Controlling factors for gas hydrate occurrence in Shenhu area on the northern slope of the South China Sea

Temperature and pressure on seafloor of the northern slope in the South China Sea are suitable for gas hydrate formation, but bottom simulation reflector (BSR), an indication of gas hydrate occurrence, only occurred in limited areas of the slope. Drillings in the BSR-distributed area (the District S) on the northern slope of the South China Sea suggested that gas hydrate only occurred at Sites SH2, SH3, and SH7 with high saturation (up to 20%–40%), and there is no hydrate at Sites SH1 and SH5 although the distance between SH1 to SH3 is only 500 m. In this paper, we investigated seafloor gradient, fault development, temperature, and pressure in the District S on the northern slope of the South China Sea to understand the possible factors controlling BSR distribution and gas hydrate occurrence. The District S is a structurally fractured continental slope zone and its seafloor gradient varied greatly. The BSR-occurred areas have an average gradient of 19.89×10−2 whereas the BSR-free zone has the average gradient of 10.57×10−2. The calculated relative structural intensities from fault densities and displacements show that the BSR-distributed areas tend to occur in the areas with a moderately high structural intensity, where faults frequently developed close to the seafloor that are possibly favored for lateral migration of gases. On the basis of temperatures and pressures at drilling sites, hydrate-occurred Sites SH2, SH3, and SH7 are located within the thermodynamically stable area for methane hydrate, and hydrate-absent Sites SH1 and SH5 are out of the thermodynamically stable area for methane hydrate formation, suggesting that both BSR and the thermodynamic stability are necessary for hydrate occurrence in the subsurface.