One goal of Korea’s Second Ulleung Basin Gas Hydrate Expedition, UBGH2, is to examine geotechnical properties of the marine sediment associated with methane gas hydrate occurrences found offshore eastern Korea in the Ulleung Basin, East Sea. Methane gas hydrate is a naturally occurring crystalline solid that sequesters methane in individual molecular cages formed by a lattice of water molecules. Offshore Korea, gas hydrate is found in thin, coarse-grained sediment layers that are interbedded with fine-grained sediment. Methane gas hydrate is a potential energy resource, but whether extracting methane from gas hydrate in these layered marine environments is technically and economically viable remains an open research topic as of 2019. This data release provides insight about a poorly quantified aspect of this process: the reaction of fine-grained sediment particles (fines) to the change in pore water chemistry that occurs when methane is extracted from gas hydrate. Fines are an issue for production because they can get resuspended in the flow of fluid and gas toward an extraction well. As fines move, they can cluster and subsequently clog pore throats in the sediment, reducing permeability (which controls how easily methane can flow toward the extraction well). The type of fine-grained particle and the chemistry of the surrounding pore water are the two main factors in determining the cluster structure (the size and fabric of the cluster) and the cluster formation and settling rates. Data in this study provide insight into both factors. Fine particles interact with each other primarily in response to electrical forces, and changes in pore water chemistry can significantly alter how those forces are communicated between particles. In marine systems, in situ pore water is an electrically conductive brine. As gas hydrate dissociates, however, fresh water is released along with the methane, making the pore water less conductive. Depending on the type of fine-grained particles involved, the pore water chemistry change enhances or diminishes the clustering and changes the rates at which the clusters form and settle. For this data release, specimens from the UBGH2 expedition were observed during sedimentation (settling) tests in pore fluids of differing chemistry. The results included in this data release can (1) provide insight into the types of fines present, which can be difficult to quantify if using the more standard x-ray diffraction method for identifying fines and (2) indicate whether the in situ fines are likely to increase or decrease their capacity to clog pore throats as the pore water transitions from higher to lower salinity during gas hydrate dissociation.