated in an inert atmosphere beforehand, it comprises at least the following steps:

[0018] 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 CO2 released during said heating sequence in an oxidizing atmosphere are continuously measured;

[0019] B) the carbon present in said sample is characterized and quantified 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 parameter representative of an organic carbon content defined by a formula of the type: and / or a parameter representative of a mineral carbon content defined by a formula of the type: oxyMinC (%) = [S5 * ^], where S4CO2 and S4CO represent said quantities respectively of CO2 and CO released during said sequence of temperatures up to an intermediate temperature (TLO) of said sequence of temperatures, and where S3 and S4'CO represent said quantities of CO2 and CO released during said sequence of temperatures beyond said intermediate temperature (TLO) of said sequence of temperatures, said intermediate temperature (TLO) of said sequence of temperatures being between 540 and 600°C.

[0020] According to one implementation of the invention, said initial temperature (TO') may be 200°C.

[0021] According to one implementation of the invention, said final temperature (TF') may be 850°C.

[0022] According to an implementation of the invention, said intermediate temperature (TLO) of said temperature sequence may be 580°C.

[0023] According to one implementation of the invention, said temperature sequence may comprise an isothermal plateau of a 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 being 3 minutes.

[0024] According to one implementation of the invention, said temperature sequence may comprise at least one intermediate isothermal stage whose temperature is between said initial (T'O) and final (T'F) temperatures of said temperature sequence, said at least one intermediate isothermal stage having a predetermined duration of between 1 and 5 minutes, and preferably being 3 minutes.

[0025] According to one implementation of the invention, said temperature sequence may comprise at least 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 isothermal stage at a temperature between 440 and 480°C, and preferably 460°C and / or a fourth intermediate isothermal stage at a temperature between 540 and 600°C, and preferably 580°C.

[0026] According to an implementation of the invention, said temperature sequence may comprise a number of thermal gradients defined by NG= NII+1 where NG is said number of thermal gradients of said temperature sequence and Nil is the number of said intermediate isothermal stages of said temperature sequence.

[0027] According to one implementation of the invention, at least one of said thermal gradients of said temperature sequence may be between 15°C / min and 40°C / min, and is very preferably 25°C / min.

[0028] According to an implementation of the invention, in step B), at least said parameter representative of said organic carbon content and said parameter representative of said mineral carbon content of said sample can be determined, and a parameter representative of a total carbon content is determined from a sum of said parameters representative of said organic carbon and mineral carbon content respectively.

[0029] List of figures

[0030] Figure 1 schematically illustrates an example of implementation of the temperature sequence under an oxidizing atmosphere of the process according to the invention.

[0031] 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.

[0032] Figure 3A (respectively Figure 3B) shows total carbon contents determined by a method according to the prior art (respectively the method according to the invention) as a function of the total carbon contents measured by a reference method for 15 samples.

[0033] Figure 4A (respectively Figure 4B) shows mineral carbon contents determined by a method according to the prior art (respectively the method according to the invention) as a function of the mineral carbon contents measured by a reference method for 15 samples.

[0034] Figure 5A (respectively Figure 5B) shows organic carbon contents determined by a method according to the prior art (respectively the method according to the invention) as a function of the organic carbon contents measured by a reference method for 15 samples.

[0035] Description of the embodiments

[0036] The invention relates to a method for characterizing and quantifying carbon (and in particular quantifying organic carbon and / or mineral carbon) present in a surface formation.

[0037] The term "superficial deposit" means a continental or coastal formation, loose or secondarily consolidated, resulting 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.

[0038] Soil refers to the entire set of outer layers of surface formations, the properties of which are directly controlled by the mutual actions of water, air, living and dead organisms, and even human activities in the most recent periods.

[0039] 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 with orifices having a diameter of 2 mm, dried at a temperature below 40°C, then crushed until fragments having dimensions of less than 200 μm are obtained. The method according to the invention can be advantageously but not limited to 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:

[0040] - an oxidation furnace in an oxidizing atmosphere,

[0041] - means of measuring carbon monoxide (CO) and carbon dioxide (CO2).

