Origin and distribution of shallow gas-charged sediment on the inner continental shelf of central west coast of India

Abstract

High-resolution shallow seismic investigations on inner continental shelf of Goa have been carried out to map gas-charged sediment zones and investigate origin of trapped gas. Analyses of data reveal that the gas is trapped by different stratigraphic layers and is distributed in two zones. Zone-1 is restricted up to approx. 30 m water depth and in the southern part of the study area. In this zone, gas is confined within the Holocene layers above the Holocene maximum Hooding surface (MFS). Whereas, zone-2 is comprised of several isolated gas-charged patches which are distributed in the entire study area mostly between approx. 20 m and approx. 45 m water depth. In this zone, gas is confined below Holocene-MFS in incised river bed or a layer just below MFS or both. Gas-charged sediment in zone-1 is formed due to degradation of organic material which is predominantly early Holocene mangrove derived, whereas, zone-2 it is formed due to degradation of organic material which were deposited by ancestral rivers during the post-glacial sea level rise of the Late Pleistocene. The distribution pattern of zone-1 is controlled by coastline configuration, long-term longshore current pattern, Holocene rapid sea level rise and lithological character of exposed inter-tidal zone. Whereas, distribution of zone-2 is controlled by palaeo-rivers of different glacial periods and their hydraulic energy condition during the base level rise.