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Quantifying Soil Organic and Inorganic Carbon (SOC & SIC) separately in carbonate soils involves successive pretreatments and/or measurements to separate the two carbon forms. The Rock-Eval® (RE) thermal analysis has been developed in the 70’s by IFPEN to study oil bearing rocks with a ramped heating pyrolysis and oxidation. The RE has been increasingly used over the past 20 years to quantify and characterize SOC without pretreatments and with a single analysis in any kind of soils. To improve the SOC and SIC quantifications by RE, Disnar & al. (2003) and Sebag & al. (2022a; 2022b) suggested statistical corrections of the TOC and MinC standard parameters, which are SOC and SIC content estimators respectively. However, few applications have focused on SIC quantification and no adjustments of the RE standard analysis cycle have been investigated. This study aims at adjusting the RE standard analysis protocol to quantify SOC and SIC. For this, a panel of carbonate soils with a wide range of SOC and SIC contents was analyzed by RE. The SOC and SIC quantifications by RE were compared to those obtained by elemental analysis (EA, standard method to quantify soil carbon). A too high SIC amount in the RE crucible (> 4 mg) led to an underestimation of the SIC content, even after correcting the MinC parameter. The higher the SIC amount in the RE crucible, the more the SIC content was underestimated. The standard analysis cycle was too short to achieve a complete thermal decomposition of the SIC amount in the crucible leading to an underestimation of the SIC content. Thus, we propose to extend the last oxidation isotherm from 3 min to 5 or 7 min to achieve the complete SIC thermal decomposition. Moreover, the temperature limit used during the pyrolysis to distinguish the CO and CO2 emitted by organic matter thermal cracking and carbonate thermal decomposition phase is often inappropriate as the thermoresistant part of SOC and the main part of SIC decompose simultaneously after 550°C. Thus, we suggest to stop the pyrolysis phase at the onset of SIC decomposition to drag all SIC signal during the oxidation phase only. This modification of the standard analysis protocol for carbonate soils allows us to avoid some of the statistical corrections suggested by Disnar & al. (2003) and Sebag & al. (2022a; 2022b). Finally, we propose a new RE analysis protocol to simplify the calculations and improve the accuracy of SOC and SIC quantifications in carbonate soils. Disnar, J.R., B. Guillet, D. Keravis, C. Di-Giovanni & D. Sebag, 2003. Soil organic matter (SOM) characterization by Rock-Eval pyrolysis: scope and limitations. Organic Geochemistry 10.1016/S0146-6380(02)00239-5Sebag, D., V. Lamoureux-Var, I. Kowalewski, D. Pillot & H. Ravelojoana, 2022a. Procédé pour la quantification et la caractérisation du carbone dans les sols. IFP Energies Nouvelles Patent No. 3121225. France.Sebag, David; Lamoureux-Var, Violaine; Kowalewski, Isabelle; Ravelojoana, Herman; Lefrançois, Noémie, 2022b. Improved quantification of SOC and SIC in Rock-Eval® thermal analysis. SOM (8th International Symposium on Soil Organic Matter), Seoul, Korea.

Quantifying both soil organic and inorganic carbon (SOC & SIC) is essential to understand carbon (C) dynamics and to assess the atmospheric C sequestration potential in calcareous soils. The procedures usually used to quantify SOC and SIC involve pretreatments (decarbonation, decarbonatation) and calculation of the difference between C contents estimated by elemental analysis on raw and pretreated aliquots. These procedures lead to analytical bias associated to pretreatments, measurement deviations associated to the sample heterogeneity, and cumulative errors associated to calculations. The Rock-Eval® thermal analysis, used in soil sciences since the 2000s, provides two parameters estimating the organic (TOC) and inorganic (MinC) C contents of a non-pretreated aliquot with a single analysis. Nevertheless, the Rock-Eval® protocol has been standardized in the 70s by IFP Energies Nouvelles for studying oil bearing rocks and is, thus, not perfectly suited for soil study. Previous studies suggested statistical corrections of the standard parameters to improve their estimations of C contents assessed by elemental analysis but only few of them focused on the estimation of inorganic C content using the MinC parameter. Moreover, none of them suggested adjustments of the standard Rock-Eval® protocol. This study proposes to adapt this protocol to optimize SOC and SIC quantifications in soil samples. Comparisons between SOC and SIC quantifications by elemental analysis and by Rock-Eval®, with and without statistical corrections of the standard TOC and MinC parameters, were carried out on a soil panel with a wide range of SOC and SIC contents. The results show that the standard Rock-Eval® protocol properly estimates SOC contents once the TOC parameter is corrected. However, it cannot achieve a complete thermal breakdown of SIC amounts > 4 mg leading to an underestimation of high SIC contents by the MinC parameter, even after correcting it. Thus, the final oxidation isotherm is extended to 7 min to complete the thermal breakdown of SIC before the end of the analysis.

