Linking Methane Seepage to Fluid Flow Mechanisms: Evidence from AVO Characteristics of Bottom Simulating Reflectors

The presence of gas hydrates over continental margins may be inferred by various seismic indicators, including the bottom simulating reflector (BSR). Recently, the occurrence of two BSRs have been reported from many regions of the world. In this study we estimate the uncertainty in amplitude versus offset (AVO) behaviour of the single BSR and double bottom simulating reflector (DBSR) observed over two geological provinces; the Kerala-Konkan Basin, offshore India and Green Canyon, offshore USA, and attempt to infer a mechanism for the observed anomalies from the AVO patterns. Anomalous behaviour of seismic velocities within the gas hydrate stability zone (GHSZ) associated with the occurrence of DBSRs, low amplitude seismic chimneys and bright spots, indicates increased hydrate concentration and fluid venting structures underneath the DBSR locations. Such structures, if extended upward into the regional GHSZ through discrete fracture networks, may act as a passage for methane escape into the ocean. Our analysis indicates that the variability in AVO signatures for gas hydrate saturated sediments is potentially linked to the discrete zones of steeply inclined fractures that are responsible for the migration of deep gas and its escape through the seabed. Key Words: Methane hydrate; Gas dynamics; Seismic reflections; Plumbing system; Amplitude versus offset (AVO); Bottom simulating reflector