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

Abstract

In the Krishna-Godavari (K-G) offshore basin, India, a 130 m thick fracture-filling gas hydrate-bearing layer (GHBL), associated with near-seafloor paleo-cold seep activity, was identified during drilling and coring at Site NGHP-01-10 (Site 10). Our new analyses of drilling cores and pore-water show that authigenic carbonates and shells are present throughout the upper 200 mbsf at Site 10, with two separate intervals of high chloride concentrations up to 663 mM. It indicates a periodically active cold seep and relatively young hydrate system. This study combines core, well log and seismic data to gain insight into the fine characteristics and detailed formation process of such a thick paleo-cold seep and hydrate system. Seismic imaging of newly interpreted chimney-like structures, growth faults and multiple stacked mass transport deposits (MTDs) illustrates that the system is located within a geologically disturbed sediment zone. Synthetic seismogram-derived time-depth relationship between seismic and core data shows that multiple MTDs repeatedly control the paleo-cold seeps and further influence the hydrate. A new paleo-cold seep and hydrate system is identified southeast of Site 10, where seismic data reveal a buried vent characterized by a high amplitude reflection consistent with seafloor polarity, along with high density and velocity indicative of authigenic carbonates. 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 within the anticline ridge. After formation, the process of hydrate recycling is triggered by the sedimentation and has led to the upward shift of the system. Our findings indicate that there exists a periodically active cold seep and gas hydrate system at Site 10. The activity of this periodic system can account for the formation of the multilayered or thick GHBL, and facilitate an understanding of the evolution of the paleo-cold seep found around the world. Although the cold seep at Site 10 is not active and the hydrate is currently only in the chloride diffusion stage, the underlying gas accumulation means that new cold seep and hydrate systems may form in the future.