Seismic chimneys as gas migration pathways and their role in hydrate accumulation: AVA insights from the Cauvery-mannar basin
Date
2025Author
Mishra, C.K.
Dewangan, P.
Sriram, G.
Masood, P.A.
Arun, S.K.
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Gas hydrates have been identified in the Cauvery-Mannar Basin, a passive rift setting largely devoid of recent tectonic activity as evidenced by seismic data. This study provides novel insights into gas migration pathways from deep-seated reservoirs to the shallow subsurface, with particular emphasis on the role of seismic chimneys as focused conduits for fluid and gas transport. The Bottom Simulating Reflector (BSR), a key seismic indicator of gas hydrates, is observed intermittently across multiple seismic profiles, underlain by extensive gas-charged sediments. A prominent feature in one of the seismic profile is a chimney-like structure, ?650 m tall and 1000 m wide, originating from deeper gas-rich strata at depth. Formed through overpressure-induced hydrofracturing, the chimney acts as a conduit for vertical gas migration from deep-seated gas reservoirs to the Gas Hydrate Stability Zone (GHSZ). At the base of GHSZ, hydrates form impermeable layers that restrict vertical migration of free gas and promote lateral migration. Amplitude variation with angle (AVA) analysis reveals strong Class III anomalies in the updip direction of BSR, indicating preferential lateral gas migration along the base of GHSZ. Class IV anomalies observed in the updip region may signify high-concentration gas hydrates or fracture-filled deposits formed through focused gas accumulation. In this tectonically quiescent setting, hydrostatic pressure gradient drives both vertical and lateral gas migration, transporting gas from deep reservoirs to the shallow subsurface in the Mannar basin. These findings have important implications for understanding gas hydrate systems in other passive continental margin environments.
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