HAL-BRGM, les publications scientifiques en libre accès du BRGM
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Hypothesis on the karstification/dolomitization of the Barremian reservoir in Parentis Basin
No full textInternational audienceThe Parentis sub-Basin, located southwest of Bordeaux (France), has been known since the 1950s as a prolific oil reservoir. It lies at the eastern end of the Bay of Biscay. Several fields are exploited in the onshore part of the basin, the largest being at Parentis-en-Born, discovered by ESSOREP in 1954 and exploited by VERMILION REP since 2006. The field is an east-west anticlinal structure, formed during various geodynamic episodes in connection with halokinesis of the Triassic and Hettangian series. One of the reservoirs exploited is a limestone/dolomite dating from the Barremian to the Upper Jurassic characterized by a matrix porosity of 14% and a permeability of 100 mD over an average thickness of 200 metres.Numerous studies and acquisitions (3D seismic, 122 boreholes) have been carried out by ESSOREP and VERMILION REP exploration geologists to characterise the limestone/dolomite reservoir, particularly regarding the origin of the dolomitization and porosity. Models of emplacement suggest post-sedimentary dolomitization by reflux and epigenic karstification linked to early tectonic bulges.Our study aims to use modern methods and concepts to gain a better understanding of the major stages in the development of this reservoir in relation to the geodynamic events. It is based on a detailed study of cores and thin sections. A total of 160 samples were taken from 7 boreholes to conduct a diagenetic study (petrography, cathodoluminescence and initial results on strontium isotopes).The macroscopic study of the cores showed the presence of numerous mud losses (no-recovery zones), often correlated with faults (striations observed), which were previously interpreted as epigenic karstification. Numerous veins were observed in the fracture zones, composed of calcite, saddle dolomite and anhydrite. Microscopically, the paragenesis shows primary dolomitization prior to burial (euhedral to subhedral dolomite), followed by phases of circulation of magnesium-rich hydrothermal fluids (saddle dolomite -'nonplanar') and sulphates (anhydrite). Initial interpretations suggest that the void zones encountered in the boreholes are linked to the upwelling of deep-seated fluids sourced by Triassic/Lower Jurassic evaporites and dolomites, resulting in hypogenic karstification. These episodes would be linked to the resumption of the anticline during the Pyrenean phase.</p
Inferring the evolution of the seismicity rate using ETAS model
No full textInternational audienceOne of the challenges of deep geothermal energy is the occurrence of induced seismicity and understanding its occurrence. Prediction methods are not very reliable up to now and we aim to investigate how machine-learning techniques could be used for this task. The objective is to predict declustered seismicity rate as it may be more correlated to injection parameters. To de termine this declustered seismicity rate, we focus on the ETAS (Epidemic-Type Aftershock Se quence, e.g. Ogata, 1988) model to extract the declustered seismicity rate variation in time. Spatial variations are neglected so as not to have too many parameters to invert. However, the problem of ETAS is that the background seismicity rate is time independent. This is not true for the background seismicity rate related to fluid injection that is generally supposed to be de pendent on this injection scheme (e.g. Feng et al, 2022). One solution is to realize inversion over moving step windows (Aochi et al, 2021). However, it does not take advantage of all seis micity catalogue and yield parameters that are different for every step and not just for back ground seismicity rate. Kumazawa and Ogata (2014) proposed a solution for time dependent ETAS using spline functions. We focus on a similar idea, introducing a background seismicity rate as a sum of hyperbolic tangents. This has the advantage of yielding a continuous func tion and takes advantage of the hypothesis of variation of the seismicity rate by step, following injection variation. Open-access data from the 2000 hydraulic stimulation tests at Soultz-sous Forêts research site (Alsace, France) are used to analyze the evolution of seismicity. The appli cation to this case study shows that timing of variation in seismicity rate can be inferred precise ly, while amplitude of variation is less constrained. Probability of background seismicity rate vary from about 35% at the beginning of the stimulation to around 25% afterward. No dependance on magnitude of events is found for the triggering term of ETAS model for this example. This may be due to the small range of magnitude considered in such induced seismicity case stud
Characterization and simulation of the thermodynamic and physicochemical properties of natural and industrial brines
No full textInternational audienceBehavior of solids as a function of: -Temperature (TR-DRX and TGA-DSC) -Relative humidity: DVS and RH-DRX -In solution: InSituX® --> Access to solid-solid transitions and characterization of hydrated forms Discontinuous Isoperibolic Thermal Analysis (DITA): Determination of the boundaries of condensed phase domains including at least one liquid phase.</div
Tungsten Solubility And Speciation In Hydrothermal Solutions Revealed By In Situ X-Ray Absorption Spectroscopy
