Controlling Effect of Particle Size on Gas Hydrate Enrichment in Fine‐Grained Sediments.

The particle size of sediments below the seabed is a crucial factor affecting the formation and enrichment of gas hydrates. Apart from the formation and enrichment law of gas hydrate in coarse‐grained sediments (dominated by a sandy‐sized fraction), in the fine‐grained sediments (<62.5 μm) which accounts for more than 90% of offshore gas hydrate resources globally, the control effect of sediment particle size on gas hydrate is still unclear. Therefore, understanding the relationship between the fine‐grained sediment particle size and gas hydrate enrichment is essential for revealing the global distribution and dynamic evolution of gas hydrates. Here, we analyzed the vertical gas hydrate saturation, particle size parameters of sediments, whole‐rock minerals, and clay mineral components based on drilling data and sediment samples from fine‐grained gas hydrate reservoirs (GHRs) in the Shenhu area of the northern South China Sea. The results show that in fine‐grained sediments, the coarse particles cannot improve the reservoir quality or enrich the gas hydrate because many fine particles fill the intergranular pores formed by the coarse particles. Meanwhile, the fine particles were dominated by clay minerals, especially in the illite/smectite mixed layer, which significantly reduced the permeability of the sediment layer and was not conducive to the enrichment of gas hydrates. Moreover, sedimentary processes directly control the sediment particle size and mineral composition, which play an essential role in controlling GHRs at the macroscale. In the fine‐grained sediments, very fine sediments (<8 μm) have a more significant negative impact on gas hydrate enrichment. Plain Language Summary: Offshore gas hydrates have great global resources and environmental significance. Gas hydrate is the important pool that stores methane in offshore fine‐grained sediments. However, the formation and enrichment rules of gas hydrate in fine‐grained sediments still need to be clarified. We collected sediment samples from the gas hydrate reservoirs and adjacent layers in the northern South China Sea and analyzed the controlling effect of sediment particle size on the formation and enrichment of gas hydrate in the fine‐grained sediments. We found that the coarse particles in the fine‐grained sediments cannot play a positive role in the enrichment of gas hydrate. The very fine‐grained particles (<8 μm) seriously restrict the formation and enrichment of gad hydrate, which is worthy of attention in fine‐grained reservoirs. Our results on natural sediments partly conform to previous laboratory simulation results on induced gas hydrate formation, effectively establishing potential links between laboratory simulations and natural deposition records. Key Points: Pore‐filling type dominates the gas hydrate in fine‐grained reservoir, particle size between 5 and 6 Φ is beneficial to gas hydrate enrichmentIn fine‐grained gas hydrate reservoir, the very fine‐grained particles (≤8 μm) restricting the enrichment of gas hydrateDifferent sedimentary processes playing an essential role in the formation and enrichment of gas hydrate [ABSTRACT FROM AUTHOR]