Surface gas geochemistry above the natural CO2 reservoir of Montmiral (Drôme, France), source tracking and gas exchange between the soil, biosphere and atmosphere

International audienceOne of the options considered to mitigate greenhouse gas concentrations in the atmosphere is underground storage of CO2. There is a strong need for enhancing and developing methods that would help throughout the duration life of such underground storage, to ensure the safety and able to monitor the evolution of the injected CO2 plume. Among these, geochemical methods can play an important role. Here, we describe results acquired under the research programme “Géocarbone-Monitoring”, partially funded by the French National Research Agency, on the Montmiral natural analogue in South-Eastern France. Other results obtained under the same research programme in the French Massif Central are reported elsewhere in this volume.Spot sampling methods allowing a great geographical coverage and continuous measurements on selected points were undertaken in 2006 and 2007, in order to determine soil gas concentrations and fluxes as well as carbon isotope ratio determinations. One important result is that without any evidence of deep CO2 leakage, both CO2 concentrations and fluxes appear to be higher than can be explained only by biological activities. Further investigations are thus needed to understand the gas evolution better throughout the year

Geochemical Study of Natural CO2 Emissions in the French Massif Central: How to Predict Origin, Processes and Evolution of CO2 Leakage

International audienceThis study presents an overview of some results obtained within the French ANR (National Agency of Research) supported Géocarbone-Monitoring research program. The measurements were performed in Sainte-Marguerite, located in the French Massif Central. This site represents a natural laboratory for CO2/fluid/rock interactions studies, as well as CO2 migration mechanisms towards the surface. The CO2 leaking character of the studied area also allows to test and validate measurements methods and verifications for the future CO2 geological storage sites. During these surveys, we analyzed soil CO2 fluxes and concentrations. We sampled and analyzed soil gases, and gas from carbo-gaseous bubbling springs. A one-month continuous monitoring was also tested, to record the concentration of CO2 both in atmosphere and in the soil at a single point. We also developed a new methodology to collect soil gas samples for noble gas abundances and isotopic analyses, as well as carbon isotopic ratios. Our geochemical results, combined with structural geology, show that the leaking CO2 has a very deep origin, partially mantle derived. The gas rises rapidly along normal and strike-slip active faults. CO2 soil concentrations (also showing a mantle derived component) and CO2 fluxes are spatially variable, and reach high values. The recorded atmospheric CO2 is not very high, despite the important CO2 degassing throughout the whole area

Raman microprobe analysis of gaseous inclusion in diagenetically recrystallized calcites

Raman microprobe studies have been made on two core samples from the Smackover formation (Jurassic) Perry county Mississippi, at about 6.5 kilometers depth. Inclusions were found in calcite cements. Among other interesting aspects, one can see following gaseous inclusion assemblages -H2S, CH4, CO2, carbon ; H2S, CH4, CO2, H2O, carbon ; and H2S, CH4, carbon where a characteristic low temperature behavior is noted for the CH4-H2S system. The solid phase appears to be a very poorly ordered form of graphite, similar to that found in the earliest stages of the kerogengraphite transition found in metagenesis. Other series of inclusions are calcic saline aqueous. Corrected homogenization temperatures for these inclusions are near those of the samples in situ. This indicates that the inclusions have readjusted to present conditions, those of present day bottom hole temperatures and pressures. Considering the importance of early decrepitations one can assume that a readjustment occurred between early entrapment conditions and those currently prevalent at depth. The observations suggest either an evolution of gaseous inclusions up to the conditions of the upper stability of methane and methane — CO2 mixtures in the presence of H2O or the evolution of trapped hydrocarbon liquids which evolved to produce a gas -kerogen assemblage exempt of any organic molecules more complex than CH4.Les inclusions fluides ont été étudiées par microthermométrie et microsonde Raman dans des recristallisations calcitiques de sédiments carbonatés prélevés par forage à une profondeur de 6,5 km dans le Smackover (Jurassique) à Perry County (Mississippi). On a pu mettre en évidence en particulier les associations CH4-H2S -carbone, CH4-CO2-H2S -carbone et CH4-CO2-H2S-H2O -carbone, dont le comportement à basse température est caractéristique du système CH4-H2S. La comparaison des spectres Raman de la phase solide avec des travaux antérieurs sur une série de charbons montre qu'il s'agit d'un composé graphitique mal organisé, comparable au produit habituel de la maturation du kerogen lors de la métagenèse. Les fluides aqueux sont constitués par des saumures riches en ions Ca. Les températures d'homogénéisation des inclusions précoces et des inclusions tardives indiquent, après correction de pression, des valeurs de piégeage identiques et comparables à celles mesurées actuellement au fond du puits. Compte tenu de l'importance des décrépitations des inclusions précoces, on peut supposer qu'il y a eu modification de leur densité et réajustement selon les conditions actuelles de température et de pression. Ces résultats suggérent soit l'évolution in situ des hydrocarbures piégés sous forme liquide ou gazeuse vers une association composée d'un résidu graphitique et d'un gaz riche en H2S ne contenant pas d'hydrocarbures plus lourds que le méthane, soit le piégeage des gaz CH4-H2S-CO2 et leur évolution vers leurs conditions de stabilité thermique maximum.Guilhaumou Nicole, Velde Bruce, Beny Claire. Raman microprobe analysis of gaseous inclusion in diagenetically recrystallized calcites. In: Bulletin de Minéralogie, volume 107, 2, 1984. Inclusions fluides

