Anomalous porosity preservation and preferential accumulation of gas hydrate in the Andaman accretionary wedge, NGHP-01 site 17A

This paper is not subject to U.S. copyright. The definitive version was published in Marine and Petroleum Geology 58A (2014): 99-116, doi:10.1016/j.marpetgeo.2014.04.009.In addition to well established properties that control the presence or absence of the hydrate stability zone, such as pressure, temperature, and salinity, additional parameters appear to influence the concentration of gas hydrate in host sediments. The stratigraphic record at Site 17A in the Andaman Sea, eastern Indian Ocean, illustrates the need to better understand the role pore-scale phenomena play in the distribution and presence of marine gas hydrates in a variety of subsurface settings. In this paper we integrate field-generated datasets with newly acquired sedimentology, physical property, imaging and geochemical data with mineral saturation and ion activity products of key mineral phases such as amorphous silica and calcite, to document the presence and nature of secondary precipitates that contributed to anomalous porosity preservation at Site 17A in the Andaman Sea. This study demonstrates the importance of grain-scale subsurface heterogeneities in controlling the occurrence and distribution of concentrated gas hydrate accumulations in marine sediments, and document the importance that increased permeability and enhanced porosity play in supporting gas concentrations sufficient to support gas hydrate formation. The grain scale relationships between porosity, permeability, and gas hydrate saturation documented at Site 17A likely offer insights into what may control the occurrence and distribution of gas hydrate in other sedimentary settings.The financial support for the NGHP01, from the Oil Industry Development Board, Oil and Natural Gas Corporation Ltd., GAIL (India) Ltd. and Oil India Ltd. is gratefully acknowledged. We also acknowledge the support extended by all the participating organizations of the NGHP: MoP&NG, DGH, ONGC, GAIL, OIL, NIO, NIOT, and RIL