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

N. K. Thakur ¹, Nandini Nagarajan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Tectonics controlled evolution of deep crustal exposures have recently attracted much attention. The southern part of Peninsular India, consisting of highgrade metamorphic rocks, sandwiched between converging rifted margins, shows characteristically different geophysical signatures as compared to other geologically similar provinces. Based on geophysical anomalies and the tectonic framework of the region it is proposed that the subsurface structural configuration beneath the granulite terrain of South India has been perturbed by Meso-Cenozoic rifting at the continental margins and consequent crust-mantle dynamics.

Keywords

Deep Crust, Meso-Cenozoic Rifting, South India.

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M. Simhadri Naidu ¹, P. Prasada Rao ¹, B. Ashalatha ¹, N. K. Thakur ¹

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

Abstract

The Kerala-Konkan Basin on the Western continental margin of India (WCMI) with 2 to 3 km maximum sediment thickness and suitable percentage of Total Organic Content (TOC) provides an interesting avenue for the exploration of gas hydrates. The identification of characteristic feature i.e. Bottom Simulating Reflectors (BSRs) in the multi-channel seismic data provides the inference about the presence of gas hydrates in a region. However, in absence of one of the main characteristics of BSR (i.e. cross-cutting with the geological strata) in the Kerala-Konkan Basin, we have examined other evidences such as venting, pockmarks, faulting, blanking and diapirs. The seismic sections show several deep faults which can be traced up to the seafloor in a few locations. Shallow faults within the sediments in the vicinity of deep faults have been observed. The presence of several deep and shallow faults provides channels for vertical gas-fluid migration contributing to the methane in the shallow sediments. The region is characterized by compression related diapiric structures bounded by faults. The columnar fractures occurring below the seafloor and depressions are interpreted as fluid/ gas escape features. Over-pressured sediment structures, large amplitude reflections within the sand layer indicate that gas is probably abundant in this area. The present study deals with analysis of geological and tectonic processes and the structural controls derived from the re-interpretation of seismic reflection sections for understanding the formation of gas hydrates in the Kerala-Konkan Basin.

Keywords

Gas Hydrate, Faults, Fluid/Gas Migration, Kerala-Konkan Basin.

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A. R. Sridhar ¹, H. C. Tewari ¹, V. Vijaya Rao ¹, N. Satyavani ¹, N. K. Thakur ¹

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

Abstract

A five-layered crustal model with 5.9-6.2, 6.0, 6.3, 6.6 and 7.0 kms^-1velocities is derived with seismic refraction/wide-angle reflection data along the Thuadara-Sendhwa-Sindad profile in central India. The profile traverses across the western part of the Narmada-Son lineament in the N-S direction. The data are subjected to 2-D forward modelling using both travel times and amplitudes. Refracted waves (P_n from the Moho observed as first arrivals at a distance of about 200 km are used to derive the upper mantle velocity of 8.1 kms-'. The Moho is well constrained both from the P_n and wide-angle reflections from four shot points. The main features of the velocity structure is the delineation of a low velocity layer (6.0 kms^-1) in the upper crust and a 12-16 km thick high velocity (7.0 kms^-1) layer at the base of the crust. The high velocity lower crustal layer, representing the magmatic underplating in the region, may be related to the formation of the Narmada Basin and the Deccan volcanic episode. The Deccan volcanism may be a consequence of the Reunion mantle plume activity. The crustal thickness varies between 37 and 43 km along the profile and the thickest crust is found between the Narmada and Tapti rivers. The gravity mode1 constrained from the seismic velocity structure corroborates the crustal structure. Deep-seated faults responsible for the evolution of Narmada Basin are inferred from the present study.

Keywords

Underplating, Deccan volcanism, Wide-Angle reflection, Low velocity layers, Moho, Narmada-Son lineament, Central India.

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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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