Characteristics of periodically active cold seep-related gas hydrate systems in the Krishna-Godavari offshore basin, India

In the Krishna-Godavari (K-G) offshore basin, India, a 130 m thick fracture-filling and near-seafloor paleo-cold seep-related gas hydrate-bearing layer (GHBL) was encountered by drilling at Site NGHP-01-10 (Site 10) and nearby piston sampling of authigenic carbonates and shells. Our analyses of drilling cores and pore-water show that authigenic carbonates and shells are widely distributed within 200 mbsf at Site 10, with two separate intervals of high chloride concentrations up to 663 mM. This indicates that the GHBL is a young system of multistage formation related to periodically active cold seeps. This study combines core, well logging and seismic data to gain insight into the fine characteristics and detailed formation process of such a thick system. Seismic imaging of new chimney-like structures, growth faults and multiple stacked mass transport deposits (MTDs) illustrates that the system is located in the chaotic reflection strata. Synthetic seismogram shows that multiple MTDs repeatedly control the paleo-cold seeps and further influence the hydrae system. Based on a buried vent with a high amplitude reflection consistent with seafloor polarity, and its high density and high velocity similar to authigenic carbonates, a new and larger paleo-cold seep-related hydrate system is defined to the southeast of Site 10. These two thick systems probably formed in stages due to the clear stratifications on the seismic data, 2D anisotropic saturations and internal chimney-like structures. They are originated from diapirism and growth faulting, and their lateral extent depends on the fracture zone width of the anticline ridge. After formation, they are then buried by multiple MTDs and have already been upshifted by sedimentation. Although the cold seep near Site 10 is not active and the hydrate system is currently only in the chloride diffusion stage, the underlying gas accumulation means that new hydrate systems and cold seeps may form in the future. Our results suggest that the processes of formation, sedimentation, upward shift and diffusion of hydrate systems have been circulating near Site 10, which could better interpret the formation and dynamic evolution of the multilayered or thick GHBL found at drill sites around the world.