The origin of N2-H2-CH4-rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia

International audienceThe study of natural gas seepages in New Caledonia has shown the occurrence of two gas families, one is N2-H2-CH4-rich, the other is N2-rich. The N2-H2-CH4-rich gases are bubbling in hyperalkaline springs (OH–-rich) knownin the peridotite massif of the southern part of the island. This family of gas shows contents of N2 between 50 and62%, H2 between 26 and 36%, and CH4 between 11 and 16%. δ13C values of methane are ranging from −39 to−32‰. We interpret the origin of H2 as a product of fluid-rock interaction between basic-ultrabasic rocks andwater with oxidation of Fe2+ and reduction of underground water in the fracture system of the peridotitesnappe.Methane is interpreted as the result of a reduction of dissolved inorganic and/or organic carbon in subsurfaceaquifers. The second family of gas emissions was found in thermal springs in the sedimentary units locatedstructurally below the peridotites nappe. The gas is composed mostly of N2 (between 97 and 98%) associatedwith relatively high concentration of He. Both gas families show notably high N2 vs fossil noble gas contents(36Ar, 20Ne, 84Kr) suggesting that N2 is not directly issued from gas dissolved in aquifers equilibrated with atmospherebut most likely finds its origin in a deep source, probably within metamorphic sediments which are tectonicallyburied belowthe ophiolitic nappe. Weinterpret the N2-H2-CH4-rich family of gas as a result of a mixingbetween two end-members, (1) a H2-CH4-rich pole issued fromthe weathering of peridotite rocks and (2) a N2-rich pole which would be issued from the metamorphosed sediments buried below the ophiolitic units. An unusualinverse correlation between the atmospheric noble gas isotopes 20Ne and 36Ar is interpreted as the resultof a degassing of a relatively shallow confined aquifer related to a bubble flow from depth

What Pulsating H2 Emissions Suggest about the H2 Resource in the Sao Francisco Basin of Brazil

Proterozoic sedimentary basins very often emit natural hydrogen gas that may be a valuable part of a non-carbon energy infrastructure. Vents in the Sao Francisco Basin in Brazil release hydrogen to the atmosphere mainly during the daylight half of the day. Daily temperature and the regular daily tidal atmospheric pressure variations have been suggested as possible causes of the pulsing of H2 venting. Here, we analyze a ~550 m-diameter depression that is barren of vegetation and venting hydrogen mainly at its periphery. We show that daily temperature changes propagated only ~1/2 m into the subsurface and are thus too shallow to explain the H2 variations measured at 1-m depth. Pressure changes could propagate deeply enough, and at the depth at which the cyclic variations are measured hydrogen concentration will have the observed phase relationship to atmospheric pressure changes provided: (1) the pressure wave is terminated by geologic barriers at about 25% of its full potential penetration distance, and (2) the volume of gas in the vents is very small compared to the volume of gas tapped by the venting. These constraints suggest that there is a shallow gas reservoir above the water table under the ~550 m-diameter barren-of-vegetation depression. The 1D-analytical and finite-element calculations presented in this paper help define the hydrogen system and suggest the further steps needed to characterize its volume, hydrogen flux and resource potential

Genèse et mobilité de l'hydrogène naturel (source d'énergie ou vecteur énegétique stockable ?)

PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Identification and Quantification of Carbonate Species Using Rock-Eval Pyrolysis

This paper presents a new reliable and rapid method to characterise and quantify carbonates in solid samples based on monitoring the CO2 flux emitted by progressive thermal decomposition of carbonates during a programmed heating. The different peaks of destabilisation allow determining the different types of carbonates present in the analysed sample. The quantification of each peak gives the respective proportions of these different types of carbonates in the sample. In addition to the chosen procedure presented in this paper, using a standard Rock-Eval 6 pyrolyser, calibration characteristic profiles are also presented for the most common carbonates in nature. This method should allow different types of application for different disciplines, either academic or industrial

Tracking CO2 leakage with noble gases

International audienc

Contribution of siderite–water interaction for the unconventional generation of hydrocarbon gases in the Solimões basin, north-west Brazil

