科学掘削におけるメタンハイドレートの物理検層解析 : 熊野堆積盆地の例

抄録

During Integrated Ocean Drilling Program (IODP) Expedition 314 at Nankai Trough, located on the sudbuction margin offshore Kii Peninsula, five sites were drilled and logged with logging-while-drilling (LWD). A 935-m-thick basin fill was penetrated and logged in Kumano Forearc Basin at Site C0002. The basin fill consists of repeated turbidite sequences intercalated with background hemipelagic mud. A gas hydrate-bearing zone was identified in the interval of 218-401 mbsf above a bottom-simulating reflector (BSR) by indirect evidences of electrical resistivities and P-wave velocities. More than sixty sharp resistivity spikes were identified and most of them are developed in sand layers. Thicker hydrate layers are localized near the BSR and frequency of hydrate occurrence is decreasing upward. Below the BSR, a number of turbidite sequences were identified and the thickest turbidite zone occurs at 482-547 mbsf as a possible gas/fluid conduit to produce hydrates. Distribution of turbidite sequence plays an essential role as potential conduits and containers for gas hydrate generation in Kumano Basin. Detailed observations of the LWD logs demonstrate a possible lithological control on gas migration and systematic distribution of gas hydrates in turbidite-dominant marginal basins.