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Estimating the composition of gas hydrate using 3D seismic data from Penghu Canyon, offshore Taiwan

Estimating the composition of gas hydrate using 3D seismic data from Penghu Canyon, offshore Taiwan
Estimating the composition of gas hydrate using 3D seismic data from Penghu Canyon, offshore Taiwan

Direct measurements of gas composition by drilling at a few hundred meters below seafloor can be costly, and a remote sensing method may be preferable. The hydrate occurrence is seismically shown by a bottom-simulating reflection (BSR) which is generally indicative of the base of the hydrate stability zone. With a good temperature profile from the seafloor to the depth of the BSR, a near-correct hydrate phase diagram can be calculated, which can be directly related to the hydrate composition. However, in the areas with high topographic anomalies of seafloor, the temperature profile is usually poorly defined, with scattered data. Here we used a remote method to reduce such scattering. We derived gas composition of hydrate in stability zone and reduced the scattering by considering depth-dependent geothermal conductivity and topographic corrections. Using 3D seismic data at the Penghu canyon, offshore SW Taiwan, we corrected for topographic focusing through 3D numerical thermal modeling. A temperature profile was fitted with a depth-dependent geothermal gradient, considering the increasing thermal conductivity with depth. Using a pore-water salinity of 2%, we constructed a gas hydrate phase model composed of 99% methane and 1% ethane to derive a temperature depth profile consistent with the seafloor temperature from in-situ measurements, and geochemical analyses of the pore fluids. The high methane content suggests predominantly biogenic source. The derived regional geothermal gradient is 40°C km-1. This method can be applied to other comparable marine environment to better constrain the composition of gas hydrate from BSR in a seismic data, in absence of direct sampling.

BSR, Gas hydrates, Methane, Penghu channel, Taiwan
1017-0839
105-115
Sahoo, Sourav Kumar
6dab0376-36df-44c5-9f36-cb4a29d9b03b
Chi, Wu Cheng
bd2055f0-2c58-4f68-9195-a5c2728f3667
Han, Wei Chung
546831f1-9f93-4912-9403-51aec105f043
Chen, Liwen
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Liu, Char Shine
847a5109-8a90-4f20-ab2a-a59da1f978c2
Wang, Yunshuen
4559a5a1-3411-4ea3-9ddd-9f034c4b0925
Sahoo, Sourav Kumar
6dab0376-36df-44c5-9f36-cb4a29d9b03b
Chi, Wu Cheng
bd2055f0-2c58-4f68-9195-a5c2728f3667
Han, Wei Chung
546831f1-9f93-4912-9403-51aec105f043
Chen, Liwen
060ed1c9-bd67-43a5-ab62-5b03869dc30f
Liu, Char Shine
847a5109-8a90-4f20-ab2a-a59da1f978c2
Wang, Yunshuen
4559a5a1-3411-4ea3-9ddd-9f034c4b0925

Sahoo, Sourav Kumar, Chi, Wu Cheng, Han, Wei Chung, Chen, Liwen, Liu, Char Shine and Wang, Yunshuen (2018) Estimating the composition of gas hydrate using 3D seismic data from Penghu Canyon, offshore Taiwan. Terrestrial, Atmospheric and Oceanic Sciences, 29 (2), 105-115. (doi:10.3319/TAO.2017.09.11.01).

Record type: Article

Abstract

Direct measurements of gas composition by drilling at a few hundred meters below seafloor can be costly, and a remote sensing method may be preferable. The hydrate occurrence is seismically shown by a bottom-simulating reflection (BSR) which is generally indicative of the base of the hydrate stability zone. With a good temperature profile from the seafloor to the depth of the BSR, a near-correct hydrate phase diagram can be calculated, which can be directly related to the hydrate composition. However, in the areas with high topographic anomalies of seafloor, the temperature profile is usually poorly defined, with scattered data. Here we used a remote method to reduce such scattering. We derived gas composition of hydrate in stability zone and reduced the scattering by considering depth-dependent geothermal conductivity and topographic corrections. Using 3D seismic data at the Penghu canyon, offshore SW Taiwan, we corrected for topographic focusing through 3D numerical thermal modeling. A temperature profile was fitted with a depth-dependent geothermal gradient, considering the increasing thermal conductivity with depth. Using a pore-water salinity of 2%, we constructed a gas hydrate phase model composed of 99% methane and 1% ethane to derive a temperature depth profile consistent with the seafloor temperature from in-situ measurements, and geochemical analyses of the pore fluids. The high methane content suggests predominantly biogenic source. The derived regional geothermal gradient is 40°C km-1. This method can be applied to other comparable marine environment to better constrain the composition of gas hydrate from BSR in a seismic data, in absence of direct sampling.

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Accepted/In Press date: 11 September 2017
e-pub ahead of print date: 1 April 2018
Published date: 1 April 2018
Keywords: BSR, Gas hydrates, Methane, Penghu channel, Taiwan

Identifiers

Local EPrints ID: 419429
URI: http://eprints.soton.ac.uk/id/eprint/419429
ISSN: 1017-0839
PURE UUID: e3e4881a-ca4e-42cb-8c28-6bf76a494f5b

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Date deposited: 12 Apr 2018 16:30
Last modified: 08 Jan 2022 02:16

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Contributors

Author: Sourav Kumar Sahoo
Author: Wu Cheng Chi
Author: Wei Chung Han
Author: Liwen Chen
Author: Char Shine Liu
Author: Yunshuen Wang

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