[0042] 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.

[0043] 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 organic carbon quantifications with a greater measurement error compared to low carbon contents by the methods according to the prior art.The Applicant then surprisingly observed, after analyses carried out on numerous samples with varying carbon contents, that a heating sequence under an oxidizing atmosphere applied directly to the sample, without a prior heating sequence under an inert atmosphere, makes it possible to determine the different forms of carbon in a sample at least as reliably, and even more reliably in the case of samples with very low organic carbon contents, than with methods according to the prior art including a pyrolysis phase.

[0044] Since the sample to be analyzed has not been heated in an inert atmosphere beforehand, the method according to the invention comprises at least the following steps:

[0045] 1- Heating sequence under oxidizing atmosphere

[0046] 2- Characterization and quantification of the carbon present in the sample

[0047] 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 from another layer of a surface formation.

[0048] 1. Heating sequence under oxidizing atmosphere

[0049] 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. It is clear that the at least one thermal gradient according to the invention does not necessarily connect the initial and final temperatures according to the invention.

[0050] 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.

[0051] Generally, the temperature range for the initial temperature TO' 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.

[0052] Generally speaking, the temperature range for the final temperature TF' of the temperature sequence under an oxidizing atmosphere ensures total decomposition of all forms of organic carbon, even those which are very refractory, but also decomposition of carbonates.

[0053] According to one implementation of the invention, the temperature sequence 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.

[0054] Figure 1 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 predetermined non-zero duration and a final temperature TF', the two temperatures being linked by a thermal gradient.

[0055] 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 for measuring the quantity of CO and / or CO2 may be used.

[0056] According to the invention, an intermediate temperature (hereinafter referred to as TLO) is defined between the initial temperature T'O and the final temperature T'F of the heating sequence in an oxidizing atmosphere, more precisely between 540 and 600°C, and preferably 580°C. This temperature will be used during step 2) to define quantities involved in the determination of the TOC and MinC parameters modified according to the invention.

[0057] Figure 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 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 the intermediate oxidation temperature TLO, here substantially equal to 580 °C, is called S4CO2 (respectively S4CO).Conventionally, the portion of the CO2 (respectively CO) curve to the right of the intermediate oxidation 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 residue 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 residue during heating in an oxidizing atmosphere.

[0058] Figure 2 shows non-zero CO2 values ​​above a temperature of about 650°C, which is characteristic of carbonate soils and therefore of the presence of mineral forms of carbon. It can also be observed that the CO curve ends around the intermediate temperature TLO.

[0059] 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 540 and 600°C, and preferably 580°C.These isothermal stages 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 under an oxidizing atmosphere, the different 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 process according to the invention described below because it makes it possible to achieve complete combustion of one class before starting 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 Nil is the number of intermediate isothermal stages in 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.

[0060] 2) Characterization and quantification of the carbon present in the sample

[0061] During this step, the aim is to characterize and quantify the carbon present in the soil sample, and more precisely to determine at least one parameter representative of an organic carbon content and / or a mineral carbon content from the quantities of CO and CO2 measured continuously during step 1) described above.

[0062] According to the invention, new TOC and MinC parameters are defined, modified compared to the prior art. More precisely, a parameter denoted oxyTOC and / or an oxyMinC parameter is determined according to formulas of the type: oxy

[0063] JMinC = [s5 * — l L 440J where S4CO2 and S4CO represent the quantities of CO2 and CO respectively released during the sequence of temperatures in an oxidizing atmosphere up to the intermediate temperature defined in step 1), and S5 and S4'CO represent the quantities of CO2 and CO respectively released during the sequence of temperatures in an oxidizing atmosphere beyond the intermediate temperature defined in step 1).

