Migration of methane associated with gas hydrates of the Shenhu Area, northern slope of South China Sea

摘要

Shenhu Area is located in the northern slope region of the South China Sea. Pre-drilling reservoir prediction by the analysis of both 2D and 3D seismic data indicates the presence of hydrate-bearing sediments of approximately 70 m thickness. Data also suggests a strong spatial correlation between gas hydrates and gas chimneys. Evidence suggests that these gas chimneys created pathways for the migration of gas and fluids into the gas hydrate stability zone (GHSZ). In this study, we analyze reflection seismic data collected in the vicinity of a well where an abundance of gas hydrate is known to exist. Between the base of the gas hydrate stability zone and the top of the gas chimney, a thin layer of fine grained sediments is observed. A number of small faults (and possibly additional micro-fractures) breach the stratum and connect the gas chimney and GHSZ directly. Fluids, including free gas, may migrate upwards through these fractures. Additionally, small faults within the flanks of the gas chimney may effectively promote gas migration to the GHSZ. Amplitude variation with offsets analysis and instantaneous frequency analysis provided evidences of fluid pathways. A low-frequency anomaly over the center of the gas chimney indicates high seismic attenuation. This is interpreted to be caused by the migration of fluids (probably methane). Based on the results of this study, we propose an exploration for the origin of formation of Shenhu Area gas hydrate formation through the migration of methane.