Influence of Clay‐Containing Sediments on Methane Hydrate Formation: Impacts on Kinetic Behavior and Gas Storage Capacity.

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]