Theoretical simulation of the evolution of methane hydrates in the case of Northern South China Sea since the last glacial maximum

Abstract

The distribution and inventory of gas hydrates in a region is determined by the sediment characteristics, methane supply and evolution of the reservoir. In recent decades, the geo-environmental constraints and sources of methane have been intensively investigated, and numerous experimental and numerical simulation tools have been developed to evaluate the inventory of hydrates. However, information regarding the evolution of hydrate reservoirs remains limited. This study developed a simulator to theoretically model the evolution of specific reservoirs since the last glacial maximum (LGM). The LGM was a recent cold epoch that occurred approximately 18,000 years ago. Since the LGM, the earth’s climate system has experienced a continuous increase of surface temperature and rising sea level. Given a sufficient supply of methane and a transport system, hydrates may form in marine sediments if sea level rises or melt if the temperature increases. A one-dimensional simulator that represents the sediment was developed and uses the current hydrate profiles as the initial conditions and reliable paleoenvironmental data obtained in sites located in the northern shelf of the South China Sea (SCS) as boundary conditions. Two types of hydrate profiles were reversely simulated till the LGM: (1) a Gaussian profile, which was observed in the Shenhu area and (2) a trapezoidal profile, which was observed in the Dongsha area, SCS. The evolution and past quantities of local hydrate reservoirs were estimated. The model results demonstrated that shallow (500–700 m below the seafloor, or mbsf) moderate-saturation (50 % pore volume, or v:v) hydrate deposits will form if they are subjected to recent climate changes. The inventory of hydrates in the NSCS increased by only 0–7 % over the past 18,000 years under various scenarios. A sensitivity analysis was performed to identify the most pertinent parameters that control the formation and dissociation of hydrates, including the grain size, temperature gradient and deposition depth. The distribution and depth of the reservoir were determined to be the most critical factors in the evolution of the studied hydrates.