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International audience; Different regimes are usually observed for fluid migration through an immersed granular layer. In this work, we report a puzzling behavior when injecting water at a constant flow rate through a nozzle at the bottom of an immersed granular layer in a Hele-Shaw cell. In a given range of parameters (granular layer height and fluid flow-rate) the granular bed is not only fuidized, but the particle-laden jet also exhibits periodic oscillations. The frequency and amplitude of the oscillations are quantified. The Strouhal number displays a power-law behavior as a function of a non-dimensional parameter, J, defined as the ratio between the jet velocity at the initial granular bed height and the inertial particle velocity. Fluid-particle coupling is responsible for the jet oscillations. This mechanism could be at the origin of the cyclic behavior of pockmarks and mud volcanoes in sedimentary basins.

Microfaults formed in continental carbonates reveal poorly known mechanisms of shear localization induced by early diagenesis during compaction. These faults are characterized by sinuous shape, bed-controlled, pervasive distribution, no calcite precipitation, and mainly disaggregation processes. Two main sets were described: (1) The first set is composed by normal-sense, high-angle microfaults affecting the top of carbonate beds showing undulating pedogenic bed surface. They show porosity increase and are sometimes organized in polygonal patterns. Their occurrence seems related to overconsolidation of pedogenic surface and density inversion – phreatic loading – fluid expulsion processes in the surficial carbonate bed. (2) The second set is composed by low-angle compactive microfaults with large slickenlines and incipient shear-offset. Their organization within two conjugate systems (normal-sense set and strike-slip set) almost contemporaneous is consistent with a NS extension following the slope induced by the basin subsidence to the south. Their occurrence seems related to vertical loading below few meters depth and occurred by shear-enhanced compaction and incipient pressure-solution process. The presence of such structures gives news information concerning dilatant or compactive shear processes and rheological properties of micritic carbonates during early diagenesis.

Detailed sedimentological investigations on numerous outcrops and drill-core sections in the 2.1-Ga-old Franceville basin, Gabon, provide evidence for a large-scale injectite network. The injectites were formed by the injection of sands through a thickness of about 150 m of the FB1 Member, and now cover a minimum area of 70 km 2 corresponding to the Bangombe plateau, but are also recognized close to Franceville 35 km farther south-east. The injectite lithology corresponds to a mud- or carbonate-supported sandstone characterized by a loose and uncompacted fabric contrasting with the host-rock. Because the injectites are often parallel to subparallel to the stratification, they were misinterpreted as depositional beds by previous authors. At outcrop, the injectite bodies exhibit sill, dyke, wing and protrusive geometries emplaced during early burial within poorly compacted material, with partial wall erosion and dissociation. The source of the sand material is attributed to a channel-levee turbidite depositional system located in the lower part of the FB1 Member. These deposits were buried by a thick biochemical muddy cap deposited during a starvation phase, which increased the seal capacity of the system. This type of architecture has a high potential to develop compaction disequilibrium during burial. Moreover, in the case of the Francevillian injectites, the abundance of microbial organic matter favoured early methane degassing and lateral charging of the sand reservoir. The overpressure in the channels was periodically released during early burial, through the effect of rapid sedimentary and tectonic loading, and possible seismic activity. The fluidized sand was injected according to a symmetrical wing pattern. Thus, the injected masses caused a local decrease in the grade of the lateritic manganese ore deposit of the Bangombe plateau.

International audience; The giant Jurassic-aged pockmark field of Beauvoisin developed in a 800 m wide depression for over 3.4 Ma during the Oxfordian; it formed below about 600 m water depth. It is composed of sub-sites organized in clusters and forming vertically stacked carbonate lenses encased in marls . This fine-scale study is focused on a detailed analysis of petrographical organization and geochemical signatures of crystals that grew up in early to late fractures of carbonate lenses, surrounding nodules, and tubes that fed them. The isotopic signature (C, O and Sr) shows that at least three different episodes of fluid migration participated to the mineralization processes. Most of the carbonates precipitated when biogenic seepage was active in the shallow subsurface during the Oxfordian. The second phase occurred relatively soon after burial during early Cretaceous and thermogenic fluids came probably from underlying Pliensbachian, Late Toarcian or Bajocian levels. The third phase is a bitumen-rich fluid probably related to these levels reaching the oil window during Mio-Pliocene. The fluids migrated through faults induced by the emplacement of Triassic-salt diapir of Propiac during the Late Jurassic and that remained polyphased drain structures over time.

