Numerical Analyses on Controlling Factors for Hydrate Differential Accumulation in the Shenhu Drilling Area, South China Sea

Abstract

In 2007, the first marine gas hydrate drilling expedition of the Guangzhou Marine Geological Survey (GMGS-1) was carried out in the Shenhu area on the northern continental slope of the South China Sea, collecting geophysical and drilling data from eight different sites. No hydrate sample was recovered at site SH5, even though it locates close to site SH2 where the hydrate was drilled. The reason for the different distribution of hydrate is not thoroughly understood. On the basis of the actual exploration data, the key factors controlling hydrate formation in the Shenhu area were analyzed by employing the numerical modeling. To evaluate the key controlling factors among gas flux, water depth, temperature gradient, and cap rock location, sensitive studies were conducted. The results show that the gas flux is the prerequisite factor determining the hydrate quantity. The seawater depth does not affect the hydrate formation significantly. The temperature gradient plays an important role in hydrate quantity and saturation. The cap rock location controls the gas migration to the gas hydrate stability zone (GHSZ). According to the sensitivity analyses and drilling data, we proposed a mechanism of hydrate differential accumulation in the Shenhu area. The relatively high geothermal gradient and the cap rock located below the GHSZ cause the absence of hydrate at site SH5, leading to the differential accumulation.