Possible migration front of gas-related fluid inferred from 3D seismic in the eastern Nankai Trough

High resolution 3D seismic survey, “Tokai-oki to Kumano-nada”, was conducted for methane hydrate exploration in the eastern Nankai Trough by METI in 2002. Our study focuses on a series of accordion-shaped reflectors with horizontal axis of fold back. They are connected to the edge of BSRs and alternate their polarities at every fold back hinge. We call the reflectors “Foldback Reflectors (FBRs)” in this study. Sedimentary horizons are successive across these series of reflectors with no fault displacement as a general rule. FBR generally corresponds to lateral seismic facies boundary between BSR distribution area and outside of the BSR area. The formation beneath the BSR shows dimmed facies characterized by relatively low amplitude and lack of high frequency components in contrast to outside of the BSR area with normal facies. Seismic velocity analysis suggests that FBRs correspond to velocity boundaries, where the dimmed faceis below the BSR coinsides with relatively low velocity. The polarities of FBRs are also consistent with such velocity changes. Such dimmed facies with low velocity and low amplitude anomaly suggests effects of gas components in the pore water. In this area, FBRs are mostly developed in the well-stratified formation but not in the area of frequent fractures and the area of major lateral lithological change. The observed FBRs are clustered in northern slope of the uplifted outer ridge, whereas few FBRs are developed in the southern slope of the outer ridge with frequent compressive and strike-slip deformations related to major fault systems including the Kodaiba faults and the Tokai faults. The estimated strike directions of each FBRs are probably controlled by the dip direction of crossing formation. Another important character of FBRs is that it never crosses major unconformities into lower strata. In addition, high amplitude layers are sometimes recognized at hinges of foldbacks convex to the outside of the BSR area. These high amplitude layers probably having higher permeability are interpreted as conduits of gas-related fluid from the BSR distribution side to the outside of the BSR area. From these facts, FBR can be regarded as an important proxy indicating fluid distribution and possible migration front of gas-related fluid. This study used data provided by METI, JOGMEC and MH21 Research Consortium for Methane Hydrate Resources in Japan.