One 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. Une des options envisagées pour réguler les concentrations de gaz à effet de serre dans l’atmosphère est le stockage souterrain du CO2. Dans ce domaine existe un fort besoin de renforcer et de développer des méthodes susceptibles d’être utilisées tout au long de la durée de vie de ces stockages souterrains, afin de s’assurer de leur sécurité et de pouvoir suivre l’évolution du panache de CO2 injecté. Parmi elles, les méthodes géochimiques peuvent jouer un rôle important. Nous décrivons ici les résultats acquis dans le cadre du programme de recherche « Géocarbone-Monitoring » financé en partie par l’Agence Nationale de la Recherche sur l’analogue naturel de Montmiral dans le Sud-Est de la France. D’autres résultats obtenus dans le cadre de ce même programme de recherche sont rapportés dans un autre article présent dans ce volume. Des méthodes d’échantillonnage ponctuelles permettant une grande couverture géographique et des mesures en continu sur certains points sélectionnés ont été entreprises en 2006 et 2007, afin de permettre la détermination des concentrations en gaz du sol et des flux, ainsi que des déterminations des rapports isotopiques du carbone. Sans aucune preuve d’une fuite de CO2 profond, à la fois les concentrations et les flux de CO2 semblent être plus élevés que ne pouvant s’expliquer par l’effet des seules activités biologiques. Des études plus approfondies sont donc nécessaires pour mieux comprendre l’évolution des gaz tout au long de l’année.

Geochemical Study of Natural CO2 Emissions in the French Massif Central: How to Predict Origin, Processes and Evolution of CO2 Leakage - This study presents an overview of some results obtained within the French ANR (National Agency of Research) supported Geocarbone-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.

This 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. Cette étude présente les principaux résultats de campagnes de monitoring géochimique menées en 2006 et 2007 dans le cadre du projet Géocarbone-Monitoring, sur le site de Sainte-Marguerite, situé dans le Massif Central. Ce site constitue un « laboratoire naturel » pour l’étude des interactions CO2/fluides/roches et des mécanismes de migration du CO2 vers la surface, à l’échelle des temps géologiques. Le caractère particulièrement émissif de cet « analogue » permet également de tester et valider des méthodes de mesure et de surveillance des futurs sites de stockage de CO2. Au cours des campagnes de terrain, nous avons analysé des flux de CO2 entre le sol et l’atmosphère, et nous avons prélevé et analysé à la fois des gaz des sols, et du gaz provenant de sources carbo-gazeuses, présentes dans toute la région. Un dispositif de « monitoring continu » dans le temps a également été testé, afin d’enregistrer conjointement les teneurs en CO2 de l’atmosphère et dans le sol en un point précis. Nous avons pu mettre au point un suivi géochimique basé sur la composition isotopique des gaz rares prélevés dans les sols. L’ensemble de nos résultats, confronté à la géologie de terrain, nous a permis de mettre en évidence l’origine mantellique du CO2. Ce CO2 remonte rapidement à la surface à l’état gazeux, le long de failles normales et/ou décrochantes, actives actuellement. Les teneurs et flux de CO2 dans le sol sont spatialement variables et élevés, et montrent également une origine mantellique. Les teneurs atmosphériques semblent faiblement augmenter par rapport à l’important dégazage observé dans la région.

Surface Gas Geochemistry above the Natural CO2 Reservoir of Montmiral (Drome, France), Source Tracking and Gas Exchange between the Soil, Biosphere and Atmosphere - One 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 "Geocarbone-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.

We have devised an experimental method for calculating isotopic mass balances for the C-12 and C-13 in the pyrolysis products from a type II kerogen during artificial maturation in a closed system. The method also enables a study of the isotopic evolution of cracking products versus kerogen transformation ratios.The type II kerogen (Paris Basin) was at the onset of catagenesis. Pyrolysis was carried out in a confined closed system under isothermal conditions, at several temperatures (275-350 degrees C) and times at a constant pressure of 10 Mpa. Under these conditions, the kerogen transformation ratios covered a range from 1% to 87%. Pyrolysis effluents were separated into hydrocarbon gases (C-1-C-5), non-hydrocarbon gases (CO, CO2, H-2, H2S), C-6-C-14 products soluble in pentane (C-6-C-14 saturated, C-6-C-14 aromatics), C14+ products soluble in pentane (C14+ saturated, C14+ aromatics, resins 1). C14+ products soluble in dichloromethane (asphaltenes and resins 2) and residue insoluble in dichloromethane.Validation of mass and atomic carbon balances plus delta C-13 measurements was checked carefully in order to obtain accurate isotopic mass balances. Mass balances on pyrolysis products were higher than 97.7% of the initial kerogen amount. The delta C-13 fractionation observed between different pyrolysis products reached 18 parts per thousand, but only 2 parts per thousand between different C14+ fractions within the range of pyrolysis conditions. (c) 2005 Elsevier Ltd. All rights reserved.