International audience; Constraining the thermal, burial and uplift/exhumation history of sedimentary basins is crucial in the understanding of upper crustal strain evolution and also has implications for understanding the nature and timing of hydrocarbon maturation and migration. In this study, we use Vitrinite Reflectance (VR) data to elucidate the paleo‐physiography and thermal history of an inverted basin in the foreland of the Atlasic orogeny in Northern Tunisia. In doing so, it is the primary aim of this study to demonstrate how VR techniques may be applied to unravel basin subsidence/uplift history of structural domains and provide valuable insights into the kinematic evolution of sedimentary basins. VR measurements of both the onshore Pelagian Platform and the Tunisian Furrow in Northern Tunisia are used to impose constraints on the deformation history of a long‐lived structural feature in the studied region, namely the Zaghouan Fault. Previous work has shown that this fault was active as an extensional structure in Lower Jurassic to Aptian times, before subsequently being inverted during the Late Cretaceous Eocene Atlas I tectonic event and Upper Miocene Atlas II tectonic event. Quantifying and constraining this latter inversion stage, and shedding light on the roles of structural inheritance and the basin thermal history, are secondary aims of this study. The results of this study show that the Atlas II WNW‐ESE compressive event deformed both the Pelagian Platform and the Tunisian Furrow during Tortonian‐Messinian times. Maximum burial depth for the Pelagian Platform was reached during the Middle to Upper Miocene, i.e. prior to the Atlas II folding event. VR measurements indicate that the Cretaceous to Ypresian section of the Pelagian Platform was buried to a maximum burial depth of ~3 km, using a geothermal gradient of 30°C/km. Cretaceous rock samples VR values show that the hanging wall of the Zaghouan Fault was buried to a maximum depth of

Highlights: • The Giant Gjallar Vent is still active in terms of fluid migration and faulting. • The Base Pleistocene Unconformity acts as a seal to upward fluid migration. • Seal bypass in at least one location leads to a new phase of fluid venting. The Giant Gjallar Vent (GGV), located in the Voring Basin off mid-Norway, is one of the largest (~ 5 × 3 km) vent systems in the North Atlantic. The vent represents a reactivated former hydrothermal system that formed at about 56 Ma. It is fed by two pipes of 440 m and 480 m diameter that extend from the Lower Eocene section up to the Base Pleistocene Unconformity (BPU). Previous studies based on 3D seismic data differ in their interpretations of the present activity of the GGV, describing the system as buried and as reactivated in the Upper Pliocene. We present a new interpretation of the GGV’s reactivation, using high-resolution 2D seismic and Parasound data. Despite the absence of geochemical and hydroacoustic indications for fluid escape into the water column, the GGV appears to be active because of various seismic anomalies which we interpret to indicate the presence of free gas in the subsurface. The anomalies are confined to the Kai Formation beneath the BPU and the overlying Naust Formation, which are interpreted to act as a seal to upward fluid migration. The seal is breached by focused fluid migration at one location where an up to 100 m wide chimney-like anomaly extends from the BPU up to the seafloor. We propose that further overpressure build-up in response to sediment loading and continued gas ascent beneath the BPU will eventually lead to large-scale seal bypass, starting a new phase of venting at the GGV.

Sequence stratigraphy in the seismic interpretation workflow helps in the understanding of the basin analysis and the spatial distribution of reservoirs, seal and source rocks. Classical methods consist in identifying seismic discontinuities corresponding to the reflection terminations (downlap, toplap, onlap, and truncation) to define stratigraphic sequences and their system tracts. Such task is a labour intensive process mainly based on a limited number of auto-tracked horizons. Recently new approaches have been proposed to optimize this workflow. In this paper, we propose to analyze the thickness variations of a geological model, computed with a global approach based on a minimization process between the seismic relationships (Pauget et al 2009). Given the fact, the geological model is continous; the variation of the thickness can be computed for any seismic voxel. The thinning zones of the geological model enhance stratigraphic discontinuities and provides to the interpreter a high level of precision in the identification of the sequences. We have applied this method on the block F3, located in the Dutch sector of the North Sea, presenting relevant large-scale sigmoidal bedding. The analysis of the thickness attribute enhanced zones of convergence of the seismic reflections packages corresponding to the observed downlaps and toplaps. A sub division into stratigraphic sequences could be achieved by mapping and thresholding the thickness values. Convergence zones of the different reflection packages were modeled in three dimensions for a better understanding of the spatial depositional process. This case study has shown the rapidity, robustness and the accuracy of the geo-model approach in the analysis of the stratigraphic sequences. These results suggest that the method could be used to optimize the level of detection of subtle traps, seals and reservoirs, at an early stage in the interpretation process.

