Scenario of gas-charged sediments and gas hydrates in the western continental margin of India

Abstract

Echosounding, high-resolution shallow seismic data were collected along track lines spaced at 20 km interval across the western continental margin of India. A detailed analysis of the underway data revealed the occurrence of methane-bearing gas-charged sediments and pockmarks in the shelf and gas hydrate horizons along the slope and rise. The presence of the zones of incoherent reflections accompanied by lack of acoustic penetration and the presence of discontinuous high-intensity reflections in the form of acoustic masking are seen extending from the seafloor down to 5-10 m. Some are dome-shaped extending tot he seafloor as observed off Coondapur and south of Mumbai. Others are inverted U-shaped acoustic masking with rounded edges discernible off Karwar. Mormugao and Mumbai. Significant contributions of gas from the slope sediments to the upper strata and the overlying waters are indicated by buried/exposed pockmarks and the prominent plumes are represented by strong echoes in the sediments. These pockmarks occur as conical and dish-shaped incisions often truncating the strata above the sub-bottom reflector. Distinct gas plumes rise from a few meters to about 70 m above the sea floor. The existence of an acoustic turbid zone beneath the pockmarks and a continuous supply of gas from below towards the pockmarks indicate the sediments as gas-charged. The presence of bottom simulating reflectors (BSRs) in slope-rise areas off Goa suggest the presence of gas hydrates in water depths is greater than 500-2200 m. Over the Laccadive ridge complex, BSR lies at about 460 milliseconds below the seabed where the water depth is found to be nearly 2000 m. At places, it found that chaotic and /or scattered hyperbolic reflections occur below BSR suggesting the presence of gas-charged sediments or free gas while the distinct blanking zones as well as acoustic voids appear above the BSR. It is concluded that the venting of gas causes the presence of pockmarks and seeps. Formation of gas hydrates in the deeper waters are probably caused by the rapid sedimentation and organic carbon-rich sediments coupled with an environment of high pressure and low temperature.