Detailed structure and geological background of Foldback Reflectors near Methane-hydrate BSRs inferred from 3D seismic in the eastern Nankai Trough

Abstract:

Understanding subsurface gas/fluid behavior is significant for revealing hydrocarbon pathways or for discussions on global carbon cycle. Conventional hydrocarbon indicators in seismic profiles are in practice useful for detection of gas hydrates or gas in shallow sedimentary sequences. We herein study uniquely distinct seismic reflectors imaged in 3D seismic data acquired in the eastern Nankai Trough. The reflectors termed here as Foldback reflectors (FBRs), which occur along the edge of BSR and have accordion shaped characteristics. The FBRs resemble but differ from vertically stacked stratigraphic traps, in terms of that all the reflectors cut the formational sequences. FBRs are distributed in thick, well-stratified and less deformed sediments of the north-western slope of the uplifted outer ridge. Two-dimensional thermal structure taking into account topographic effects on the seafloor in our model undoubtedly shows that FBRs are developed well below depths of the base of gas hydrate stability (BGHS), even with changing hydrocarbon proportion including methane and ethane. P-wave velocity strongly suggests that the FBRs serve as a boundary of gas distribution below the BGHS. The polarity reversed at the edge of each FBR is consistent with a velocity model. Relatively fast regional uplift possibly produces the trapped free gas in pore spaces that is related to dissociation of gas hydrates. Morphology of FBRs is controlled by lithology and structure of their surrounding formation. The edge of FBRs might be corresponded to the boundary of the sandy/muddy seismic unit that is inferred from an instantaneous amplitude attribute analysis. Previously reported distribution of gas hydrates in this region can be well-constrained by lithology which is consistent with the seismic unit defined in this study. FBRs strike almost in the same direction with hosting formation, and they are always developed in downdip of the formation below BSRs. It is thus inferred that the regional uplift and the layer-parallel fluid migration play important roles in forming FBRs. Therefore, FBRs are regarded as an important proxy for gas distribution and fluid migration in the gas hydrate bearing areas.

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 [K1]“formational sequences”?

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