Highly saturated free gas accumulations below the gas hydrate stability zone in the Qiongdongnan Basin, South China Sea

ABSTRACT Free gas is commonly observed in shallow, unconsolidated marine sediments below the base of gas hydrate stability zone (BGHSZ), exhibiting high amplitude bright spots, enhanced reflections, and/or velocity pull-down features in seismic data. Although previous drilling expeditions indicate low to moderate free gas saturations (i.e., concentrations of free gas within porous marine sediments) below the BGHSZ, recent drilling reveals widely distributed, highly saturated, free gas layers within submarine sedimentary fan systems in the Qiongdongnan Basin. By integrating borehole well log data and seismic attribute analysis (including seismic variance analysis, rms statistical analysis, and spectral decomposition), we characterized the spatial distributions of free gas and gas hydrate, along with sedimentologic mass transport deposits (MTDs) and clay-rich channel systems. We identified free gas below the BGHSZ within channelized deposits or fan-like submarine systems. The distribution of the free gas accumulations was more extensive than that of gas hydrate due to the efficiency of various sealing mechanisms within the imaged depositional systems. The formation of gas hydrate reduces the permeability of a sedimentary section to the migration of gas, inhibiting the upward migration of gas and leading to the accumulation of free gas. Additionally, MTDs and overlying clay-rich channel systems act as seals, further trapping free gas within the sedimentary section beneath the BGHSZ. We suggested that unconsolidated, sand-rich reservoirs at shallow depths can host highly saturated free gas layers, where the gas enrichment mechanisms differ between horizontal and inclined sedimentary layers. We demonstrated that the co-location of gas hydrate and free gas accumulations represents an appealing target for potential energy extraction in the Qiongdongnan Basin.