Velocity modeling and attribute analysis to understand the gas hydrates and free gas system in the Mannar Basin, India

Abstract

The Mannar basin, located between India and Sri Lanka, is poorly explored for hydrocarbon compared to other petroliferous basins in the Indian offshore. 2D multichannel seismic data was acquired onboard R/V Samudra Ratnakar for exploring gas hydrate deposits in this basin. The bottom-simulating reflectors (BSRs) are observed in the seismic section, indicating the possible gas hydrate deposit. Several pull-down structures are identified below the BSR, suggesting the presence of free gas-bearing sediments. We also observed some chimney-like features, which extend from a deeper horizon up to the seafloor. An automated version of migration velocity analysis was used to estimate P-wave velocities along selected seismic lines. High interval velocities (>1.7 km/s) are observed above the BSR for ~100 ms TWT across the chimney locations, indicating the presence of gas hydrates for the first time in the Mannar basin. Low velocities are observed below the BSR, indicating a widespread region of free gas-bearing sediments. Seismic attributes (instantaneous amplitude and frequency) analysis are used to demarcate the extent of these regions. Chimneys are identified as vertical zones of reduced amplitude, while the gas-bearing sediments below the BSR show high amplitudes and low-frequency. A deeper horizon ~3.3 s TWT shows signatures associated with a deep-seated gas reservoir. We propose that chimneys and small sub-vertical fractures provide pathways for the migration of deep-seated gases to shallower regions. The hydrate deposits of the Mannar basin are unique as the organic-rich sediments are not supplied through any major rivers. Instead, they are derived from the high biomass due to the unusual physiography of the Mannar basin.