Abstract Sands penetrated at shallow depths (< 1 km) in the Gulf of Mexico can flow to the surface and undermine seafloor installations. Here we explore aspects of natural fluid expulsion structures from these "shallow-water sands". Highresolution seismic data shows no obvious sand injection structures from the Blue Unit, a shallow-water sand, in the Ursa area, but perhaps nascent dike formation. Also, channel sands stratigraphically above the Blue Unit are overpressured, with a possible pressure source in the latter. In the Ursa area the maximum principal stress is vertical and constrained by the integrated unit weight of the sediments at IODP Site U1324. The stress necessary to form hydrofractures (the minimum principal stress plus negligible tensional strength of the sediment) is about 85% of the maximum principal stress indicating an approximately isotropic state of stress. High Poisson's ratios (0.46-0.49), determined from wireline compressional and shear wave velocities at Site U1324, are also consistent with a nearly isotropic state of stress at shallow depths. Estimated maximum fluid pressures are about 80% of the total overburden stress, less than necessary to hydrofracture the sediment. Therefore, the fluid pressure is insufficient to form sand injectites. The formation of sand injectites, as observed in the geologic record, may require very high fluid pressures, approaching lithostatic values. Diverse orientations of ancient injectites support the necessity of very high fluid pressures and a nearly isotropic state of stress. Introduction Drilling operations in the Gulf of Mexico, especially in deep water, have encountered unconsolidated and overpressured sand at shallow depths. Once penetrated these sands flow into the wells, washout, erode around the exterior of casing strings, and locally flow to the surface [1]. Surface features associated with this ?shallow water flow' behavior include cracks, faults, and gully-like features [2]. In areas where this behavior has occurred, the seafloor appears to be undergoing subsidence associated with sediment loss at depth. In the Ursa drilling area (Figure 1) the shallow-water sands are overpressured which initiates the flow into the boreholes [1, 3]. The shallow-water sands in the Ursa area readily flow into boreholes and to the surface if the borehole is normally pressured (i.e. the hole is uncased or an underbalanced mud is used). Therefore, an obvious question is whether there are any natural injections from the shallow-water sands or other evidence of fluid flow. Given the overpressured nature of the shallow-water sands [3], naturally occurring injections and other fluid migration features could be expected. These "injectites" are common in some petroleum systems where they complicate sand reservoir geometry and have important implications for hydrocarbon production [4]. Because fluid pressure and sand geometries are well-constrained, the Ursa area may be an important natural laboratory to ascertain the limiting fluid pressure and stress conditions for the occurrence of injectites and other fluid flow features. Fluid Migration and Sand Injection from Blue Unit Sand in IODP Drilling Area During the last 70 ka, rapid deposition, directly south of the Mississippi River accumulated a sand and mud sequence, the Blue Unit [5].

Le developpement des energies alternatives intermittentes se heurte a des difficultes economiques et politiques souvent insoupconnees. Apres la crise de l’euro, l’Europe connaitra-t-elle celle de l’electricite ?

Which EU policies for electricity grids ?Le developpement des energies alternatives se heurte a des difficultes economiques et politiques souvent insoupconnees. Aux genereux tarifs de rachat mis en place pour favoriser le developpement de l’eolien et du photovoltaique, et aux subventions requises pour garantir les capacites d’appoint (par ailleurs fortement emettrices de CO2) necessaires a l’equilibre du systeme electrique, il faut ajouter le developpement des reseaux de transport de l’electricite des lieux de production vers les lieux de consommation (en Allemagne, la longueur necessaire serait de 4 500 km).La construction couteuse de ces reseaux se heurte a l’opposition des populations, a tel point que l’Allemagne n’a pu construire en dix ans que 100 km de ces nouvelles lignes...Enfin, le renforcement des interconnexions entre les Etats europeens encourage chacun d’entre eux, comme l’Allemagne ou le Danemark, a faire supporter par ses voisins l’intermittence resultant de ses seules decisions politiques nationales. Apres la crise europeenne de l’euro, connaitrons-nous celle de l’electricite ?The development of alternative energy sources has encountered often unexpected political and economic difficulties : the generous rates for purchasing this electricity (set to give a boost to wind and photovoltaic power) ; the subsidies necessary for maintaining output during spikes in demand (which cause a sharp increase in CO2 emissions) so as to avoid sharp fluctuations on the grid ; and, not to be forgotten, the development of networks for transporting electricity from places of production to places of consumption. In Germany, 4.500 km of lines are needed. The steep investments required for this have run up against opposition from public opinion with, as a consequence, the construction in ten years of only 100 km of new lines. Furthermore, reinforced interconnections between EU member states have led each country, such as Germany or Denmark, to transfer onto its neighbors the responsibility for coping with the intermittence of production that ensues from its own national decisions. After the European euro crisis, will there be an electricity crisis ?

International audience