Sand encased by fine-grained sediment regulates methane migration through the gas hydrate stability zone

Abstract

Sand encased by fine-grained sediment can often be present in the shallow subsurface along continental margins. However, its role in methane migration through the gas hydrate stability zone (GHSZ) in mud-dominated sediments has not been fully investigated. Here, we use 3-D seismic and drilling data in the northern South China Sea, and numerical modeling to examine how sand encased by fine-grained sediment regulates methane migration through the GHSZ. Our analysis of well logging data at Site W01, an area of active methane emission, reveals the occurrence of a 12.5-m-thick sand layer that contains gas hydrates within the GHSZ. The 3-D seismic data show that there are two gas chimneys, one located below and the other above the sand layer at Site W01. The lower gas chimney exhibits a high-amplitude rim at the base of the sand, suggesting that it contributes methane for gas hydrate formation within the sand. The upper gas chimney extends from the sand layer to the seafloor, acting as a pathway for the current methane emissions. The sand probably arrests the upward propagation of the lower gas chimney and captures upward-migrating methane. If the overpressure generated by the methane flux is insufficient to breach the overlying fine-grained sediment, the methane would be diverted to migrate laterally within the sand. Meanwhile, a highly heterogeneous distribution of gas hydrates forms in the sand, restricting further methane flow. As hydrate accumulation clogs the sand and creates a self-sealing effect, the pore pressure gradually increases due to the continuous accumulation of free methane gas. Eventually, this pressure can lead to hydraulic fracturing, resulting in the formation of a new gas chimney in the overlying fine-grained sediment. This study provides new insights into the characteristics of gas hydrate reservoirs in sand and the migration of free methane gas through the GHSZ along continental margins.