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.

Previous gas hydrate production tests conducted by the Guangzhou Marine Geological Survey (GSGM) in 2017 and 2020 indicated the great potential of gas hydrates in the Shenhu Sea area in the Pearl River Mouth Basin (PRMB), China. In this study, the effects of deposition processes in submarine canyons and the distribution of gas chimneys on gas hydrate accumulation were investigated using high-resolution two- dimensional (2D) and three-dimensional (3D) seismic data. Four intact submarine canyons were identified in the study area. Five deepwater depositional elements are closely related to submarine canyons: lateral accretion packages (LAPs), basal lags, slides, mass transport deposits (MTDs), and turbidity lobes. MTDs and lobes with multiple stages outside the distal canyon mouth reveal that the sedimentary evolution of the canyon was accompanied by frequent sediment gravity flows. Gas chimneys originating from Eocene strata are generally up to 3 km wide and distributed in a lumpy or banded pattern. The analysis of seismic attributes confirmed fluid activity in these gas chimneys. Gas hydrates are mainly distributed in ridges among different canyons. Based on the gas sources of gas hydrates and depositional evolution of submarine canyons, depositional processes of sediment gravity flows in submarine canyons and the distribution of gas chimneys significantly affect the accumulation of gas hydrates. Based on these findings, this study establishes a conceptional model for the accumulation of gas hydrate, which can provide guidance in the prediction for favorable gas hydrates zones in the area and nearby.

The China Geological Survey (CGS) has carried out a large number of surveys and core drilling over the deepwater area of Qiongdongnan Basin (QDNB) in the South China Sea and discovered the natural gas hydrate system controlled by the gas chimney with a high geothermal gradient. The complex hydrate system consists of a sandy hydrate reservoir distributed around a lateral transition gas-hydrate mixed zone and a free gas zone in the middle. The hydrate and gas are distributed in the same layer, which is thin but potentially valuable for commercial exploitation. In this paper, a geological model of the target hydrate system in QDNB was established based on the results of several rounds of drilling. The method of numerical simulation was utilized to assess the production capacity of the target hydrate system and clarify the evolution of hydrate and gas saturation distribution with different well positions. The simulation results indicate that the producer well built in the center of the highly-saturated hydrate zone has a limited gas production capacity, with a cumulative production of only 7.25 × 10⁶ m³ in 9 years. The well built at the boundary of the hydrate zone can rapidly link up the gas in the transition zone through a large production pressure differential, but it lacks control over the hydrates and its dissociated gas in the transition zone—the cumulative gas production volume from hydrate accounts for only 12.3%. As for the wells built in the transition zone and gas zone, they can directly invoke the free gas production capacity. Free gas is produced as the formation pressure reduces and hydrate is induced to dissociate, making the gas from the hydrate the subsequent production capacity. The cumulative production can exceed 6 × 10⁸ m³ in 9 years. The stable production duration can extend to 2645 days, and the cumulative proportion of gas at the wellhead from hydrate reaches close to 30%. It is necessary to avoid the free water layer. The bottom water coning would improve the water production by 40% and shorten the stable production duration. In summary, the complex hydrate system of this type in the QDNB has the potential for industrialized exploitation. In the future, the well group can be used for the further improvement of the hydrate utilization rate.