We present data of major and noble gas composition from mainly newly reported, natural springs - related gas seeps across Grande Terre, the main island of New Caledonia (NC). Two types of springs are investigated: (1) Ophiolitic Springs which emerge from the NC Ophiolite, recognised as the youngest large and well-preserved ophiolite, including seven sites in the Massif du Sud and one site in a chromite mine tunnel located in the Ti & eacute;baghi Massif in the north of Grande Terre; (2) Four Basement Springs which emerge from the pre-Late Cretaceous basement in the eastern central part of Grande Terre. Our results reveal that both seep types are dominated by air-like N-2 (60-95%; delta N-15 = -0.2 to + 0.1 parts per thousand vs Air), and contain CH4 (up to 23%) whose delta C-13 generally range from -40 parts per thousand to -20 parts per thousand VPDB in the Ophiolitic Springs and is lighter in the Basement Springs (delta C-13(CH4) = -70 to -35 parts per thousand VPDB). Ophiolitic Sites also show elevated H-2 content reaching 35%, with delta D values ranging from -740 to -710 parts per thousand VSMOW, whereas Basement Sites display very low levels of H-2 (<300 ppm). The gas composition and isotopic signatures at Ophiolitic Sites are characteristic of serpentinization environments, suggesting that this process is the primary pathway for H-2 generation; consistent with previous studies. This is confirmed by the elevated dissolved OH- which results in elevated pH (up to 11.83), among other factors. Isotopic data for CH4 and CO2 indicate that CH4 in the Ophiolitic Sites likely originates from both biogenic and abiotic processes, while microbial methanogenesis is the major source of CH4 in the Basement Springs and at the Ti & eacute;baghi Tunnel. The H-2-CH4-H2O isotopic fractionation factors suggest equilibration at approximately 50-100 degrees C, indicating a low-temperature serpentinization system within the New Caledonia Ophiolite, which aligns with previous studies. Consistent with the regional geology and tectonics, helium isotopic data (He-3/He-4) reveal a predominantly radiogenic signature (similar to 0.3 Ra) at the Basement Sites where the crust is thicker, and a significant mantle-derived component (similar to 13-37% mantle helium) in the southernmost coastal Prony region. For the central part of the Massif du Sud peridotite nappe where crustal and mantle-derived contributions are absent, we propose that the air-like signatures revealed by He-3/He-4, He-4/Ne-20, Ar-36/N-2 and Kr-84/N-2 ratios reflect atmospheric entrainment and the degassing of air-saturated groundwater during water table fluctuation, which also accounts for the predominance of the air-like N-2 in the gas emissions.

We studied a rock sample belonging to the Mid-Continent Rift in Kansas (USA) that is partially hydrated with the formation of serpentine and amphibole. At the mineral scale, the original mineral assemblage of olivine and orthopyroxene (+ plagioclase + alkali feldspar) is partially replaced by serpentine, magnetite, biotite, amphibole, with locally some apatite and quartz. The replacement tends to preserve the shape of the original olivine and orthopyroxene, supporting a pseudomorphic replacement. Scanning Electron Microscopy (SEM), Electron Probe Micro-Analyzer (EPMA) mineral compositional map, and mass balance equations were used to quantify the transfers of mass during fluid infiltration. Here, the hydration reactions involve the decrease of rock density and, under the assumption of volume preservation, require a significant loss of mass through the fluid phase. Indeed, this study demonstrates that the formation of the newly formed hydrated minerals results from similar mass transfers between parent mineral, fluid, and product minerals. This study also demonstrates that the redox conditions are similar during the formation of all of these hydrated minerals. These observations confirm the close relationship between local stress generated by hydration reactions, mass transfers, and the scale over which the system is open.