International audience; Soil organic matter (SOM) is a complex heterogeneous mixture formed through decomposition and organo-mineral interactions, and characterization of its composition and biogeochemical stability is challenging. From this perspective, Rock-Eval® is a rapid and efficient thermal analytical method that combines the quantitative and qualitative information of SOM, including several parameters related to thermal stability. This approach has already been used to monitor changes in organic matter (OM) properties at the landscape, cropland, and soil profile scales. This study was aimed to assess the stability of SOM pools by characterizing the grain size fractions from forest litters and topsoils using Rock-Eval® thermal analysis. Litter (organic) and topsoil samples were collected from a beech forest in Normandy (France), whose management in the last 200 years has been documented. Fractionation by wet sieving was used to separate large debris (> 2 000 μm) and coarse (200–2 000 μm) and fine particulate OM (POM) (50–200 μm) in the organic samples as well as coarse (200–2 000 μm), medium (50–200 μm), and fine (< 50 μm) fractions of the topsoil samples. Rock-Eval® was able to provide thermal parameters sensitive enough to study fine-scale soil processes. In the organic layers, quantitative and qualitative changes were explained by the progressive decomposition of labile organic compounds from plant debris to the finest organic particles. Meanwhile, the grain size fractions of topsoils presented different characteristics. The coarse organo-mineral fractions showed higher C contents, albeit with a different composition, higher thermal stability, and greater decomposition degree than the plant debris forming the organic layer. These results are consistent with those of previous studies that microbial activity is more effective in this fraction. The finest fractions of topsoils showed low C contents, the highest thermal stability, and low decomposition degree, which can be explained by the stronger interactions with the mineral matrix. Therefore, it is suggested that the dynamics of OM in the different size fractions be interpreted in the light of a plant-microbe-soil continuum. Finally, three distinct thermostable C pools were highlighted through the grain size heterogeneity of SOM: free coarse OM (large debris and coarse and fine particles), weakly protected OM in (bio)aggregates (coarse fraction of topsoil), and stabilized OM in the fine fractions of topsoil, which resulted from the interactions within organo-mineral complexes. Therefore, Rock-Eval® thermal parameters can be used to empirically illustrate the conceptual models emphasizing the roles of drivers played by the gradual decomposition and protection of the most thermally labile organic constituent

International audience; The Mozambique and Madagascar margins present major rivers that are responsible for the discharge of large amounts of terrestrial organic matter (OM) which can influence carbon cycling in marine environments. Therefore, the Mozambique channel represents a unique case to study the fate of the organic carbon in deepwater domains. Using a new and extensive data set of sedimentary OM collected from sediment traps, seafloor sediments and core sediments, we address the origin of the OM that is transported and deposited in the Mozambique Channel, its degradation state and preservation conditions. A Rock-Eval 6 survey allowed us to characterize the origin and amount of OM from shallow to deep-water turbidite systems, between 500 and 4400 m water depth. Rock-Eval 6 performed on suspended sediments within particle traps at 47 m above the seabed show that the OM is transported into the deep-water domain with relatively high TOC (between 1.5 and 2.5%). However, the OM is largely oxidized close to the water-sediment interface (Oxygen Index >300 mg CO 2 /g TOC). Seafloor sediments sampled to a maximum depth of 40 cm show lower TOC values compared to those collected from particle traps suggesting that the degradation of the OM is mainly active at the water-sediment interface. Small concentrations of OM are preserved within the recent sediments of the distal area of the Zambezi turbidite system below 2500 m water depth (TOC < 0.5%). Rock-Eval results show that core sediments from the Majunga slope (NW margin of Madagascar) and the Zambezi slope (Mozambique margin) contain the highest concentration of terrestrial OM (TOC between 1 and 2%). However, the OM within core sediments from the deep-water domain is largely oxidized and degraded, probably due to the conjugate effect of low sediment accumulation rates (SARs) and high permeabilities of the coarse-grained sediments. Consequently, the deep-water domain of the Mozambique Channel does not seem to be an important sink of terrestrial OM. This process is reinforced by important bottom water currents which induce the remobilization and transport of seafloor sediments that lead to higher oxygen exposure time in the uppermost centimeters of sediments.

