Heat flow and gas hydrate saturation estimates from Andaman Sea, India.

Shankar, Uma and Riedel, Michael (2013) Heat flow and gas hydrate saturation estimates from Andaman Sea, India. Marine and Petroleum Geology, 43 . pp. 434-449. DOI 10.1016/j.marpetgeo.2012.12.004.

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Abstract

Gas hydrate was recovered in the Andaman Sea along the eastern coast of the Andaman Islands during the India National Gas Hydrate Program (NGHP) Expedition-01 at Site NGHP-01-17. Coring confirmed gas hydrate occurs predominantly in discrete volcanic ash layers. Pore water chemistry, electrical resistivity and P-wave velocity logs are used to estimate gas hydrate saturations at Site NGHP-01-17. Gas hydrate saturation estimated from chloride concentrations shows values up to ∼85% of the pore space for distinct ash layers from ∼270 m below seafloor to the base of gas hydrate stability zone (BGHSZ). Gas hydrate saturations estimated from the electrical resistivity and acoustic velocity logs using standard empirical relations and modeling approaches are comparable to each other, but saturations are only ∼20% of the pore space on average. This much lower gas hydrate saturation estimate from the log data is a result of overall reduced resolution of the logging tools relative to the typically 20–30 cm thick hydrate-bearing ash layers. Available 2D multi-channel seismic data were also analyzed and a bottom-simulating reflector (BSR) was imaged along several seismic profiles. The depth of the BSR is more than 600 m along the seismic line crossing Site NGHP-01-17, which makes this one of the deepest BSRs observed worldwide. To understand the unusual depth of the BSR, we mapped its depth and estimated heat flow from the BSR depth using a simple conductive model. BSR-derived heat flow values range from ∼12 to ∼41.5 mW/m2 from the study area and follow the bathymetry trend of dominant North–South ridges and can be explained with the east-ward trending increase in heat-flow toward the current seafloor spreading center. We also modeled the BGHSZ to analyze the linkage between gas hydrate occurrences in the Andaman Sea and its relation to the tectonic activity. Our analysis suggests an extensively variable BGHSZ in the Andaman Sea controlled mainly by overall low geothermal gradients. Consistent local minor variations were observed with lower heat flow values over prominent topographic highs and higher values in valleys/troughs due to focusing and defocusing effects of the topography.

Document Type: Article
Keywords: Gas hydrates; Andaman Sea; Electrical resistivity log; Archie analysis; P-wave velocity logs; Rock physics modeling; Gas hydrate saturation; Heat flow
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.marpetgeo.2012.12.004
ISSN: 0264-8172
Date Deposited: 19 Oct 2015 08:24
Last Modified: 19 Oct 2015 08:24
URI: http://oceanrep.geomar.de/id/eprint/30059

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