Tectonic influence on gas hydrate stability zone thickness: ‐Seismic study from Indian offshore

Abstract

Deep offshore areas of Andaman forearc basin and Krishna-Godavari basin are the two Indian areas where distinct Bottom Simulating Reflector (BSR), the seismic indicator of gas hydrates is observed. The tectonic settings of Andaman and Krishna-Godavari offshore are very diverse. Krishna-Godavari is a rifted passive margin basin in the east coast of India whereas Andaman forearc basin is in the intra-oceanic subduction zone. An interesting aspect of BSR identified in these two areas, is the large difference in the depth of occurrence of BSR below seafloor. As BSR is the seismic reflector from the base of hydrated layer, the depth of BSR below seafloor is the thickness of the hydrate stability zone (HSZ). It was noticed from seismic sections that depth of BSR below seafloor in Andaman offshore is almost double as that of Krishna-Godavari area for the identical water depths. An effort is made in this study to understand this large difference in the hydrate stability zone thickness between these two areas. The major factors, which affect the thickness of hydrate stability zone, are sea bottom temperature, geothermal gradient and composition of gases forming hydrates. Analysis of available data from both these areas rules out the possibility of large difference in HSZ thickness on account of variations in sea bottom temperature and composition of gases. Available well data show that geo-thermal gradient in Andaman offshore is much lower in comparison to that of Krishna-Godavari offshore. This low geothermal gradient in Andaman is due to the relatively cool plate being suducted. Hence on the basis of available data it can be concluded that the higher thickness of hydrate stability zone in Andaman can be attributed to lower geothermal gradient, which in turn is dependent on the tectonic setting. The present study of HSZ thickness of two tectonically diverse offshore basins has brought out the influence of tectonic settings on gas hydrate stability zone thickness.