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 gas hydrate overlying free gas. Here we document the discovery of a novel flat-lying transitional 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 gas hydrate. Observations at this site show that temperature has a large control on hydrate formation in sands and may be a key mechanism which allows gas migration and hydrate formation in high-saturation hydrate filled sands.