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Gas bubbles are widespread in seafloors and lakebeds and typically found in shallow and fine‐grained sediments. Sediment properties control gas nucleation and gas migration. Gas migration and pathways have been studied mostly in clean coarse particles or fine‐grained matrices. Nevertheless, both cases show very distinct geo‐behaviors. Pore habit is defined by the counteracting effects of effective stress and pore‐throat‐dependent capillary pressure. In this article, we explore gas nucleation by CO2 gas exsolution and its consequent gas‐driven fractures (open‐mode discontinuities) or pore invasion in binary sediments as a function of fines content (FC). We conducted physical test analogies for different FC subjected to gas exsolution. Our results highlight that the pore habit of gas in gassy sediments depends on its capability to invade a neighboring pore (capillarity) and burial depth (effective stress). We show that the load dominant fraction in binary soils can be used to estimate the dominant pore throat size. We then proposed a robust methodology to predict the pore habit of gassy sediments from its properties as defined in recent developments in soil behavior and characterization. Finally, we applied it to a real case offshore Vancouver Island. Key Points: Fines content determines the global behavior of gassy and non‐gassy sedimentsThe pore habit in gassy sediments depends on its capability to invade a neighboring pore (capillarity) and burial depth (effective stress)The analysis proposed helps understand subsurface gas activity in natural marine environments, potential passageways and stagnant bubbles [ABSTRACT FROM AUTHOR]

On Earth, natural hydrates are mostly encountered in clay‐rich sediments. Yet their formation processes in such matrices remain poorly understood. Achieving an in‐depth understanding of how methane hydrates accumulate on continental margins is key to accurately assess (a) their role in sustaining the development of some chemosynthetic communities at cold seeps, (b) their potential in terms of energy resources and geohazards, and (c) the fate of the methane releases, a powerful greenhouse gas, in this changing climate. This study investigated the formation of methane hydrates and their gas storage capacity (GSC) in clay‐rich sediments. A set of hydrate experiments were performed in matrices composed of sand, illite‐rich clay, and montmorillonite‐rich clay at different proportions aiming to determine the role of mineralogy on hydrate formation processes. The experiments demonstrate that a clay content of 10% in a partially water saturated sand/clay mixture increases the induction time by ∼60%, irrespective of the nature of the clay used. The increase in water saturation in the two matrices promotes hydrate formation. Micro‐Raman spectroscopic analyses reveal that increasing the clay content leads to a decrease in the hydrate small‐cage occupancy, with an impact on the storage capacity. Finally, the analyses of collected natural samples from the Black Sea (off Romania) enable us to estimate the GSC of the deposit. Our estimates is different from previous ones, and supports the importance of coupling multiscale properties, from the microscale to the geological scale, to accurately assess the total amount of methane hosts in hydrate deposits worldwide. Plain Language Summary: Natural gas hydrates are amongst the largest methane reservoirs on Earth. They are sensitive to temperature increase. Societal and environmental concerns surrounding natural gas hydrates pertain to their decomposition, and in particular to the amount of gas they may release and its fate: can it trigger geohazards, or jeopardize the development of unique chemosynthetic communities encountered on continental margins? In‐depth knowledge of hydrate formation processes and properties are essential to provide reliable answers to these questions. The majority of hydrate deposits are characterized by clay‐rich sediments. We led a comprehensive study coupling fine microstructural analyses of both natural and synthetic hydrate samples with macroscale laboratory experiments within different matrices to show how clays can affect their formation kinetics and storage capacity. We found that even a small amount of clay can significantly change the formation kinetics of hydrates, and the matrix mineralogy affects their storage capacity. This study is crucial with a view to accurately assessing the amount of methane trapped in hydrate deposits, and for improved prediction of the consequences of their decomposition on the environment in a changing climate. Key Points: Clay content and clay mineralogy significantly affect the induction time of methane hydrate formation as well as the hydrate morphologyIllite‐rich clays reduce the cage occupancy of hydrates and thus the resulting gas storage capacity (GSC)Microscale analyses underpins the need to characterize microscopic properties of hydrates to estimate GSC [ABSTRACT FROM AUTHOR]