No full textInternational audienceTungsten (W) concentrations in fluids in equilibrium with crystalline tungsten oxide are used to determine thermodynamic parameters for W solubility and W species in hydrothermal fluids. The solubility data were measured in situ at high pressures and temperatures using X-ray absorption. X-ray spectroscopic data measured in situ – with X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) – were applied to characterize the symmetry and the type of atoms of the first coordination shell of W aqueous complexes present in the fluid at given temperatures and pressures. Experiments were performed at up to 400 °C and at pressures of 40, 50 and 60 MPa. With this dataset, we were able to improve constraints for the already-suggested fluid species WO42-, HWO4-, H2WO40, NaWO4- and NaHWO40. Further, we were able to introduce the H3WO4+ species that is found to be dominant in acidic fluids. No evidence was found for W species involving Cl‑ as a ligand. The ionic W species found in the fluid are characterized by a tetrahedral complex at alkaline conditions. In neutral to acidic conditions, W complexes with distorted octahedral symmetry are formed. These complexes may be polymerized at temperatures ≤200 °C and W concentrations >10-3 mol kg‑1 H2O. X-ray spectroscopy as well as thermodynamic modeling suggests that polytungstate species are not relevant at equilibrium concentrations found in the solubility experiments of this study (≤10-3 mol W kg‑1 H2O in equilibrium with tungsten oxide) or at concentrations reported for natural systems. Using the thermodynamic properties of the species mentioned above, in situ data on the solubility of scheelite can be successfully described. Thermodynamic modeling shows that scheelite solubility and wolframite solubility strongly increase with increasing salinity, especially up to 1 m NaCl (m denotes molality), and vary with pH, which is consistent with earlier reports. Overall, this study provides improved thermodynamic properties for a set of W fluid species that cover a wide range of fluid compositions, which is necessary for understanding the complex processes of W enrichment and mineralization in hydrothermal systems
The 1956 eruption of Bezymianny volcano (Kamchatka)—part I: petrological constraints on magma storage and eruptive dynamics
No full textInternational audienceBezymianny volcano (Kamchatka, Russia) is an andesitic island arc stratovolcano that started to erupt in 1955 after ~ 1000 years of dormancy. On March 30, 1956, the climactic phase of the eruption was preceded by a 4-month-long emplacement of a shallow cryptodome, which triggered a flank collapse violently decompressing the magma into a laterally directed blast followed by an explosive phase emplacing extensive pumice concentrated pyroclastic density currents (pumice C-PDC). Aiming at constraining the plumbing system below Bezymianny volcano prior to the 1956 eruption, we performed a multiphase textural and petrological study using dense to vesiculated clasts of the blast and pumice samples from the post-blast C-PDC deposits. We inferred the pressure and temperature conditions of magma storage using sample vesicularity, amphibole destabilization rims, volatile contents in melt inclusions, microlite textures, and phase compositions (phenocrysts, microlites, and glasses). We propose a three-level magma storage characterized by a deep reservoir (≥ 200–350 MPa, ≥ 840 °C, ~ 4.0–8.0 wt% H2O and CO2 up to 1500 ppm, where amphibole is stable), a shallow reservoir (50–100 MPa, 850–900 °C, 1.5–4.0 wt% H2O and CO2 < 250 ppm, where amphibole is unstable and quartz crystallizes) in which the pre-cryptodome magma resided and from which the post-blast pumiceous magma originated, and a subsurface cryptodome (< 25 MPa, ~ 900 °C, cristobalite crystallized) from which the blast was initiated. This plumbing system provides the framework for constraining the timescales of the 1956 eruptive dynamics (companion paper). The three-stage architecture proposed for the 1956 andesitic reservoir compares to the present-day plumbing system emitting mafic lavas, thus suggesting that the timescales of the eruptive dynamics (e.g., magma residence time and ascent rate) may be the key to determining evolved or mafic magmas
Mapping Alluvial Terraces in Watersheds Using Gaussian Mixture Model on Relative Height
No full textInternational audienceAlluvial terraces in watersheds are geomorphic features formed by river incision and sediment deposit, representing former floodplain levels. They serve as valuable records of fluvial dynamics, climatic changes, and tectonic activity. Mapping methods for terraces that rely on field-acquired data, often involving physical or chemical analyses, are not feasible for large-scale applications. When aiming to map at the national scale, the development of a methodology that eliminates the need for such detailed information enhances scalability and broadens applicability. We proposed a semi-automatic predictive mapping method for watershed terraces using 25m Digital Earth Model (DEM) provided by the IGN (French geographical service) and derived variables such as curvature, slope, and the difference from a base level (Raingeard et al., 2019). This method demonstrated meaningful results in the Pyrenean Piedmont for the Baïse and Ousse rivers, with the predicted map showing strong alignment with the geological reality. In this study, we propose an automated approach for identifying alluvial terraces using relative height. Relative height is defined as the difference between the elevation derived from a DEM and the base level. Our methodology is based on the hypothesis that terraces are represented as flat areas in the relative height, where pixels exhibit similar statistical distributions. To capture these patterns, we employ a Gaussian Mixture Model, a probabilistic framework that approximates data as a combination of multiple Gaussian distributions. In this context, each Gaussian distribution corresponds to a specific alluvial terrace. We conducted experiments on the study areas used by Raingeard et al. (2019), and the results are consistent with both the semi-automatic method and the geological reality. These outcomes provide promising prospects for the predictive mapping of superficial deposit