International audience; Marine sediments near continental margins contain sedimentary organic matter (SOM) which is subject to the metabolic activity of micro-organisms during early diagenesis resulting in production of biogenic methane. This process occurs at microscopic scale and anaerobic conditions. Here, we apply a new numerical approach to simulate biogenic methane production offshore Aquitaine (Bay of Biscay) where gas seeps have been recently observed as the result of microbial activity. This new approach accounts for: (1) degradation of a labile-SOM fraction to methane, (2) first order kinetics of the thermal degradation of a thermo-labile-SOM fraction into labile fraction at greater burial and (3) decrease of SOM reactivity with time. First, the organic matter is characterized through pyrolysis using Rock-Eval performed on cuttings collected from two wells located within the methane seepage area. The microbial system is fed from a type III continental-derived SOM which is immature (average Tmax < 425 °C). The basin model is built and calibrated on seismic and well data. It accounts for the consumption of methane required to precipitate methane-derived authigenic carbonates which are found widely distributed on the seafloor as the result of the anaerobic oxidation of methane during upward migration. A sensitivity analysis is performed on the main model input parameters to quantify their impact on the biogenic gas production and expulsion/migration processes. Results led to a reference scenario for microbial gas production in offshore Aquitaine. With this model the generated methane is predominantly dissolved in water and transported by advective processes. Migration is mainly vertical from the source rock layers to the seafloor and controlled by sediment porosity and strata geometry. Modelling can reproduce natural processes such as gas migration at emission points (gas seeps) which have been previously mapped in the offshore Aquitaine Basin. Our results suggest that the biogenic methane is sourced by a present-day active system with a mean flow rate of 27 Mg/y which is relatively lower than flux modelled during the early Pleistocene reaching up to 41 Mg/y. Calculated total methane lost to the seafloor along the Aquitaine Shelf is in accordance with methane flow rate estimated from in situ measurements and acoustic signatures of bubbling sites, and ranges between 0.87 Tcf/My and 1.48 Tcf/My. Here we propose a new workflow to assess and predict biogenic gas occurrences in offshore environment at the basin scale where gas is sourced by recent continental-derived organic matter. This new approach can help to better assess the total biogenic methane budget emitted naturally in the shelf area of oceans that may reach the atmosphere with a negative impact on climate and environment.

An advanced method for pyritic and organic sulphur quantification in organic rich sediments was developed using a Rock-Eval which was designed for sulphur quantification in addition to carbon quantification (Rock-Eval S). This new method involves data processing of parameters obtained via Rock-Eval S analysis of rock samples. It was applied to sedimentary mixtures with known organic and pyritic sulphur contents. The results show that this method allows an accurate quantification of both pyritic and organic sulphur in organic rich sediments that do not contain sulphates. This new method provides important analytical advantages, including (i) rapid analysis time (2 hours per sample); (ii) no chemical preparation steps, which are laborious and hazardous due to the use of strong acids; (iii) automatic analysis; and (iv) simultaneous quantification of organic/inorganic carbon and organic/inorganic sulphur in a single run.

An advanced method for pyritic and organic sulphur quantification was developed using a Rock-Eval 6 adapted for sulphur quantification in addition to carbon (Rock-Eval 6S). It consists in data processing of sulphur parameters obtained via Rock-Eval 6S analysis of a rock sample. This Rock-Eval 6S method for pyritic and organic sulphur quantification was applied on sets of organic rich sedimentary rock samples from the Grey Shale Member in North Yorkshire and from the Kimmeridge Clay Formation in Dorset. Results from the Rock-Eval 6S method were compared to results from another independent method based on elemental analysis. A good correlation between both methods is observed except for samples with low pyritic and organic sulphur contents. This new Rock-Eval 6S method for pyritic and organic sulphur quantification provides important advantages compared to the elemental analysis method, including: rapid analysis time (2 hours per sample); no chemical preparation steps, which are heavy and dangerous procedures due to the use of strong acids; automatic analysis; combined quantification of organic/inorganic carbon and organic/inorganic sulphur in a single run.

The hypothesis of a low temperature natural “vulcanization” process as a potential mechanism for the formation of insoluble bitumen and H2S in reservoirs which have not undergone biodegradation, high thermal stress, or thermochemical sulfate reduction (TSR) has been investigated using laboratory simulation experiments. The experiments, involving elemental sulfur (32S and 34S), a low maturity oil and water at 200 °C in sealed gold tubes, clearly showed that elemental sulfur is able to induce the formation of H2S, polar compounds (resins and asphaltenes) and insoluble bitumen most likely by way of sulfur cross-linking processes. Specific organic sulfur compounds (thiophenes, dithiophenes and thienothiophenes) formed by the reaction of sulfur and hydrocarbons may be potential markers of such a process, but remain to be detected in natural samples.