Raman microprobe analysis on gaseous inclusions from the diagenetically altered Terres Noires (S-E, France)

Raman microprobe determinations have been made on primary gaseous inclusions in quartz crystals found in concretions from diagenetically altered Jurassic sediments from S-E France. The gas compositions vary as a general function of the diagenetic grade of the enclosing sediments. Two major types of gas inclusions were found, a wet gas in the western part of the basin and a dry gas in the eastern part. The proportions of CH4, C2H6, C3H8, CO2, H2S and N2 were determined to within one percent of the amount present. The gas mixtures and fluid phase assemblages indicate that these materials were entrapped during the evolution of the organic and mineral rock assemblages induced by increases of temperature and pressure. There is no evidence for change of the organic material posterior to its entrapment as a fluid inclusion. UV fluorescence optical observations indicate that organic liquids are present in the primary inclusions and also in secondary inclusions situated in healed cracks. These secondary inclusions are found in the whole area. UV fluorescence measurements indicate that this trapped material could be an extract from a more evolved organic material than that found in primary inclusions. These secondary inclusions could have been formed during tectonic event of the Alpine orogenies.La composition des phases gazeuses des inclusions primaires piégées au cours de la diagenèse dans les quartz et les célestites des septarias minéralisées des Terres Noires jurassiques du S-E (France) a été déterminée par microsonde Raman. L'estimation quantitative des espèces CH4, C2H6, CO2, H2S et N2 a été faite avec une précision de 1 %. On peut y observer une variation en fonction du degré de diagenèse subie par les sédiments encaissants. Deux types principaux de mélanges gazeux sont présents : un gaz de type sec dans la zone orientale du bassin, et un gaz de type humide dans la zone occidentale. La répartition et la composition des inclusions primaires contenant de la matière organique montrent qu'elles ont été piégées durant l'élévation de température et de pression qui a accompagné la diagenèse, sans que l'on puisse observer les traces d'une évolution de leur contenu postérieure au piégeage. L'observation microscopique en éclairage UV permet de distinguer une famille d'inclusions fluides organiques secondaires présentes dans les fractures recicatrisées des cristaux de l'ensemble de la zone étudiée. Ces inclusions présentent en fluorescence des teintes nettement différentes des inclusions liquides primaires : elles pourraient correspondre à une matière organique plus évoluée, piégée lors d'événements tectoniques postérieurs liés à l'orogenèse alpine.Guilhaumou Nicole, Jouaffre Didier, Velde Bruce, Bény Claude. Raman microprobe analysis on gaseous inclusions from the diagenetically altered Terres Noires (S-E, France). In: Bulletin de Minéralogie, volume 111, 6, 1988. pp. 577-585

Morphology, origin and infrared microthermometry of fluid inclusions in pyrite from the Radka epithermal copper deposit, Srednogorie zone, Bulgaria

