Probable Occurrence of Gas Hydrates Along the Continental Margins of India

The gas hydrates have gained the importance of being potential to be an alternate energy resource and are know to occur world wide in the sediments of the outer continental margins and polar regions associated with permafrost and are stable under certain temperature-pressure conditions. A gas hydrate stability zone (GHSZ) thickness map of Indian offshore has been created from spatial analysis of physical parameters that influence the formation and preservation of gas hydrates by using Arc/Info GIS software. These parameters include bathymetry and heatflow/geothermal gradient data obtained from published literature, digital databases and unpublished sources, and the seabed temperature from extrapolation of the best-fit polynomial of hydrothermal profile. In addition to the measured heatflow/geothermal data, the values were predicted in the regions devoid of data by considering the Indian offshore comprising of three provinces such as: oceanic, stretched continental crust and the crust influenced by the hotspot. Heatflow versus age relationship is used in case of oceanic crust in Arabian Sea, Bay of Bengal, and Andaman Sea in the regions beyond the shelf edge. The stretched crust criterion is applied only in the northern Arabian Sea between shelf and the inferred oceanic crust. A regression line is fitted between the available heatflow data and the corresponding age of the crust to compute heatflow values, in the region where the crust influenced by the Reunion hotspot. Seabed temperature data were also predicted in deficient areas based on the regression analysis of available bathymetry versus seabed temperature relationship. The thickness of gas hydrate stability zone was computed both in TWT (msec) and meters, by solving the gas hydrate phase and pressure-temperature equations at each GIS grid node. The GHSZ thickness map is reliable to the extent it can be used as an important indicator for gas hydrate potential of an area. Large volumes of multi-channel seismic reflection data acquired by oil majors in India have been examined and inferred the probable presence of bottom simulating reflections (BSR) in the offshore areas of India. The depths of the BSRs on several seismic profiles in the offshore regions of Goa and Saurashtra (west coast), Krishna-Godavari and Mahanadi (east coast) of India and the Northern Andaman Sea are within the zone of computed gas hydrate stability. Assuming the depth of BSR observed from seismic data as the base of the gas hydrate stability zone, the geothermal gradient values have been computed. Most of the values are in close agreement with the observed geothermal gradient data. Sediment thickness and Total Organic Carbon (TOC) content maps were prepared from the compiled data, which were used as constraints in assessing the GHSZ thickness and the probable occurrence of gas hydrates in the marine sediments in Indian offshore. The non-geophysical proxies, derived from the analysis of 5m long sediment cores, such as chlorine anomalies, reduction trend of sulphate concentration, enrichment of methane and abundance of sulfate reducing bacteria, fomenters and nitrate reducing bacteria with depth provide indirect clue for the occurrence of gas hydrates at deeper depths. The high-resolution seismic data and sediment cores longer than 20m, perhaps would enhance the understanding of occurrence of gas hydrates in the marine sediments.