The sulfur isotopic composition (δ34S) of petroleum is believed to be affected mainly by sulfur incorporation reactions into the sedimentary organic matter during the early diagenesis. However, secondary processes could affect the original δ34S of oil under the effect of thermal maturity or of the microbial activity of biodegraded reservoirs. In this study, the different processes that may affect the δ34S of in-reservoir oils were assessed based on the sulfur content and isotopes of a series of oil and core samples coming from various reservoirs of the Lower Cretaceous Mannville Group, Western Canada Sedimentary Basin (WCSB). Based on the molecular study, these samples appear to have reached various levels of maturity and biodegradation, ranging from 0 to 6.5 on the biodegradation scale of Peters and Moldowan. In addition, mixing of organic matter coming from different source rocks was identified based on the comparison with extensive correlation studies performed in the WCSB. Investigation of the δ34S shows a trend that seems a priori correlated to the level of biodegradation. However, a careful interpretation of molecular and sulfur isotope data leads to the conclusion that the observed δ34S variations have rather to be ascribed to contributions of oils generated by various source rocks. Alternatively, variations of δ34S could neither be related to maturity differences nor to kinetic effects during organic sulfur compounds biodegradation. In the case of some specific core samples showing a common origin based on biomarker study, δ34S variations might not be related to different sources but to secondary sulfur incorporation/exchange processes occurring within the reservoir. These processes would involve reduced sulfur species from bacterial sulfate reduction formed in situ or migrated into Mannville reservoirs. This hypothesis is supported by laboratory experiments showing sulfur exchange/incorporation under plausible conditions for shallow reservoirs.

Thermochemical sulfate reduction (TSR) is usually associated with significant sour gas (H 2 S) generation and calcite precipitation in deep and hot carbonate reservoirs as a result of reaction between sulfate minerals (primarily anhydrite) and petroleum fluids at elevated temperature (beginning at ∼110-140°C). Thermal enhanced oil recovery (TEOR) techniques are widely applied during production to increase heavy oil recovery in mature fields by lowering oil viscosity. The reservoir rock may reach high temperature (150 °C < T< 300 °C) during steam and hot water flooding for secondary oil recovery that induces TSR. Experimental (pyrolysis in gold tubes) and numerical (0D fluid-rock interaction numerical model: ArXim) simulations were undertaken with a simple petroleum system assimilated to n-alkane model compounds in the presence of sulfate to assess the feasibility of "induced TSR" and to tentatively estimate the sulfur risk in terms of H 2 S and organosulfur compound (OSC) formation during steam and hot water stimulated enhanced recovery operations. The oxidation of organic matter (by reaction with both S° and sulfate) can be easily reproduced under laboratory conditions, as confirmed by the presence of organic sulfur in the soluble and insoluble residue. For the experiments performed in the presence of sulfate, higher amounts of H 2 S, polar compounds and organic residue are produced, and aromatic fractions are enriched in condensed aromatics (sulfur-containing and non-sulfur-containing). Although sulfate reduction from 3% to 5% is deduced from a sulfur balance calculation, no secondary carbonate is detected using X-ray diffraction (XRD). Numerical simulations suggest no limiting thermodynamic factor for the precipitation of carbonate under our experimental conditions and indicate the replacement of sulfate by carbonate. According to numerical data, the lack of carbonate can be explained by its slow growth and/or high energy barrier of nucleation and/or insufficient residence time in laboratory experiments.

The nor-triterpenoid, 24-nor,28-lupanoic acid, methyl ester 1a, present as the acid in an African crude oil, was isolated and identified using NMR. This compound represents a key intermediate in the formation of nor-lupane derivatives which are widespread petroleum markers indicative of terrestrial inputs.

