Methane flux of ODP Sites 190-1178 and 131-808 (Table 1)

A two-dimensional transient model of methane hydrate accumulation is designed and applied in the Nankai accretionary prism. Model results show that the formation and distribution of methane hydrate and free gas is related to value of the thermal flux, the fluid flux, the methane flux supplied from below, and methane source within the sediment. The duration of the methane flux is of significant importance. The older the sediments, implying a longer period of methane flux supply from below and within the sediments of the model, the larger the volume percentage of free gas will be. This can explain the occurrence of BSR in a area with lower methane flux observed, but absence of the BSR in the other area with higher methane flux observed. Modeling also predicts that the maximum saturation of methane hydrate is about 1% of the pore space in the area of ODP Site 1178, and the methane hydrate occurrence zone is probably 90–200 m below the sea floor (mbsf), which is supported by both direct and indirect evidence for hydrate. In the area near ODP Site 808, the fault zones having high fluid flow and methane flux may possibly result in methane hydrate formation without BSR occurrence.