Natural hydrogen (H2) emanations in intracratonic areas offer potentially exploitable carbon-free energy. To date, H2 seepages have been detected in more than sixty sites with exploration ongoing in many locations. One mechanism of natural hydrogen generation is the oxidation of Fe2+ in Fe-rich lithologies, and estimating the potential for hydrogen generation by this pathway is an important aspect of characterizing H2-generating rocks. However, accurate estimation of Fe2+ can be challenging due to large-scale heterogeneities and small sample sizes used in conventional analysis. Here, we propose a correlative imaging technique to assess H2 generation potential in Fe2+-rich source rocks by integrating 2D chemical information with 3D volumes of the rock imaged using X-ray computed tomography (micro-CT). The advantage of this method lies in its ability to analyze a whole drill core of the source rock to obtain the most representative values while preserving sample integrity. Our method, validated on fractured monzo-diorite from a natural H2-emitting well in Kansas, USA, yields an estimate of 707.93 +/- 49.18 mol (H2)/ton (source rock), as the upper limit. The proposed method could be useful in characterizing source rocks and estimating their natural H2 generation potential in the early stages of natural H2 exploration.

Most of the ongoing discussions about natural hydrogen (H2) exploration focus on surface hydrogen occurrences and generation processes. However, the trapping process is the most critical issue with regard to both discovering significant accumulations of hydrogen in the subsurface and providing profitable clean energy production for humankind. The extremely high mobility of hydrogen molecules is due to their very small size that confers very diffusive properties, and this results in frequent surface seepages that have recently been recorded worldwide. During this study, we performed a detailed characterization of the caprocks (dolerites) that retain hydrogen in the Bourakebougou natural H2 field. Our investigation on the sealing capacity of these dolerites revealed that their thickness is important in addition to the density and size of their fractures. The role of aquifers has also been highlighted as contributing to the retention of H2 in subsurface. Indeed, hydrogen being poorly soluble in water at low pressure and temperature conditions, it is less likely to diffuse easily to the surface at shallow depth. This study highlights that the exploration of H2 fields should not be based only on the presence of an H2 generation process, but also based on the presence of a very efficient trapping system.

The North American Mid-Continent rift (MCR) is a 1.1 Ga aborted rift, which has recently become an area of intense focus for energy resource exploration following the report of H2 emissions. To document the nature of the producing rocks, we conducted a multi-scale study on preserved drill-core samples from the DR1-A well located in the same area as the H2-producing wells in Kansas. We showed that this well reaches an unmapped part of the MRS composed of fayalite-bearing monzo-diorites in which we identified atypical veins of iddingsite, a complex mixing of Fe-rich phyllosilicates. Combining scanning and transmission electron microscopy (SEM and TEM) with scanning transmission X-ray microscopy (STXM) allowed us to differentiate at least two types of sub-micro-meter Fe-rich veins. A central reduced vein cutting the fayalite, other mafic minerals, the plagioclase, and to a lesser extent the alkali feldspar, contains a complex mix of serpentine, chlorite, and mica. Furthermore, a border vein, with a higher degree of Fe-oxidation, is found to be contained only within the fayalite. This external vein mainly contains iron and silicon, together with a few percent of potassium and calcium, and can be divided into two sub-veins composed of Fe3+-rich interstratified chlorite-smectite and Fe3+-rich serpentine in direct contact with the fayalite. Textures and microstructures of these phyllosilicates suggest that they have crystallized from a late magmatic and differentiated fluid, which precipitation produced the central vein together with the exsolution of an H2O-enriched fluid phase. This exsolved fluid, chemically far from equilibrium with the fayalite, appears to have induced a deuteric alteration of the fayalite, leading to the crystallization of the external veins enriched in ferric iron. These observations bring new perspectives on the history of formation of iron-rich clay minerals, which may, somehow, be related to H2 production.

Mega‐accretionary prisms do not exhibit the characteristics of the classical Coulomb wedge theory. In the example of the Barbados accretionary prism, a system of elongated extensional trenches (“basin‐&‐range” type) developed in the thickest zone of the tectonic wedge and this system has been active since at least Miocene times. Synchronously with this extension, compression occurred at the front of the accretionary prism (frontal accretion zone), and compression was also active at the inner edge of the prism (characterized by back‐thrusting). The interpretation proposed in this study invokes a spreading of the superficial part of the accretionary prism due to a viscous response associated to a tectonic thickening of the deepest parts of the prism. This interpretation is consistent with the fact that extension is limited to the axial part of the prism and coincides with the zone where the prism is the thickest. The deep zone of viscous behavior is interpreted as shale‐rich sedimentary domains intensively deformed by penetrative deformation. Analog sand‐silicone models monitored by X‐ray CT scanner have been used to simulate the distribution of the deformation in the Barbados accretionary prism as observed on available seismic reflection data. A bilayer model with an upper brittle zone and a lower ductile part is best suited to account for observed deformation. Compressive structures at the front and at the rear of the prism correlate with the zones of high concentration of maximum shear strain.