Pyrite samples from the Radka epithermal, replacement type, volcanic rock-hosted copper deposit, Bulgaria, have been studied using near-infrared (IR) microscopy. Two generations of pyrite based on their textures, composition and behaviour in IR light can be distinguished. Electron microprobe analyses, X-ray elemental mapping and Fourier transform infrared spectroscopy were used to study the relationship between crystal zoning, trace element contents and IR transmittance of pyrite. The observed crystal zoning is related to variable arsenic contents in massive fine-grained and colloform pyrite from the early pyrite-quartz assemblage, and cobalt contents in pyrite crystals from the late quartz-pyrite vein assemblage. There is a negative correlation between trace element content and IR transmittance of pyrite. The IR transparency of pyrite is thus a sensitive indicator of changes in trace element concentrations. Fluid inclusions have only been found in the second pyrite generation. Scanning electron microscopy observations on open fluid inclusion cavities permitted the crystallographic features of vacuoles to be determined. A characteristic feature of primary fluid inclusions in pyrite is a negative crystal habit, shaped mainly by {100}, {111} and {210}. This complicated polyhedral morphology is the reason for the observed opacity of some isometric primary inclusions. Secondary fluid inclusion morphology depends on the nature of the surface of the healed fracture. Recognition of the primary or secondary origin of fluid inclusions is enhanced by using crystallographically oriented sections. Microthermometric measurements of primary inclusions indicate that the second pyrite generation was deposited at maximum P-T conditions of 400 °C and 430 bar and from a fluid of low bulk salinity (3.5-4.6 wt%), possibly KCl-dominant. There are large ranges for homogenisation temperatures in secondary inclusions because of necking-down processes. Decrepitation features of some of pyrite-hosted inclusions and of all inclusions in associated quartz indicate reheating of the veins to 500-550 °C. The late cobalt-rich quartz-pyrite vein assemblage in the Radka deposit may be the shallow manifestation of deeper and genetically related porphyry copper mineralisation. This is a common observation of many intermediate- to high-sulfidation epithermal replacement-type ore bodies in this ore district and possibly the Cretaceous Banat-Srednogorie metallogenic belt in general

Synthesis and anion exchange properties of a Zn/Co double hydroxide salt

International audienceIn this study, a ZnCo―SO4 double hydroxide salt (DHS) has been synthesized by the reaction of CoSO4*7H2O with solid ZnO in aqueous solution. The resulting pink ZnCo―SO4 DHS was characterized by X-ray Diffraction, Infrared Spectrometry, UV/Visible Spectrometry and Thermal analysis. Characterizations show that its structure is similar to the one described for the layered Zn Sulfate Hydroxide Hydrate, i.e. brucite layers with 1/4 empty octahedra presenting tetrahedrally coordinated divalent atoms above and below the empty octahedra. Co atoms are located in the octahedra and zinc atoms in tetrahedra and octahedra. In this structure the exchangeable anions are located at the apex of tetrahedra. SEM and TEM observations show that the DHS is composed by aggregates of hexagonal plates of several hundreds of nanometers. Due to its interest for industrial or environmental applications, the exchange of sulfate groups by carbonates has been investigated. Results show a limited exchange capacity and a higher affinity for sulfates compared to affinity for carbonates

Synthesis and anion exchange properties of a Zn/Ni double hydroxide salt with a guarinoite structure

International audienceIn this study, the first route to synthesize a compound with the guarinoite structure (Zn,Co,Ni)6(SO4)(OH,Cl)10·5H2O is reported. Zn/Ni guarinoite is obtained from the reaction of NiSO4·7H2O with solid ZnO in aqueous solution. The resulting green Zn/Ni guarinoite ((Zn3.52Ni1.63)(SO4)1.33(OH7.64)·4.67H2O) was characterized by X-ray diffraction, infrared spectrometry, UV–Visible spectrometry and thermal analysis. It is shown that its structure is similar to the one described for the layered Zn sulfate hydroxide hydrate, i.e. brucite layers with View the MathML source empty octahedra presenting tetrahedrally coordinated divalent atoms above and below the empty octahedra. Ni atoms are located in the octahedra and zinc atoms in tetrahedra and octahedra. In this structure the exchangeable anions are located at the apex of tetrahedra. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations show that the Zn/Ni guarinoite is composed of aggregates of hexagonal plates of several hundreds of nanometers. Due to its interest for industrial or environmental applications, the exchange of sulfate groups by carbonates has been investigated. Results show a limited exchange and a higher affinity of the Zn/Ni guarinoite for sulfates compared to carbonates