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

K. L. Kaila ¹, Kalachand Sain ²

Affiliations
1 2-16-106, Prashantinagar, Uppal Road, Hyderabad - 500 039, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The Indian subcontinent has been divided into six crustal velocity zones based on P-wave velocity structure obtained along 20 Deep Seismic Sounding (DSS) profiles, covered in India from 1972 to 1992. Zone - 1 consists of two parts: zone  - 1.a comprises the Dharwar craton, Cuddapah basin, Godavari graben and the Koyna region, and zone- 1 .b consists of the western margin of the Bengal basin and the Singhbhum craton. Zone - 2 covers the West Bengal sedimentary basin and the Mahanadi coastal basin. Zone - 3 includes Bundelkhand craton and the Vindhyan basin. Zone - 4 covers Cambay basin north of the Narmada lineament and the Aravalli - Delhi fold belt. Zone - 5 lies south of the Nannada lineament covering the Tapti graben extending up to the west coast of India. Zone - 6 comprises the northwest Himalaya up to the Nangaparbat including Jammu and Srinagar sedimentary basins. Detailed crustal velocity structures in these six zones have been described with their bearing on regional tectonics. Velocity structure of the lower crust in some of these zones, suggests that crustal underplating has played a very important role in the evolution of the Indian continental crust and lithosphere. Some petrological inferences have also been drawn for the Indian crust from crustal velocity zonation.

Keywords

Geophysics, Crustal Velocity Structure, Tectonics, Indian Subcontinent.

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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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kalachand Sain ¹

Affiliations
1 Gas Hydrate Project, NGRI, Hyderabad, IN

Abstract

No Abstract.

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Kalachand Sain ¹, Harsh Gupta ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad-500007, IN
2 National Geophysical Research Institute, Uppal Road, Hyderabad-500007, IN

Abstract

At present India produces only one-third of her oil requirements. The escalating demand for energy and the rising price of oil are compelling factors to look for an alternate source of energy for sustainable growth. Gas-hydrates, crystalline form of water and methane, seem to be a viable source of energy. Bathymetry, seafloor temperature, sedimentary thicknesses, rate of sedimentation and total organic carbon (TOC) indicate good prospects of gas-hydrates within the vast offshore regions of India. The energy potential of gas-hydrates is estimated to be twice the energy contained in the total fossil fuel reserves. Several oil companies and national institutes are engaged in gas-hydrate investigations making use of geophysical, geochemical, geological and microbiological data. Based on analysis of available seismic data, geological characteristics, geochemical and microbiological proxies, gas-hydrates have been identified in the continental margins of India (the Bay of Bengal and the Arabian Sea). The drilling and coring under the auspices of the Indian National Gas Hydrates Program (NGHP) has validated the ground truth in the Krishna-Godavari (K-G) and Mahanadi basins and in the Andaman region of the eastern offshore. It is necessary to delineate the gas-hydrates and free-gas bearing sediments and evaluate the resource potential in prospective areas. The National Geophysical Research Instimte (NGRI) has built requisite expertise in special processing, modelling and inversion of marine seismic data based on traveltime tomography; waveform inversion; amplitude versus offset (AVO) inversion; AVO attributes and pre-stack depth migration coupled with effective medium theory or rock physics modelling. The blanking, reflection strength, instantaneous frequency and attenuation attributes have been found to be vital for identifying gas-hydrates without BSR and/or ascertaining whether a BSR is related to gas-hydrates. The traveltime tomography of large offset multi-channel or ocean bottom seismic data can be used for demarcating the zone of gas-hydrates and free-gas bearing sediments. The pre-stack depth migration of seismic data using the large wavelength velocity tomograms may help to understand the genesis of gas-hydrates. All these approaches and their application to available marine seismic data are presented here with a view to investigate gas-hydrates along the continental margins of India.

Keywords

Gas-Hydrates, Free-Gas, BSR, Identification, Estimation, Indian Margins.

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Laxmidhar Behera ¹, Kalachand Sain ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The average crustal velocity and Moho depth, estimated from the data of more than 5000 km deep seismic sounding (DSS) profiles and a number of broadband seismological stations covering various geological and tectonic units of the Indian shield have been presented. The average crustal velocity varies from 62 to 65 km/s except at few anomalous regions. The average crustal thickness varies from 40 to 42 km except in Dharwar craton (35-45 km) and southernmost part of the southern granulite terrain (35-38 km). The crust is thinner towards both eastern and western coasts of the shield. The crust to the north of the west coast is again thinner (-25 km) than that of the east coast (-35-37 km) with a prominent up-Warp at the eastern flank of the West-Bengal Basin (-30-32 km). The shallowing of the crust towards the coast is due to underplating and breakup of the Indian shield from the Gondwanaland.

Keywords

DSS, Broadband Seismology, Moho, Crustal Structure, Indian Shield.