In today's race to find ways to produce cheap and green hydrogen, the natural hydrogen wells in Bourakebougou offer a promising solution and are a good example of how H2 can be produced in the natural environment. Not only has one well been successfully exploited to generate electricity for the local village, but twenty-four other exploratory boreholes have also demonstrated the presence of natural H2 in the surrounding area. The Bourakebougou H2 field offers a unique opportunity for geoscientists to determine the key characteristics of natural hydrogen reservoirs. This paper presents the coring, logging, and geochemistry studies that were performed to better characterize the nature of the Bourakebougou H2-bearing reservoirs. The shallowest main reservoir, in which there is the highest content of H2, is made of dolomitic carbonate (Neoproterozoic cap carbonate). These carbonates are largely karstified and show a high degree of heterogeneity in porosity (0.21-14.32%). Based on the analysis of the drilling imagery of the carbonated reservoirs, the accumulation of hydrogen occurs in the karst (void) representing a secondary porosity in the rock matrix. Other reservoirs, especially the deepest ones, are porous sandstone rocks with much more homogeneous porosities (4.52-6.37%) compared to the massive carbonates. For the wells analysed, the neutron tool reacted in a specific way when there is the presence of hydrogen. Hence, it stands out as being the primary tool to detect the presence of natural hydrogen beyond simple gas logging. When comparing a H2 reservoir system to classical oil and gas reservoir systems, the results show that the hydrogen reservoir is a dynamic system that is progressively recharged in H2-rich gas at the production timescale.

The Djibouti area is located in the Afar depression which corresponds to the core of a major hot spot at the junction of three rifts: The Red Sea oceanic rift, the Gulf of Aden oceanic rift and the East African continental rift. In the extension of the Gulf of Aden, the opening of the Gulf of Tadjourah in the last 30 million years was followed by the establishment of parallel NW-SE oriented depressions (Gobaad, Hanle, Gaggade, Assal-Ghoubbet). This region is characterised by high heat flow, complex volcanic and tectonic activity, and a dense network of faults favorable to the development of hydrothermal activities. A field survey was conducted to study the gas composition and origin in thermal springs and fumaroles along a transect running from the Lake Abbhé to Djibouti. Hydrogen traces were detected at fumaroles and hot springs associated with hydrothermal systems at the edges of the Gobaad, Hanlé, Gaggade, Assal-Ghoubbet and Arta area. The processes governing to the origin of H2 and its consumption at different levels of the magmatic-hydrothermal systems are discussed. H2 is interpreted as related to several processes including, chemical equilibrium in magmatic gases, alteration of FeII-rich rocks, and oxidation of volcanic H2S. Methane associated with the hydrogen has mostly an abiotic origin and could result from the reaction of hydrogen with carbon oxides. Also, helium emanations have been observed notably on the sides of the Lake Abbhé in boiling hot springs. Gas samples have shown R/RA isotopic ratios of helium up to 10, which is are characteristic values for helium of mantle origin characterized an 3He enrichment with respect to air values.

