Influence of geological structures on hydrate accumulation in subsea shallow formations

Abstract

For seepage-type hydrate reservoirs, hydrate accumulation involves natural gas supply, migration in fault/fracture channels, and hydrate generation in shallow formations under seabeds. The relationship between these three steps controls the characteristics of hydrate accumulation. In this paper, a mixing-flux hydrate reaction kinetic model was used to study this accumulation process under the geological context of the Shenhu area in the South China Sea. The hydrate accumulation characteristics in four geological structures were compared. The influences of both fault systems and gas sources on hydrate accumulation in layered and anticline structures were evaluated. Simulation results show that the layered and the anticline systems with the fault-fractures are the most favorable in forming large-saturation, high-abundance hydrates in a short time. The following ranks are gas chimney and mud volcano systems. The faults in the geological systems complexify methane gas migration and help form the hydrate reservoir in the hydrate stability zone. Specifically, some faults help transport methane to the shallow subsea formation, and other ones enable methane to contact more reservoir spaces in the hydrate stability zone. Faults connected to the seafloor may induce partial and full leakage of methane gas into the ocean. The good match between gas sources and fault systems is conducive to the formation of commercial gas hydrate reservoirs. The presented work enhances our understanding of the influence of both geological systems and sedimentary structures on methane migration, accumulation, and gas hydrate generation.