Seismic evidence of free gas migration through the gas hydrate stability zone (GHSZ) and active methane seep in Krishna-Godavari offshore basin

摘要

Seismic amplitude versus offset (AVO) analysis was performed on the multichannel seismic data in the vicinity of Site NGHP-01-10, where thick fracture-filled gas hydrate is reported, to understand the fluid migration in fracture dominated gas hydrate system. Amplitude preserved common image gathers (CIG) are used in extracting the seismic amplitude with offset for seafloor, bottom simulating reflectors (BSR), and different horizons within the gas hydrate stability zone (GHSZ). The extracted amplitudes are fitted with three-term Shuey's AVO approximation to estimate the intercept and gradient values. Based on the intercept and gradient values, two distinct classes of seismic AVO responses are observed; class IV AVO (decreasing negative amplitudes with increasing offsets) and class III AVO (increasing negative amplitudes with increasing offsets) for the BSR and other horizons above the BSR. The large negative intercept and gradient values which indicate class III AVO ranges from - 0.2 to - 1.8 for the inline seismic profile and from - 0.01 to - 0.3 for the crossline profile. The presence of class III AVO anomaly within the gas hydrate stability zone (GHSZ) provides evidence of existence of free gas which may be migrating along the fault system. Some of the active faults are exposed on the surface and show gas venting signatures such as gas-flares and pockmarks in the multibeam data.