Offshore the emissions of dihydrogen are highlighted by the smokers along the oceanic ridges. Onshore in situ measurements in ophiolitic contexts and in old cratons have also proven the existence of numerous H2 emissive areas. When H2 emanations affect the soils, small depressions and vegetation gaps are observed. These depressions, called fairy circles, have similarities with the pockmark and vent structures recognized for long time in the sea floor when natural gas escapes but also differences. In this paper we present a statistic approach of the density, size, and shape of the fairy circles in various basins. New data from Brazil and Australia are compared to the existing database already gathered in Russia, USA, and again Brazil. The comparison suggests that Australia could be one of the most promising areas for H2 exploration, de facto a couple of wells already found H2, whereas they were drilled to look for hydrocarbons. The sum of areas from where H2 is seeping overpasses 45 km2 in Kangaroo Island as in the Yorke Peninsula. The size of the emitting structures, expressed in average diameter, varies from few meters to kilometers and the footprint expressed in % of the ground within the structures varies from 1 to 17%. However, globally the sets of fairy circles in the various basins are rather similar and one may consider that their characteristics are homogeneous and may help to characterize these H2 emitting zones. Two kinds of size repartitions are observed, one with two maxima (25 m and between 220 m ± 25%) one with a simple Gaussian shape with a single maximum around 175 m ± 20%. Various geomorphological characteristics allow us to differentiate depressions of the ground due to gas emissions from karstic dolines. The more relevant ones are their slope and the ratio diameter vs. depth. At the opposite of the pockmark structures observed on the seafloor for which exclusion zones have been described, the H2 emitting structures may intersect and they often growth by coalescence. These H2 emitting structures are always observed, up to now, above Archean or Neoproterozoic cratons; it suggests that anoxia at the time the sedimentation and iron content play a key role in the H2 sourcing.

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 deep-water 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 CO2/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.

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 degrees 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.

This study focuses on the respective role of (1) the occurrence and the nature of evaporitic layers and (2) the fluid pressure conditions on the decollement processes and the structure of the foreland fold and thrust belt of the Western Alps. The decollement in the Jura Mountains is predominantly localized in halite‐bearing layers. Anhydrite‐bearing units which are not associated to halite are not hosting major decollement. The shift of decollement level between Mid‐Triassic halite unit and Upper Triassic halite unit has induced local tectonic subtractions and, elsewhere, tectonic duplications at depth. Available fluid pressure measurements show that fluids are not overpressured in the Jura. Even below the salt decollement, they remain in hydrostatic conditions. South of the Jura, the absence of halite is correlated with no efficient role of the Triassic layers in terms of decollement. The available pressure measurements show that the decollement is associated with high overpressure. Because of the low friction of halite, the Jura thrust wedge shows a narrow angle (3°–4°). The relatively high friction behavior of the decollement south of the Jura is responsible for stacks of tectonic units associated with a relatively wide angle of the tectonic wedge (12°–13°). The structural change between the Chartreuse and Vercors massifs is not controlled by the properties of the decollement but by the change of thickness of the sedimentary pile involved in the tectonic wedge. The change of thickness is controlled by paleogeographic heritage during Jurassic and Cretaceous times.

Evidences for active fluid seepages have been discovered along the Zambezi continental slope (offshore Southern Mozambique). These seepages are mostly associated with pockmarks which are aligned along a trend parallel to the slope and running closely upstream of the headwall scarp of a wide zone of slope destabilization. Fluid seepages are interpreted as a potential trigger for the slope destabilization. Acoustic anomalies within the water column have been interpreted as related to moderate bubble seepages mostly located outside and only punctually inside the destabilization zone. Exploration with the SCAMPI towed camera system in the widest pockmark (diameter 200 m wide) has shown fluid seepages associated to authigenic carbonate crusts and possibly bacterial mats. These fluid seepages are also associated to the presence of chemiosynthetic organisms (Vesicomyidae and Thyasiridae bivalves, Siboglinidae tubeworms). The sampled gas in the sediment corresponds mainly to CH4 of microbial origin, generated by hydrogenotrophic methanogenesis from a substrate of organic origin, i.e. a conventional process of genesis of microbial gas in the marine domain. No evidence for thermogenic gas was detected. Another type of pockmarks has been observed within the core of the slope destabilization zone. Most of these pockmarks are inactive in terms of fluid seepage at present time and are associated to carbonate buildups forming chimney geometries. They probably correspond to diagenetic chimneys of former fluid migration pathways that have been exhumed during the mass sliding and the surrounding depression are related to recurrent activity of strong lateral slope currents which have scoured the sediments around. The spatial organization of the slope destabilization features is considered as representative of the temporal evolution of the landslide giving information about the dynamics of slope instability processes. This proposed evolution started by scattered seepages of formation water with dissolved gas. Then free gas seepages appeared notably in the upper part of the slope. This was followed by progressive shallow deformation in the sediments downslope of the main gas seepages. Finally, the whole slope was destabilized forming imbricated landslides exhuming locally former diagenetic chimneys.