In the context of reducing production costs for the oil industry, this paper aims to suggest a way for predicting wax precipitation at an early stage of the reservoir characterization and production process. In this respect, reservoir core extracts and production oil-saturated fractions have been herein analyzed by high temperature gas chromatography (HTGC) in order to quantify the high molecular weight hydrocarbons (HMWHC) which are at the origin of wax deposits in the tubing and surface facilities. Samples of oils and associated reservoir cores from two adjacent but non-communicating fields are considered in this paper. Wax deposition is documented for Field P but not for Field A. It is demonstrated that the reservoir core extract of Field P is substantially enriched in HMWHC when compared to the produced oil, whereas for Field A, with no production problems, both profiles (oil and reservoir core extracts) are similar and depleted in HMWHC. These results suggest that fields exhibiting a risk of wax precipitation could be identified during the exploration phase as soon as reservoir cores are available. Moreover, it can be envisioned that such additional data provided by the saturates distribution can potentially be used in future thermodynamic models (derived from those currently applied for assessing wax precipitation in pipelines during the transport of produced oil) in order to quantitatively predict, at an early stage of reservoir characterization, paraffins precipitation in reservoirs and tubings during production. The preliminary computed results obtained for the produced oil and the reservoir rock extract of Field P indicate that the temperature of paraffin crystallization onset is significantly under-estimated when using the composition of tank oil (42°C)-heavy paraffins are already deposited. On the other hand, the predictive calculation, using the core extract (52°C) and which can be performed as soon as a core sample is available, will allow us to minimize the risk of paraffin deposition during production and transportation. Corresponding author (I. Kowalewski)

The molecular structural units of Boscan and Duri asphaltenes have been investigated in ruthenium-ion-catalyzed oxidation (RICO) and pyrolysis. From the RICO reactions, homologous series of n-alkanoic acids (C1−C31), representing aromatic-attached n-alkyl side chains, α,ω-di-n-alkanoic acids (C4−C26), representing polymethylene bridges connecting two aromatic units, and benzenecarboxylic acids, indicating the major modes of aromatic condensations in the asphaltene molecules and the minor role of pericondensed aromatic structures in them, were detected and measured. The RICO reaction also yielded a nondistillable oxidized residue. Pyrolysis of this material after methylation yielded a homologous series of n-alkanes and n-alk-1-enes, n-alkanoic and n-alkenoic acid methyl esters, and free n-alkanoic acids with strong even-to-odd carbon preference. These products prove the presence of naphthenic-attached n-alkyl side chains and bridges, polymethylene bridges connecting aromatic to naphthenic systems, and n-al...

Two bridgehead-alkylated 2-thiaadamantanes (1,5-dimethyl and 1-methyl) have been identified in petroleum; these compounds give clues to sulfurisation processes affecting petroleum which has undergone high thermal stress.

The use of bio-resistant compounds is a promising method to evaluate hydrocarbon mass changes linked to bacterial activity in petroleum reservoirs. One of the difficulty lies in finding stable elements or compounds well represented in crude oils that are completely oblivious to bacterial activity. Another is measuring them with high enough analytical accuracy. We have done tests on different case studies using nickel and vanadium (complexed in porphyrin structures and free formed cations), as they generally show a constant ratio in crude oils of same origin and where absolute metal concentration increases with biodegradation. It is thus possible, where the composition of the initial pole is known (non-biodegraded oils or rock extracts) to calculate a concentration factor which can be used to calculate the hydrocarbons lost by biodegradation. This method, applied to case studies, has given biodegradation weight loss ranging from 20% to 80%. At 20% it corresponds to the onset of LMW n-alkanes loss, and 80% to a reinforced stage of biodegradation (a GC trace showing only unresolved cyclo and isoalkanes, and no measurable biomarkers. Interpretation problems arise however because of low metal content in certain oils, and secondary processes (mixing, thermal maturity etcO). The analytical limit yields sometimes unrealistic concentration factors because of metal detection limit by conventional ICP or Atomic Absorption. Higher precision magnetic sector ICP-MS instruments detecting metals to a level concentration of parts per trillion should be tested as well as making cross checks with other super resistant biomarkers for example.

Oil-gas phase equilibria is likely to represent one of the main causes of observed compositional heterogeneities in fields. Our research has been focused mainly on the light C 14 -hydrocarbons fraction and can be divided into two parts. The first part was performed with a fluid mixture of a Middle East crude oil and a synthetic gas, and includes PVT experiments, high resolution-gas chromatography (HR-GC) and 13 C gas chromatography-combustion-isotopic ratio mass spectrometry (GC-C-IR-MS) analyses. Our observations show that under a given set of PT conditions there is a preferential separation of aromatics into the oil phase and isoalkanes into the gas phase. Moreover, the data from 13 C GC-C-IRMS suggest a small isotopic fractionation between the two phases, with the gas being relatively isotopically lighter. The second part is theoretical, and uses Equation of State (EOS) calculations which successfully simulate the previous experiment.