[0064] Note that the formulas for the standard parameters TOC and MinC, as defined in (Behar et al., 2001), if they were applied to the protocol of the process according to the invention (i.e. to a protocol comprising only a single heating, in an oxidizing atmosphere, and not also heating in an inert atmosphere) would be written as follows:

[0065] Thus, in the method according to the invention, and unlike the prior art, the term S4'CO is taken into account in the calculation of the modified TOC parameter, which corresponds to the CO generated by combustion of the organic compounds in the sample residue during heating in an oxidizing atmosphere. This quantity was not taken into account in the methods according to the prior art. Indeed, the methods according to the prior art only include S4CO and S4CO2 in the calculation of the TOC measured in the oxidation phase. However, the Applicant was able to observe, via analyses carried out on numerous samples, that when the combustion of the organic matter is incomplete, part of the carbon is released in the form of CO instead of CO2. The Applicant was then able to observe, via analyses carried out on numerous samples, that taking into account the proportion of carbon associated with the S4'CO peak makes it possible not to underestimate the Corg content.

[0066] According to an implementation of the invention according to which both the parameter representative of the organic carbon content oxyTOC and the parameter representative of the mineral carbon content oxyMinC have been determined, a parameter representative of the total carbon content can be determined by the sum of the parameter oxyTOC and the parameter oxyMinC.

[0067] Thus, the present invention makes it possible both to characterize the carbon present in the sample (by the organic carbon and / or mineral carbon contents; a non-zero organic carbon content makes it possible, for example, to qualify the type of sample analyzed) and to quantify the carbon present in the sample (by the total organic carbon and / or organic carbon and / or mineral carbon content).

[0068] Example

[0069] The characteristics and advantages of the method according to the invention will appear more clearly on reading the application example below.

[0070] Fifteen soil samples, including 11 samples with very low organic carbon content, were selected. Each sample was analyzed three times (triplicate) with: the process noted RE-650 hereafter, which corresponds to the process described in the aforementioned patent application WQ2022 / 200093. As a reminder, this process includes, upstream of an oxidation phase, a pyrolysis phase, the final temperature of which is 650°C. the process noted RE-OxyPure hereafter, which corresponds to the process according to the invention in order to determine an average content of mineral carbon, organic carbon and total carbon for each sample.

[0071] Each of the 15 samples was further analyzed twice (to obtain averages) with an elemental analyzer. Soil carbon contents were determined on 20 mg aliquots of finely ground soil by dry combustion (ISO 10694:1995) using a CNSOH elemental analyzer, specifically the Thermo Scientific™ EA IsoLink™ IRMS system from Thermo Fisher Scientific. Total soil carbon (Ctotal) was measured on unpretreated soil samples. In contrast, total soil organic carbon (Corg) was measured after sample pretreatment with 2N hydrochloric acid to remove carbonates from the sample (decarbonation). Total soil inorganic carbon (Cmin) was then obtained by calculating the difference between Ctotal and total Corg.

[0072] Figure 3A (respectively Figure 3B) presents in the form of points the total carbon contents Ctotal-RE-650 (respectively Ctotal-RE-OxyPure) determined by the RE-650 method (respectively the method according to the invention RE-OxyPure) as a function of the total carbon contents Ctotal-AE measured by an elemental analysis for each of the 15 samples. Each figure also presents the slope P and the coefficient of determination R 2 of a straight line determined by linear regression applied to all the points of the figure considered. It can be observed in these figures that the two processes RE650 and RE-OxyPure present similar performances for the quantification of Ctotal in comparison with the Ctotal contents measured by elemental analyzer, with coefficients of determination R 2 greater than 0.99 and slopes of the regression lines close to 1.

[0073] Figure 4A (respectively Figure 4B) presents in the form of points the mineral carbon contents Cmin-RE-650 (respectively Cmin-RE-OxyPure) determined by the RE-650 method (respectively the method according to the invention RE-OxyPure) as a function of the mineral carbon contents Cmin-AE measured by an elemental analysis for each of the 15 samples. Each figure also presents the slope P and the coefficient of determination R 2 of a straight line determined by linear regression applied to all the points of the figure considered. As for total organic carbon, it can be observed in these figures that the RE650 and RE-OxyPure processes present similar performances for the quantification of Cmin in comparison with the Cmin contents measured by elemental analyzer, with coefficients of determination R 2 greater than 0.99 and slopes of the regression lines close to 1.

