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

Gas composition, concentration and stable isotope ratios (Table 2, 3, 4)

The deployment of CCS (carbon capture and storage) at industrial scale implies the development of effective monitoring tools. Noble gases are tracers usually proposed to track CO2. This methodology, combined with the geochemistry of carbon isotopes, has been tested on available analogues. At first, gases from natural analogues were sampled in the Colorado Plateau and in the French carbogaseous provinces, in both well-confined and leaking-sites. Second, we performed a 2-years tracing experience on an underground natural gas storage, sampling gas each month during injection and withdrawal periods. In natural analogues, the geochemical fingerprints are dependent on the containment criterion and on the geological context, giving tools to detect a leakage of deep-CO2 toward surface. This study also provides information on the origin of CO2, as well as residence time of fluids within the crust and clues on the physico-chemical processes occurring during the geological story. The study on the industrial analogue demonstrates the feasibility of using noble gases as tracers of CO2. Withdrawn gases follow geochemical trends coherent with mixing processes between injected gas end-members. Physico-chemical processes revealed by the tracing occur at transient state. These two complementary studies proved the interest of geochemical monitoring to survey the CO2 behaviour, and gave information on its use

Biogenic methane leakage on the Aquitaine Shelf:fluid system characterization from source to emission

International audienceThe recent discovery of biogenic methane emissions associated with methane-derived authigenic carbonate mounds along the Aquitaine Shelf edge offshore SW France (140 to 220 m water depth) questions about the initiation and temporal evolution of this fluid system (80 km N-S and 8 km E-W). Based on a multi-data study (including multibeam echosounder, subbottom profiler, single channel sparker seismic, 80 traces air gun seismic data and well cuttings and logs), different scenarii are proposed for the organic matter source levels and migration pathways of the methane. Several evidence of the presence of gas are observed on seismic data and interpreted to be linked to the biogenic system. Single channel sparker seismic lines exhibit an acoustic blanking (between 75-100 ms TWT below seafloor and the first multiple) below the present-day seepage area and westwards up to 8 km beyond the shelf-break. An air gun seismic line exhibits chaotic reflections along 8 km below the seepage area from the seabed down to 700 ms TWT below seafloor. Based on 1) the local geothermal gradient about 26 °C/km and 2) the window for microbial methanogenesis ranging from 4 to 56 °C, the estimation of the bottom limit for biogenic generation window is about 1.5 km below seafloor. Cuttings from 3 wells of the area within the methanogenesis window show average TOC (Total Organic Carbon) of 0.5 %; however, one well shows some coal levels with 30-35 % TOC in the Oligocene between 1490 and 1540 m below seafloor. Geochemical analysis on crushed cuttings evidenced heavy hydrocarbons up to mid-Paleogene, while shallower series did not evidence any. In the first scenario, we propose that methane is sourced from the Neogene prograding system. The 0.5% average TOC is sufficient to generate a large volume of methane over the thickness of this interval (up to 1 km at the shelf break area). In the second scenario, methane would be sourced from the Oligocene coals; however their spatial extension with regard to available data is too limited to supply the gas system along 80 km from north to south. The third scenario corresponds to methane production in the early Paleogene and Cretaceous source levels; but evidence for heavy hydrocarbons is not consistent with the isotopic signatures of the gases seeping at the seabed. The first scenario is therefore the most coherent one even if the TOC is relatively low in the Neogene formations. Regarding the fluid system geometry and the associated source level position, migration pathways may involve 1) upslope migration from the base of the Neogene clinoforms, 2) sub-vertical migration through faults and fractures at the shelf edge, and 3) groundwater circulation from onshore forcing methane migration westward through hydrodynamism. The PhD thesis of Guillaume Michel as well as the oceanographic expeditions Gazcogne1 (http://dx.doi.org/10.17600/13020070 and Gazcogne2 (http://dx.doi.org/10.17600/13030090 are co-funded by TOTAL and IFREMER as part of the PAMELA (Passive Margin Exploration Laboratories) scientific project

U/Pb dating of geodic calcite: new insights on Western Europe major tectonic events and associated diagenetic fluids

