Spatial distribution of gas hydrate deposits in Krishna-Godavari offshore basin, Bay of Bengal

To delineate the occurrences of gas hydrates and free gas in the Krishna-Godavari (KG) basin, Bay of Bengal, we used 3D seismic data to construct a velocity model of high spatial density. We used an automated version of migration velocity analysis to estimate P-wave velocities using Kirchoff prestack time migration. We manually checked all common image gathers (CIGs) to assess the reliability of the auto-picking algorithm. The velocity model was validated against available downhole information from drilling Sites NGHP-01-02, NGHP-01-03, and NGHP-01-10. The model shows a) anomalously high velocities at Site NGHP-01-10 where significant gas hydrates were reported and b) background velocities at Sites NGHP-01-02 and NGHP-01-03 where a negligible amount of hydrate was recovered. A bottom simulating reflector (BSR) is identified in the 3D seismic data, and an average velocity map is prepared above the BSR. Background velocities of marine sediments in the KG basin vary from 1.43 km/s to 1.6 km/s; velocities higher than 1.65 km/s indicate gas hydrate bearing sediments while velocities lower than 1.43 km/s indicate free gas bearing sediments. High velocities observed below seafloor mounds and ridges suggest the presence of gas hydrate in a structurally deformed area. Slope basins are generally devoid of gas hydrates. Free gas is observed at depths of up to several hundred meters below the BSR suggesting the migration of deeper fluid/gas into the gas hydrate stability zone (GHSZ). Zones of gas hydrates and free gas are preferentially oriented along major faults indicating the role of faults in the transportation of free gas and formation of gas hydrates deposits. We identify a free gas bearing horizon at depths (> 300 m) below the BSR, which might contribute to the lateral migration of free gas. Gas hydrate deposits in KG basin are classified into two categories: i) leaky deposits which occur in the areas of active seafloor faults, and are characterized by heterogeneous distribution of gas hydrates, and ii) non-leaky deposits which are formed in areas of subsurface faults, and show a relatively homogeneous distribution of gas hydrates. Site NGHP-01-10 is a good example of leaky gas hydrate deposits above a deep-seated gas reservoir.