Factors Controlling Short‐Range Methane Migration of Gas Hydrate Accumulations in Thin Coarse‐Grained Layers.

Wei, Li; Cook, Ann; Daigle, Hugh; Malinverno, Alberto; Nole, Michael; You, Kehua

Natural gas hydrate is often found in marine sediment in heterogeneous distributions in different sediment types. Diffusion may be a dominant mechanism for methane migration and affect hydrate distribution. We use a 1‐D advection‐diffusion‐reaction model to understand hydrate distribution in and surrounding thin coarse‐grained layers to examine the sensitivity of four controlling factors in a diffusion‐dominant gas hydrate system. These factors are the particulate organic carbon content at seafloor, the microbial reaction rate constant, the sediment grading pattern, and the cementation factor of the coarse‐grained layer. We use available data at Walker Ridge 313 in the northern Gulf of Mexico where two ~3‐m‐thick hydrate‐bearing coarse‐grained layers were observed at different depths. The results show that the hydrate volume and the total amount of methane within thin, coarse‐grained layers are most sensitive to the particulate organic carbon of fine‐grained sediments when deposited at the seafloor. The thickness of fine‐grained hydrate free zones surrounding the coarse‐grained layers is most sensitive to the microbial reaction rate constant. Moreover, it may be possible to estimate microbial reaction rate constants at other locations by studying the thickness of the hydrate free zones using the Damköhler number. In addition, we note that sediment grading patterns have a strong influence on gas hydrate occurrence within coarse‐grained layers. Plain Language Summary: Our paper focuses on gas hydrate, a "frozen" form of methane found below the seafloor. We want to understand why gas hydrate concentrates in some sediments and not in others and what factors control the concentration of hydrate. We use a numerical model to study gas hydrate and dissolved methane concentrations around a thin layer of sand (~3 m thick) and the hydrate concentration in the mud surrounding the sand. We can use this information to understand where hydrate may concentrate in nature. Key Points: We show in diffusion‐dominant gas hydrate system that the amount of POC content has the strongest influence on hydrate volume within thin coarse‐grained layersThe microbial reaction rate has most strong influence on the thickness of hydrate‐free zones in the surrounding fine‐grained sediments of the coarse‐grained layersThe distribution of hydrate within the coarse‐grained layer is highly affected by the sediment grading patterns

GAS hydrates; MARINE sediments; METHANE; CARBON cycle; GRAIN size

Geochemistry, Geophysics, Geosystems: G3, 2019, Vol 20, Issue 8, p3985

1525-2027

Academic Journal

10.1029/2019GC008405