Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Uma Shankar ¹, Maheswar Ojha ¹, Kalachand Sain ¹, Ramesh Khanna ¹, M. Sudhakar ², Abhishek Tyagi ²

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN
2 National Center for Antarctic and Ocean Research, Headland Sada, Goa-403804, IN

Abstract

New data have been collected using the multibeam echosounder (Hydrosweep) and high resolution subbottom profiler (Parasound) systems in deep water of the Western Continental Margin of India (WCMI) during the 41^st cruise of R/V Academic Boris Petrov from 17 to 26 November, 2006. The six meters gravity coring along with CTD (Conductivity-Temperature-Depth) measurements are also carried out. Two sites in the Saurashtra and Kerala-Konkan offshore basins have been covered to find out features related to gas hydrates during this short cruise. High resolution multibeam echosounder and sub-Bottom profiling delineate the fine-Scale structure of the sedimentary layer of about 50-100 m thickness below the seafloor. Gravity corer is operated at five stations, out of which four gravity cores of more than 5 m length are recovered successfully. Gas and pore waters from cores have been collected for performing the laboratory studies. The Rosette system is used for temperature and salinity measurement in the water columns. The gravity cores collected on sites show the evidence of sediment fluidization and contain certain amount of gas. The gas is collected from sediments using the technique developed in the V.I.Vernadsky Institute of Geochemistry and Analytical Chemistry for the chemical and isotopic analysis for future research. The preliminary results show that the continental slope and rise of the oceanic margin of the Western India are prospective for exploration of gas hydrate. The more definite conclusion can be drawn after carrying out laboratory studies.

Keywords

WCMI, Gas Hydrates, Hydrosweep, Parasound, and CTD.

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Uma Shankar ¹, Kalachand Sain ¹, Michael Riedel ²

Affiliations
1 CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Natural Resources Canada, Pacific Geoscience Center, Geological Survey of Canada, 9860 W. Saanich Rd. Sidney, B.C. V8L 4B2, CA

Abstract

The passive eastern Indian margin is rich in gas hydrates, as inferred from the wide-spread occurrences of bottom-simulating reflectors (BSRs) and recovery of gas hydrate samples from various sites in the Krishna Godavari (KG) and Mahanadi (MN) basins drilled by the Expedition 01 of the Indian National Gas Hydrate Program (NGHP). The BSRs are often interpreted to mark the thermally controlled base of gas hydrate stability zone (BGHSZ). Most of the BSRs exhibit moderate to typically higher amplitudes than those from other seismic reflectors. We estimate the average geothermal gradient of ∼400C/km and heat flow varying from 23 to 62 mW/m²in the study area utilizing the BSR's observed on seismic sections. Further we provide the BGHSZ where the BSR is not continuous or disturbed by local tectonics or hidden by sedimentation patterns parallel to the seafloor with a view to understand the nature of BSR.

Since, gas hydrate bearing sediment has higher electrical resistivities than that of the host sediment, we estimate two levels of gas hydrates saturations up to 25% in the depth interval between 70 to 82, and less than 20% in the depth interval between 90 to 104 meter below the seafloor using the resistivity log data at site 15 of NGHP-01.

Keywords

Gas Hydrates, Bottom Simulating Reflectors, Geothermal Gradient, Resistivity Log, Saturation, KG Basin, Eastern Indian Margin.

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