Foldback reflectors near methane hydrate bottom-simulating reflectors: Indicators of gas distribution from 3D seismic images in the eastern Nankai Trough

摘要

Understanding of fluid behavior and gas distribution in the shallow subsurface are important considerations in gas hydrate formation and the global carbon cycle. Estimation of gas distribution based on reflection seismic surveys, however, is difficult because the boundary of a gas-bearing zone is indistinct and not systematically defined. This study reports distinctive features related to gas-hydrate distribution and possible fluid migration in high-resolution 3D seismic-reflection data from sediments of the eastern Nankai Trough. These features, here termed foldback reflectors (FBRs), descend in accordion shaped reflectors near the edges of bottom-simulating reflectors (BSRs). FBRs generally correspond to lateral boundaries between two seismic facies, a dimmed' facies with relatively low amplitude and subdued high-frequency components beneath the BSR and the contrasting facies around the BSR. The dimmed facies corresponds to areas of anomalously low velocity consistent with a small amount of free gas. FBR is mostly developed in well-stratified formations in uplifted regions. Dip directions of the FBR appear to be restricted by orientation of the host formations. Edges of the FBR often correspond to high-amplitude layers. Such occurrences of FBR suggest that regional uplift and layer-parallel fluid migration are related to the formation of FBR as well as BSR.