Near-surface seafloor properties affect offshore mining and infrastructure engineering. Shallow seafloor sediments experience extremely low effective stress, and consequently, these sediments exhibit very low in-situ density, shear wave velocity, and shear stiffness. We combined data extracted from the literature with new laboratory and field results to develop a comprehensive understanding of shallow seafloor sediments. First, we explored the sediment-dependent self-compaction characteristics starting with the asymptotic void ratio at the interface between the water column and the sediment column. The asymptotic void ratio depends on the particle size and shape in coarse-grained sediments and on mineralogy and pore fluid chemistry in fine-grained clayey sediments; overall, the asymptotic void ratio correlates with the sediment-specific surface and compressibility. Second, we developed a fork-type insertion probe to measure shear wave velocity profiles with depth. Detailed data analyses confirm the prevalent role of effective stress on shear wave velocity Vs=α(σm′/kPa)β , and the inverse relationship between α and β parameters reveals that electrical interactions alter the velocity profile only in very high specific surface area sediments at very low effective stress and shows that ray bending affects the computed velocities only in the upper few centimeters (for the probe geometry used in this study). Probe insertion causes excess pore fluid pressure and effective stress changes; the ensuing time-dependent diffusion detected through shear wave velocity changes can be analyzed to estimate the coefficient of consolidation. Shear wave velocity profiles and velocity transients after insertion provide valuable information for sediment preclassification and engineering design. [ABSTRACT FROM AUTHOR]

For example, fly ash specimens exhibit relatively high liquid limit (LL), low plastic limit (PL), and plot significantly lower than the A-line on the Casagrande chart. These three liquid limits place fly ash specimens next to diatoms and volcanic ash (Fig. LLbrine, LLker, and LLDW are the liquid limits determined using the fall cone method for sediment pastes prepared with brine, kerosene, and deionized water, respectively [values corrected for mass density - refer to Jang and Santamarina ([2])]. [Extracted from the article]

Hydrate-bearing sediments are relevant to the organic carbon cycle, seafloor instability, and as a potential energy resource. Sediment characteristics affect hydrate formation, gas migration and recovery strategies. We combine the physics of granular materials with robust compaction models to estimate effective stress and capillary pressure in order to anticipate the pore habit of methane hydrates as a function of the sediment characteristics and depth. Then, we compare these results to an extensive database of worldwide hydrate accumulations compiled from published studies. Results highlight the critical role of fines on sediments mechanical and flow properties, hydrate pore habit and potential production strategies. The vast majority of hydrate accumulations (92% of the sites) are found in fines-controlled sediments at a vertical effective stress between σ′ z = 400 kPa and 4 MPa, where grain-displacive hydrate pore habit prevails in the form of segregated lenses and nodules. While permeation-based gas recovery by depressurization is favored in clean-coarse sediments, gas recovery from fines-controlled sediments could benefit from enhanced transmissivity along gas-driven fractures created by thermal stimulation. • Hydrate-bearing sediments are relevant to the organic carbon cycle, seafloor instability, and as a potential energy resource. • Sediment characteristics affect hydrate formation, gas migration and recovery strategies. • Majority of hydrate accumulations (92% of the sites) are found in fines-controlled sediments where grain-displacive hydrate growth prevails. [ABSTRACT FROM AUTHOR]

Marine plastic pollution has become a major concern as it threatens marine life and human health. Most of the plastic that enters the ocean is either consumed by animals and/or trapped in sediments. However, there is little information on how sediment properties might be affected. In this article, we explore the impact of microplastic inclusions in marine settings by using PVC plastic chips and two soil samples as analogues. We conducted a comprehensive experimental study to investigate changes in compressibility, strength, stiffness, thermal and hydraulic conductivity, and particle migration by varying plastic content. Results show that as low as 1% of plastic content by volume can lead to irreversible consequences in sediment behavior while coarse particles display a heightened sensitivity than pure fines. As plastic content in sediment increases year-by-year, we anticipate significant repercussions in marine life, the future landscape of the seafloor and subsurface phenomena. [ABSTRACT FROM AUTHOR]

The article presents information on available tools and advancement in pressure core technology for undertaking various oceans drilling projects for recovery of hydrates bearing sediments. The pressure core technology for characterization of hydrate bearing sediments requires coring, manipulation and testing under pressure and temperature (P-T) conditions within the ocean bed. The pressure core system derives preservation of sediments and production of natural gas.