International audienceHydrocarbon gases with unconventional carbon isotopic signatures were observed in the Solimiies sedimentary basin in north-west Brazil. Siderite contents measured with a new Rock-Eval methodology in the drill-cuttings samples of the Famenian source rock were found to decrease with the increase of gas maturity and with the occurrence of the gas isotopic anomalies. Triassic diabase intrusions induced heating of the source rock, which likely resulted in the gradual oxidative dissolution of siderite as suggested by the observation of etch pits on the siderite surfaces. It is proposed that ferrous iron from the carbonate was involved in a redox reaction with water producing ferric iron and Hy, then reducing CO2 and yielding an inverse correlation between siderite content and gas maturity. Alternatively, hydrogenation of highly mature kerogen by H-2 derived from siderite could explain the production of C-13-rich CH4. Mass balance considerations suggest that these mechanisms may account for a significant fraction of the hydrocarbon gases generated from the Famenian source rock in the Solimbes basi

Chemical and isotopic analysis of hydrocarbon gas at trace levels: Methodology and results

International audienceIsotopic mass spectrometry coupled online with gas chromatography (GC-C-IRMS) permits measurement of relative proportions of gaseous hydrocarbon (CH4 to C4H10) and CO2, and determination of carbon isotope ratio of hydrocarbon molecules. Access to these parameters provides valuable information about the source and the genesis of naturally-occurring gas, as well as on post-formation physico-chemical processes which might have taken place in the geological environment. In particular, it is possible to distinguish hydrocarbon gas of bacterial origin from that of thermogenic origin based on proportion and carbon isotope ratio of methane as measured by GC-C-IRMS. However, in samples containing very low amounts of hydrocarbons (from 1 ppm to 1000 ppm), accurate measurement of isotope ratios is often impossible due to the limitations of conventional GC-C-IRMS techniques using direct sample introduction. A technique was developed to overcome this limitation. It is based on a novel approach allowing pre-concentration of hydrocarbons prior to GC-C-IRMS analysis. The pre-concentration step consists in selective trapping of hydrocarbon molecules on a cold adsorbent phase, and removal of non-adsorbed gases (N2, O2, Ar,...). In a second step, pre-concentrated alkanes are desorbed, and released in an inert carrier gas, focused through a capillary and introduced into the GC-C-IRMS for chromatographic separation and measurement of concentration and carbon isotope composition of each individual carbon molecule. In order to achieve sufficient accuracy, several operating conditions are of prime importance, including sufficient signal intensity, well defined peak shape and low signal/noise ratio. Accurate measurements can be performed on samples as small as 10 cm3 of bulk gas in standard conditions, with concentrations as low as 1 ppm of methane, 0.5 ppm of ethane and 0.3 ppm of propane and butane. Total analytical uncertainty on δ13C measurements ranges from ± 0.2‰ to ± 1.5‰, depending on the hydrocarbon molecule

Natural hydrogen continuous emission from sedimentary basins: The example of a Brazilian H2-emitting structure

International audienc

Reduced gas seepages in ophiolitic complexes: Evidences for multiple origins of the H 2 -CH 4 -N 2 gas mixtures

International audienceThis paper proposes a comparative study of reduced gas seepages occurring in ultrabasic to basic rocks outcropping in ophiolitic complexes based on the study of seepages from Oman, the Philippines, Turkey and New Caledonia. This study is based on analyses of the gas chemical composition, noble gases contents, stable isotopes of carbon, hydrogen and nitrogen. These seepages are mostly made of mixtures of three main components which are H2, CH4 and N2 in various proportions. The relative contents of the three main gas components show 4 distinct types of gas mixtures (H2-rich, N2-rich, N2-H2-CH4 and H2-CH4). These types are interpreted as reflecting different zones of gas generation within or below the ophiolitic complexes. In the H2-rich type, associated noble gases display signatures close to the value of air. In addition to the atmospheric component, mantle and crustal contributions are present in the N2-rich, N2-H2-CH4 and H2-CH4 types. H2-bearing gases are either associated with ultra-basic (pH 10–12) spring waters or they seep directly in fracture systems from the ophiolitic rocks. In ophiolitic contexts, ultrabasic rocks provide an adequate environment with available Fe2+ and alkaline conditions that favor H2 production. CH4 is produced either directly by reaction of dissolved CO2 with basic-ultrabasic rocks during the serpentinization process or in a second step by H2-CO2 interaction. H2 is present in the gas when no more carbon is available in the system to generate CH4. The N2-rich type is notably associated with relatively high contents of crustal 4He and in this gas type N2 is interpreted as issued mainly from sediments located below the ophiolitic units