[1] Compaction of siliciclastic sediments is of interest for the study of numerous transport processes occurring in sedimentary basins. Mechanical compaction of sand/ clay mixtures depends on the clay content, the effective stress history, and both the mechanical compaction coefficients and the depositional porosities of the two endmembers (clean sand and pure shale). The porosity/depth profiles of siliciclastic sediments result from the superposition of two kinds of spatial variations. The first component corresponds to compaction of the mixture with depth of burial. The second component corresponds to small-scale variations of the clay content during deposition and clay infiltration processes. The porosity/depth data are bounded by a minimum porosity/depth envelope corresponding to clay content at the limit between the shaly sand domain and the sandy shale domain. There are two possible upper bounds corresponding to small or large clay contents. The porosity of the clean sand endmember decreases with the depth of burial until a critical porosity of 0.25-0.40 is reached. This critical porosity corresponds to the porosity of random assemblages of more or less spherical grains and depends on the grain-size distribution of the sand grain assemblage. The critical porosity of the shale end-member is much smaller because of the high aspect ratio of the clay particles. The model developed here is applied to downhole measurements made in a borehole that penetrates 3 km of Mio-Pleistocene shaly sand series. The compaction model agrees well with the observed compaction profile.

A numerical model is developed to study the temperature effect on stability and phase transformation of gas-hydrate. The model uses a mathematical formulation based on the enthalpy form of the conservation law of energy. The use of the enthalpy form instead of the temperature form as often done in the literature has made the problem numerically simpler. The model is then applied to describe the effect of sea bottom temperature variations on the stability of gas hydrate occurrences and on the seafloor reflectivity in sediments of the Congo continental slope. Indeed, a migrating seafloor reflectivity front is observed on 3D seimic images from two surveys performed 6 months apart and interpreted as a migrating gas hydrate stability zone. Seawater temperature variations were recorded over a 230-day period. At the seafloor, the amplitude of these variations could explain a 75m shift in water depth of the upper limit of the gas hydrate stability zone. However, the response of the sediment to a perturbation applied at the seafloor is not immediate because of the effects of thermal diffusion and of latent heat of hydrate dissociation. Thermal modelling shows that the depth of penetration of these perturbations is around 2 m for the saturated sediment and around 6 m for hydrate-bearing sediments (hydrate fraction of 0.1). The switch between gas and gas hydrate generated by these temperature fluctuations concern only the first meter of sediment. These appear too small to explain the migration of the seafloor reflectivity, considering that the wavelength of the seismic signal is significantly larger (around 10 m). However, for temperature fluctuations with larger wavelength the switch between gas and gas hydrate will be more important and consequently could explain the reflectivity contrast as a possible migrating gas or gas hydrate front.

No abstract

No abstract

Résumé -Surpressions : origine, approches conventionnelle et hydromécanique -On rencontre souvent des régimes anormaux de pression dans les bassins sédimentaires. Les relations entre la contrainte verticale effective et la porosité ont été appliquées, depuis 1970, dans la région de la Gulf Coast, afin d'évaluer ces surpressions. Des résultats ont été obtenus en faisant appel à la sismique et à la modélisation de bassin dans les bassins tertiaires de sable argileux à contrainte verticale dominante et en déséquilibre de compaction. Cependant, les surpressions d'origines différentes et/ou additionnelles (contrainte tectonique, génération d'hydrocarbures, contrainte thermique, transfert lié aux failles, fracturation hydraulique) ne peuvent pas être quantifiées en utilisant cette approche. En plus des méthodes conventionnelles, une approche hydromécanique est proposée. Pour toute profondeur, la limite supérieure est contrôlée par les conditions de fracturation hydraulique ou par la réactiva-tion de failles. La fracturation hydraulique suppose un système ouvert par période, en régime de contrainte effective mineure proche de zéro. Une connaissance approfondie des régimes de contraintes tectoniques actuels permet une estimation directe de l'évolution de la contrainte minimale. Une évaluation quantitative de la pression avec la profondeur est donc possible, puisque dans les systèmes géologiques compartimentés et/ou non drainés, les régimes de pression, quelles que soient leurs origines, ont tendance à atteindre rapidement une valeur proche de la contrainte principale mineure. Ainsi, l'évaluation de la surpression sera améliorée, puisque cette méthodologie peut être appliquée à divers environnements géolo-giques où les surpressions ont d'autres origines, les mécanismes étant souvent combinés. Cependant, les tendances de l'évolution de pression dans les zones de transition sont plus difficiles à éva-luer de façon correcte. Une recherche complémentaire sur les couvertures et les fermetures sur faille est donc nécessaire pour améliorer leur prévision. En plus de l'évaluation de la surpression, le concept de contrainte principale mineure permet de mieux appréhender le système pétrolier. En effet, les transferts d'hydrocarbures liés aux failles, les domaines de fracturation hydraulique et l'étanchéité du recouvrement dépendent d'une interaction subtile, dans le temps, entre la surpression et les régimes de contrainte principale mineure.Mots-clés : surpressions, contrainte mineure, fracturation hydraulique, transfert par faille, propriétés mécaniques. Abstract

Based on a High-Resolution 3D seismic block acquired in the Gulf of Lion in 2004–2005 we investigated fluid pipes and pockmarks on the top of the interfluve between the Hérault canyon and the Bourcart canyon both created by turbidity currents and gravity flows from the shelf to the deep basin in the north-western Mediterranean Sea. Combining the geometry of the potential fluid pipes with the induced deformation of surrounding sediments leads then to the ability to differentiate between potential fluid sources (root vs source) and to better estimate the triggering mechanisms (allochtonous vs. autochtonous cause). We linked together a set of derived attributes, such as Chaos and RMS amplitude, to a 3D description of pipes along which fluids may migrate. As previously shown in other basins, the induced deformation, creating cone in cone or V-shaped structures, may develop in response to the fluid pipe propagation in unconsolidated sediments in the near surface. The level at the top of a cone structure is diachronous. It means that stratigraphic levels over this surface are deformed at the end of the migration. They collapse forming a depression called a pockmark. These pipes are the result of repeated cycles of fluid expulsion that might be correlated with rapid sea-level rise instead of sediment loading. The most recent event (MIS 2.2 stage) has led to the formation of a pockmark on the modern seafloor. It has been used as a reference for calculating the effect of a rapid sea-level rise on fluid expulsion. As all physical and geometrical parameters are constrained, we were able to define that a + 34 m of sea level rise may account for triggering fluid expulsion from a very shallow silty-sandy layer at 9 m below seafloor since the last glacial stage. This value is consistent with a sea level rise of about 102 m during this period. This study shows that the episodic nature of fluid release resulted from hydromechanical processes during sea-level rise due to the interactivity between high pressure regimes and principal in situ stresses.

No abstract

No abstract