[0074] Figure 5A (respectively Figure 5B) presents in the form of points the organic carbon contents Corg-RE-650 (respectively Corg-RE-OxyPure) determined by the RE-650 method (respectively the method according to the invention RE-OxyPure) as a function of the organic carbon contents Corg-AE measured by an elemental analysis for each of the 15 samples. Each figure also presents the slope P and the coefficient of determination R 2 of a straight line determined by linear regression applied to all the points of the figure considered, as well as the slope P' and the coefficient of determination R 2' of a straight line determined by linear regression applied to the points of this figure associated with very low organic carbon contents (points included in the area delimited by the dotted square, such as Corg less than 0.1%). It can be observed in these figures that the Corg contents measured by the RE650 and RE-OxyPure processes are strongly correlated with the Corg contents measured by the elemental analyzer, with coefficients of determination R 2 greater than 0.98 for all samples. However, the correlations are lower for soil samples with Corg contents below 0.1% (coefficient of determination R 2 ' less than or equal to 0.95). The best correlation is obtained between the Corg contents measured by the method according to the invention RE-OxyPure and the Corg contents measured by elemental analysis (determination coefficient R 2equal to 0.9928 and slope P greater than 1 for all samples, and coefficient of determination R 2 ' equal to 0.9543 and slope P' equal to 0.8263 for samples with Corg contents less than 0.1%). Thus, the method according to the invention presents the best performance for the quantification of Corg in soils in the case of very low Corg contents. The RE-650 method according to the prior art is less reliable for the quantification of Corg in soils with very low organic carbon contents, despite the application of a statistical correction to adjust the standard TOC and MINC parameters.

[0075] Thus, the present invention allows the characterization and quantification of the forms of 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). The method according to the invention allows quantifications of organic carbon and mineral carbon at least as precise as the quantifications of the methods according to the prior art.The method according to the invention has the advantage of having a precision superior to the methods according to the prior art, and even equivalent to an elemental analysis, for the quantification of organic carbon in the case of samples with very low organic carbon contents.

Claims

CLAIMS 1. Method for characterizing and quantifying the 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 carbon present in said sample is characterized and quantified 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 parameter representative of an organic carbon content defined by a formula of the type: and / or a parameter representative of a mineral carbon content defined by a formula of the type: where S4CO2 and S4CO represent said quantities of CO2 and CO respectively released during said temperature sequence up to an intermediate temperature (TLO) of said temperature sequence, and where S3 and S4'CO represent said quantities of CO2 and CO released during said temperature sequence beyond said intermediate temperature (TLO) of said temperature sequence, said intermediate temperature (TLO) of said temperature sequence being between 540 and 600°C.

2. Method according to claim 1, wherein said initial temperature (TO') is 200°C.

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

4. Method according to one of the preceding claims, in which said intermediate temperature (TLO) of said temperature sequence is 580°C.

5. Method according to one of the preceding claims, in which said sequence of temperatures comprises an isothermal plateau of a predetermined duration at said temperature 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.

6. Method according to one of the preceding claims, in which said temperature sequence comprises at least one intermediate isothermal stage whose temperature is between said initial (T'O) and final (T'F) temperatures of said temperature sequence, said at least one intermediate isothermal stage having a predetermined duration of between 1 and 5 minutes, and preferably being 3 minutes.

7. Method according to claim 6, wherein said temperature sequence comprises at least 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 isothermal stage at a temperature between 440 and 480°C, and preferably 460°C and / or a fourth intermediate isothermal stage at a temperature between 540 and 600°C, and preferably 580°C.

8. Method according to one of claims 6 or 7, wherein said sequence of temperatures comprises a number of thermal gradients defined by NG= NI 1+1 where NG is said number of thermal gradients of said sequence of temperatures and Nil is the number of said intermediate isothermal stages of said sequence of temperatures.

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

10. Method according to one of the preceding claims, in which, in step B), at least said parameter representative of said organic carbon content and said parameter representative of said mineral carbon content of said sample are determined, and a parameter representative of a total carbon content is determined from a sum of said parameters representative respectively of said organic carbon content and mineral carbon content.

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