Shoaling of the gas hydrate stability zone inferred from 3D seismic data of the Cauvery basin

The Cauvery basin, located in the southern part of the eastern margin of India, was formed due to the frontier failed rift basin between India and Sri Lanka. This basin has an active petroleum system, and two gas discoveries were made in 2011, namely the Barracuda gas discovery and the North Dorado gas discovery. The study area is located near the Barracuda gas discovery, where bottom simulating reflectors (BSR) are observed, indicating the presence of gas hydrates and free gas. The BSR in seismic data marks the base of the gas hydrate stability zone (BHSZ). The 3D seismic data analyzed in a small region of the Cauvery offshore basin, and the analysis revealed a continuous and stable BSR at water depths between 945 m and 1000 m. The BSR is observed 90 m below the seafloor (mbsf) from seismic data and computed stability for the same region shows that the BHSZ is at 90 mbsf at 1000 m bathymetry, which agrees well with the seismic. Our study attempts to study the stability variation by changing the sea bottom temperature, the geothermal gradient, and hydrostatic pressure. The historical sea surface temperature (SST) recorded in gravity core SK157-14 shows that the SST has increased by similar to 2-2.5 degrees C post-glacial maximum (LGM). The stability calculations made using the Miles equation and this SST increase indicate that the BHSZ shallowed by 75 m post-LGM. The analysis shows that the increase in GTG by 3.7 degrees C/km resulted in shifting of he BHSZ by 20 m. Increasing the pressure by similar to 1 MPa has resulted in the deepening of BHSZ by similar to 40 m. The similarity attribute of the BSR horizon shows high similarity in the BSR horizon. Further to the seismic analysis, the well-log data available in the study area reveal the existence of free gas. From the study, we infer that the shallow occurrence of BSR and its stable behavior can be attributed to the high sea-bottom temperature, very low sedimentation rate, low geothermal gradients, and the lithology of the study area.