Migrating pathways of hydrocarbons and their controlling effects associated with high saturation gas hydrate in Shenhu area, northern South China Sea

Migration pathways of gas have close relationship with migration and accumulation of natural gas hydrate. Based on high resolution quasi-3D seismic data, and combined with practical drilling results, the authors studied the geological and geophysical features of gas migration pathways and their controlling effects associated with high saturation gas hydrate in GMGS3 drilling sites of Shenhu area. The results show that multiple types of migration pathways are developed in high saturation gas hydrate drilling sites in GMGS3 drilling area, which have favorable corresponding relation with BSR in space. Enhanced reflections occur closely beneath the BSR, and gas charging phenomena are obvious under the enhanced reflection, indicating migrating pathways exist under the gas hydrate stability zone and deep gas has migrated to shallow formations through those pathways. Deep faults, mud diapirs, gas chimneys act as vertical migrating pathways that connect deep thermogenic gases with shallow biogenic gas and pressuretemperature stability zones, and gas hydrate can be formed and accumulated just over those pathways. Shallow slump surface and high continuous sand bodies composed of channel sandstones and submarine fans provide lateral migrating pathways for the migration of shallow biogenic gas and partial thermogenic gas from deep formations, which expands the range of gas supply and increases the gas hydrate distribution scales. It is concluded that tectonic structures and regions with favorable coupling relationship between hydrocarbon migrating system and other elements are prospective targets with high saturation gas hydrate.