Discussion on the rapid formation mechanism and evolution process of methane hydrate-bearing sediments in Shenhu Area of northern South China Sea

摘要

Because of free gas migration in sediment pores, the Shenhu seafloor layers in the northern South China Sea is speculated to have experienced some rapid methane hydrate generation and accumulation. In this study, the main hydrate formation is presumed to occur rapidly in the gas-hydrate-liquid three-phase zone. A first-order reaction kinetics mechanism, which controls the hydrate generation, has been coupled into a flow-transportation-reaction model. The methane flux is chosen to be 0.5 mol.m(-2).a(-1), and the scope of the kinetic reaction coefficient ranges from 10(-1) mol.m(-3).MPa-1.a(-1) to 10(2) mol.m(-3).MPa-1.a(-1). Using these basic conditions, two geological scenarios are designed to investigate the hydrate formation within local effective MH distribution area which is 240.150 mbsf. At three moments (800 a, 4 ka and 8 ka) the state of dissolved methane and salt, layer temperature, gas and hydrate saturation, stratum permeability and capillary pressure has been displayed. The effect of kinetic coefficient on gas and hydrate content, and local temperature, has been compared. The phenomenon accompanying transient decomposition at the hydrate formation front has been found in this process. At last, the constant bottom boundary temperature is set to reflect the influence from deep tectonic activity, and the effects have been discussed. The results demonstrate the possibility of a kind of rapid dynamic evolution process and mechanism on hydrate-bearing sediment formation and aggregation. They also present interesting differences between the enclosing and open characteristics of the Shenhu hydrate system.