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C. Subrahmanyam ¹, S. I. Reddi ¹, N. K. Thakur ¹, T. Gangadhara Rao ¹, Kalachand Sain ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

Large-scale occurrence of gas-hydrates in the marine sediments of outer continental margins is by now well-known. While the methods of tracing gas-hydrate enriched zones, like identifying markers such as bottom simulating reflectors and blanking zones on seismic reflection sections, are well-established, quantification and resource assessment of gas hydrates still remain hazy. Investigations over the classical gas-hydrate enriched Blake outer ridge on the U.S. Atlantic margin have provided new insights into the environs of gas-hydrate occurrences. Deep sea drilling results in this area indicate that the estimates of gas-hydrate from geophysical measurements could be less by an order of three in comparison with those obtained from drilling. Several factors like bathymetry, sediment thickness and sedimentation rates, sea bottom temperature and total organic carbon content, which control gas-hydrate formation, indicate that the shallow sediments in the offshore regions of India could be potentially enriched in gas hydrates. National agencies like the Oil & Natural Gas Corporation, Gas Authority of India Ltd., National Geophysical Research Institute, National Institute of Oceanography and others are now actively involved in assessing the true potential of this anticipated future energy resource. The environmental aspects of gas hydrates like large-scale release of methane gas into the air and slope instabilities and submarine landslides in the continental margins resulting in catastrophic events, however, need to be kept in mind.

Keywords

Energy Resources, Gas-Hydrates, Global Distribution, Indian Offshore Occurrences.

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Praveen Kumar Singh ¹, Kalachand Sain ¹

Affiliations
1 CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

The existence of gas-hydrates in marine sediments increases the seismic velocity, whereas even a small amount of underlying free-gas reduces the velocity considerably. The change in velocities against the background (without gas-hydrates and free-gas) velocity can be used for identification and assessment of gas-hydrates. Traveltime inversion of identifiable reflections from large offset multi channel seismic (MCS) experiment is an effective method to derive the 2-D velocity structure in an area. We apply this method along a seismic line in the Kerala-Konkan (KK) offshore basin for delineating the gas-hydrates and free-gas bearing sediments across a bottom simulating reflector (BSR). The result reveals a four layer 2-D shallow velocity model with the topmost sedimentary layer having velocity of 1,680-1,740 m/s and thickness of 140-190 m. The velocity of the second layer of uniform thickness (110 m) varies from 1,890 to 1,950 m/s. The third layer, exhibiting higher velocity of 2,100-2,180 m/s, is interpreted as the gas-hydrates bearing sediment, the thickness of which is estimated as 100 to 150 m. The underlying sedimentary layer shows a reduction in seismic velocity between 1,620 to 1,720 m/s. This low-velocity layer with 160-200 m thickness may be due to the presence of free-gas below the gas-hydrates layer.

Keywords

Gas-Hydrates, Free-Gas, Velocity, BSR, Traveltime Inversion.

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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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Kalachand Sain ¹, Maheswar Ojha ¹, Nittala Satyavani ¹, G. A. Ramadass ², T. Ramprasad ³, S. K. Das ⁴, Harsh Gupta ¹

Affiliations
1 CSIR - National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 National Institute of Ocean Technology, Velachery-Tambaram Main Road, Chennai - 600 100, IN
3 CSIR-National Institute of Oceanography, Dona Paula, Goa - 403 004, IN
4 Ministry of Earth Sciences, Prithvi Bhavan, Lodhi Road, New Delhi - 110 003, IN

Abstract

Gas-hydrates are crystalline substances consisting of mainly methane and water, and occur in shallow sediments of outer continental margins and permafrost regions. They are formed at high pressure and low temperature regime when supply of methane gas exceeds the solubility limit. Unlike natural gas, oil and minerals, gas-hydrates are not stable at standard temperature and pressure (STP). One volume of gas-hydrates, when dissociated, releases 164 volumes of methane at STP. Since methane is the lowest molecular weight hydrocarbon, use of gas-hydrates as fuel will cause less pollution to the environment. These have attracted the global attention due to their natural occurrences in abundance and huge energy potential. The methane locked as gas-hydrates is envisaged as 1-120 x 10¹⁵ m³ (Boswell and Collett, 2011). Only 15% recovery from this gigantic reserve may be sufficient to meet the global energy requirement for about 200 years (Makogon et al. 2007). Thus, gas-hydrates seem to be a viable major energy resource of future, and have been identified globally either by geophysical, geochemical and geological surveys or by drilling and coring (Boswell and Saeki, 2011; Ruppel, 2011; Sain and Gupta, 2012). Besides having the energy potential, the study of gas-hydrates is also important from natural hazards point of view related to seafloor subsidence, slumps and slides (Gupta and Sain, 2011).

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