International audienceThis study presents the first application of the U/Pb dating method to highly Pb-depleted diagenetic geodic calcites of the Jurassic formations of the Paris Basin that leads to a reappraisal of the palaeohydrological history of this region. Composite U/Pb ages from multiple geodes, combined with δ18O analyses, reveal two main phases of diagenetic fluid circulations linked with major regional tectonic events. Dogger formations recorded a first diagenetic fluid episode at 147.8 ± 3.8 Ma, i.e. at the very beginning of the emersion of the basin during the Tithonian period and 30 Ma earlier than previously assumed. δ18O results confirmed that most of the calcitic cement phases that closed the porosity of these formations precipitated at the beginning of the Cretaceous period. Oxfordian formations recorded another major meteoric fluid circulation at 33.5 ± 2.8 Ma related to the initial stage of the European Cenozoic Rift System (ECRIS). Consequently, the porosity of the Mesozoic formations of the studied area may have been closed sooner than previously thought, before the main ECRIS rifting phase of Oligocene age. This study shows that U/Pb dating of secondary geodic calcite offers a new powerful way for reconstructing the coupled palaeohydrological and diagenetic history of sedimentary basins

Depicting the fluid system evolution in a major thrust and associated fracture network, from layer parallel shortening to today: the Sierra de Orba anticline, Jaca basin, Spain.

International audienceThe evolution of deformation structures and associated past fluid flow is a key to better appraise both the reservoir properties of a rock and the regional evolution of an area. Indeed, the meso-scale fracture network often develops over a long period of time, granting access to a long term evolution of a past fluid system (i.e. the temperature, origin and migration pathways of the fluids) on the one hand. On the other hand, major thrusts are believed to enable episodic fluid migrations on larger spatial scales, in relation to their activity calendar, whether this activity is over or ongoing. It is seldom however to reconstruct both the past fluid flow and the present-day fluid flow on the same structure, yet it can be very enlightening about the evolution of the thrust connectivity at depth since before a fold developed over it.In this study we reconstructed the deformation pattern and associated past fluid system related to the development of the Sierra de Orba Anticline (Spain), and we compare it to the current-day fluids resurging in the vicinity of the fault. The Sierra de Orba anticline is part of a fault-propagation fold system that developed in the northern part of the Jaca Basin, Southern Pyrenean foreland, Spain. This fold affects the sedimentary succession including the Triassic decollement level, the Upper Cretaceous to Paleocene carbonate strata and the Middle Eocene marls. The N110 striking major thrust, where the Upper Cretaceous Marboré Fm. lies unconformably onto the Eocene marls, was sampled, along with the fracture network in the hangingwall and the footwall of the thrust. The fracture network includes a sequence of prefolding sets of joints and veins, striking E-W and N060 and flexural-slip related reverse faults. Then, the network encompasses the orogeny history since likely the forebulge development until the strata tilting during folding. Syn-kinematic calcite cements were characterized petrographically in both joints and faults, then studied by means of oxygen and carbon isotopic measurements coupled with fluid inclusion microthermometry. Results highlight that the fluid system recorded an alternation between meteoric fluids (δ18O signature of the fluid: -5‰ SMOW) heated up at 70-80°C, and evolved seawater (δ18O signature of the fluid: +5 to +10‰SMOW) heated up at 70-100°C. That past fluid system around the main thrust did not record any deep-sourced fluids, unlike similar structures in the southern Pyrenean foreland. The major gas content of current day fluids was analysed from a resurgence in the footwall of the thrust. This gas is especially nitrogen-rich (>86%), and is characterized by an heavy δ13C signature of the methane content (4‰PDB). Both these features could relate to deep processes, such as an oxidation of a potential abiotic carbon that can be related to a serpentinization process. This hypothesis needs to be further evaluated by means of a study of noble gas content. Beyond regional implications, this case study illustrate how a combination of geochemical proxies can help to unravel the evolution of a fluid system at a fold-thrust scale, from its onset to today

General Statement

Subsurface sediment mobilization in SE Caribbean occurs in a context of plate boundary between the Caribbean plate and the South American plate, at the junction between the Barbados accretionary prism and the transform system of the northern Venezuela. Within this compressional and transpressional system, a several hundred kilometres-long active belt of mud volcanoes and shale diapirs develops from the Barbados tectonic wedge to the thrust belt of Northern Venezuela (Figure 1). In this system, the mud volcanoes of Trinidad and Venezuela are only the emerged part of a widely developed phenomenon in the offshore area of the Barbados prism (especially in its southern part). This has been notably spectacularly evidenced by the recent results of the CARAMBA survey of the O/V ATALANTE in 2002 (Figures 2 and 3). Structural Setting Mud domes and volcanoes developed in different structural settings (Biju-Duval et al.

Origine des eaux salées, des émanations gazeuses azotées (+Hélium) et des minéralisations de fluorine à l'extrémité nord de la Grande Faille du Bazois : Apport du couplage entre pétrologie, hydrogéologie, géochimie et géophysique

International audienc

Origin and preservation conditions of organic matter in the Mozambique Channel: Evidence for widespread oxidation processes in the deep-water domains

International audienceThe 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