Oxygen and hydrobiological profiles of homemade manure-based tea in North Africa
No full textInternational audienceHomemade manure tea (HMT) is commonly used in North Africa to enhance crop yields. Yet their physicochemical and biological characteristics remain poorly understood. This study evaluated oxygen and hydrobiological profiles of three types of HMT (bovine, ovine and poultry based, respectively noted HMTb, HMTo, HMTp) and compared them to control solutions of water and water supplemented with soluble NPK fertilizer. For these three types of HMT, oxygen and hydrobiological profiles were measured daily over a 7-day incubation period in three repeated, identical experiments, each comprising randomized treatments and five repetitions per treatment. Our results show that all HMT types rapidly transitioned to hypoxic conditions in the first 24h, shifting to anoxia between day 2 and day 7 depending on HMT type. This anoxic environment promoted denitrification and led to elevated NH 4 + concentrations, suggesting the presence of anammox and microaerobic processes. Particulate organic matter contents and bacterial densities were highest in HMTp, while ciliate densities were highest in HMTb. These findings underscore the bioactive potential of HMT as fertilizers, with HMTp showing a favorable nitrogen profile beneficial for agricultural applications. To maintain aerobic conditions longer and reduce nitrogen losses and greenhouse gas emissions, we recommend passive or mechanical aeration, applying HMT during cooler hours, and stabilizing the pH of HMT. This study offers valuable insights to refine HMT preparation protocols, enhancing their use as bioactive fertilizers
3D Characterization of Aquifers in the Cenozoic Sedimentary Formations of the Paris Basin using Petroleum Data
No full textInternational audienceIntroduction: The objective of this study is to provide a concise synthesis of the geological and hydrogeological characteristics of the main aquifers located in the Paris Basin within the Cenozoic sedimentary formations. This work is part of a multi-annual scientific program called RGF (Référentiel Géologique de la France, https://rgf.brgm.fr/page/bassin-parisien ), which aims to update the representation and cartography of French geology at the national scale. The program also funds several PhD projects in collaboration with the academic community, focusing on the geometry, facies, distribution of petrophysical properties, modeling, and mapping of the main geological formations of the Cenozoic era in the basin. Among the intended applications of this modeling work, particular emphasis is placed on the main aquifers within the basin
Flood hazard in Malawi
No full textInternational audienceThis article presents the methodology and key findings of the first national study of flood hazard conducted across Malawi as part of the 2016-2021 GEMMAP programme. After outlining Malawi's geomorphological, climatic, and hydrographic features, we detail the approach used for flood hazard assessment and mapping. Ten flood types were identified, reflecting diverse morphological, geological, and climatic settings, resulting in flood hazard maps, which cover the entire country and highlight areas at risk based on flood type and magnitude. The total area exposed to flooding in Malawi is estimated at 16% of the country's surface area (around 15,000 km2). Flooding of plains along major rivers, such as the Shire River, which affects hundreds of thousands of people, accounts for approximately 37% (5,100 km2) of Malawi’s flood-prone areas. Flooding in dambo areas comprise 34% (4,700 km2), and lake shores another 10% (1,400 km2). Although debris flow flooding is restricted to around 4.2%, it nevertheless frequently causes significant casualties and damage. The remaining surface area corresponds to the narrow flood zones on either side of the secondary rivers. Specific flood contexts are analysed in detail, including the Shire River-Lake Malawi flood connection, debris flows in mountainous regions, and river mobility in lowland areas. The methodology has been designed to be simple and pragmatic, enabling easy adaptation and application in countries with similar geographical contexts and limited flood hazard knowledg
En-échelon Rifting and Origin of the Volcanism in the Comoros
No full textInternational audienceTwo volcanic provinces have been recently discovered during the SISMAORE oceanographic cruise in the Comoros archipelago in the North Mozambique Channel between Madagascar and East Africa: N'Droundé, along the North-eastern insular slopes of Grande Comores Island and Mwezi, in the abyssal plain, north-east of Mayotte and Anjouan islands. By combining bathymetry and backscatter data, high-resolution seismic reflection and sub-bottom profiles, we have identified and mapped various tectonic (faults, forced folds) and volcanic structures (lava flows, edifices, sills, dykes) at several spatial scales on the seabed and in crosssection within the sedimentary cover. We have characterized the volcano-tectonic structures (geometry, segmentation, and kinematics) to better understand the link (geometry, chronology) between tectonic and volcanic processes. We show that volcanic and tectonic features are controlled by tectonic processes and viceversa. Ridges, volcanic cones and lava flows are set up along fissures and dikes during main rifting events to accommodate a N40°E regional extension within an E-W right lateral shear transfer zone. The volcano tectonic features are Plio-Pleistocene. This transfer zone lies between the offshore branch of the East African rift system and Malagasy grabens and may have formed when the East African rifts propagated offshore. We evidence a major rifting episode in the last Ma. The estimated volume and flux of extruded lavas show that the volcanism of the Comoros could be related to shallow tectonic processes