A Flat-Lying Transitional Free Gas to Gas Hydrate System in a Sand Layer in the Qiongdongnan Basin of the South China Sea

Most marine gas hydrate systems follow a vertical pattern with hydrate overlying free gas. Here we document the discovery of a gas to hydrate system in a horizontal sand layer in the Qiongdongnan Basin of the South China Sea. Eight wells were drilled by the Guangzhou Marine Geological Survey in 2021-2022 to investigate the occurrence and mechanisms responsible for the formation of the system. We describe a free gas-bearing sand reservoir at the center of the system sustained by advecting hot fluids and gas; away from the advecting zone, the cooler, surrounding sand reservoir is filled with hydrate. Observations at this site show that advective heat has a large control on hydrate formation in sands and may be a key mechanism which allows gas migration within the hydrate stability zone and the formation of high-saturation hydrate in sand layers. Natural gas hydrate, an ice-like substance composed of water and gas, is commonly found in sediments under the ocean. Most marine hydrate systems follow a vertical pattern where hydrate-bearing sediments overlie free gas-bearing sediments; in addition, most hydrate is hosted in marine muds at low concentration. Here we document a horizontal system that transitions from high concentrations of free gas to high concentrations of hydrate in a horizontal sand layer in the northern South China Sea. Two recent drilling expeditions are conducted to explore this unique system. Seismic and logging data suggests that multiple processes including focused fluid flow, capillary sealing and heat transfer control the formation of the system. We discover a gas hydrate system where gas transitions to hydrate in a flat-lying sand layer in the northern South China SeaCapillary sealing occurs at the sand-clay interface, preventing upward fluid advection and causes fluids to migrate laterallyAdvecting warm fluids and gas are the primary control on the hydrate and free gas system in this sand layer