Numerical Simulation of Mixed-genetic Methane Hydrate Accumulation in Shenhu Area, South China Sea

To describe the processes involved during the hydrate formation and dissociation in marine sediments, we have developed a numerical model to characterize the accumulation of mixed-genetic (i.e., biogenic and thermogenic) methane hydrate in the marine sediments by introducing the biogenic methane formation module into the existing framework of TOUGH+HYDRATE (T+H), a numerical code for the simulation of the behavior of hydrate-bearing complex porous or fractured geologic systems. As a derivate, our model inherits all the powerful features of the original T+H. In addition, in-situ conversion of organic matters to methane by bacteria is taken into account in the new added module. And upward migration of deep source fluids to the hydrate stability zone can be described as well. We consider the sedimentation that added organic matters into the system, the burial of ancient sediment and associated phenomena (e.g., evolution of temperature, sediment compaction and consequent reduction in sediment porosity and permeability, fluid expulsion). We applied the model to reproduce the hydrate accumulation pattern at a drilling site SH2 in Shenhu Area on the northern continental slope of South China Sea. The numerical simulation indicated that methane generated only by in-situ biological processes is far from enough to form such high saturation hydrate reservoir as observed in site SH2 with a hydrate saturation peak higher than 40%. Therefore, the gap is inferred to be filled by the thermogenic or mixed-genetic methane from deep source. Combining in-situ biogenic methane with that contributed by the deep source fluids, the modeling results show good agreement with observed data in terms of distributions of methane hydrate and chlorinity. In addition, the upward fluid is predicted to consist methane and water with average flux of 1.0×10^-11kg/s·m² and 2.0×10^-10kg/s·m², respectively.