Simulation of methane hydrate formation in coarse- to fine-grained sediments in the Nankai Trough, Japan

The properties of host sediments and pore water considerably affect both the occurrence and formation processes of methane hydrate. In coarse-grained layers, hydrates are generally concentrated preferentially in the pore space, and their formation is influenced by pore water salinity. To understand how geophysical and geochemical factors control the distribution of methane hydrates, we conducted numerical simulations using a one-dimensional flow model under different reservoir and fluid conditions in the Kumano Forearc Basin, Nankai Trough, Japan. Assuming an estimated range of methane flux between 0.002 and 1.9 kg m⁻² year⁻¹, three flow scenarios were considered. When the methane flux was relatively small, the results coincided with the observed hydrate distribution. In general, a low-methane flux decreases the hydrate saturation upward from the bottom of the methane hydrate stability, whereas a high-methane flux increases the saturation downward. These results also suggest that the sediment structure, such as the fracture distribution, influences the sediment stress conditions and constrains the flow regime. We further examined the effects of permeability changes in the heterogeneous lithological units on the simulation results using typical permeabilities of 10⁻¹³ m² for sand and 10⁻¹⁵ m² for mud. The results showed that hydrate saturation sharply increased and decreased in adjacent high- and low-permeability units, respectively. The consideration of complex stratigraphic conditions and variable fluid configurations provides an understanding of the environmental factors controlling hydrate generation and distribution, which is important for hydrate resource extraction and geohazard prevention.