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Seismic characterization and modelling of the gas hydrate system in the northern Bay of Bengal, offshore Bangladesh

Seismic characterization and modelling of the gas hydrate system in the northern Bay of Bengal, offshore Bangladesh
Seismic characterization and modelling of the gas hydrate system in the northern Bay of Bengal, offshore Bangladesh

The offshore Bangladesh includes the northern Bengal fan, where sediment supply from the Ganges and Brahmaputra rivers has resulted in the accumulation of up to 20 km of shallow-marine, fluvio-deltaic and slope sediments that have accumulated during rapid tectonic subsidence since the late Miocene. The high sedimentation rates, along with high organic matter content, make this area favorable for the formation of natural gas from both microbial and thermogenic sources. Here we use multichannel seismic reflection profiles and modelling of the gas hydrate stability zone (GHSZ) to present the first evidence for the occurrence of natural gas hydrate in the offshore Bangladesh. First, we analyze the sediments of the shelf and slope areas, which are characterized by downslope sediment transport features and by the presence, in places, of faults/fractures as well as widely distributed amplitude anomalies and seismic facies that we relate to the presence of gas. A high-amplitude reversed polarity reflection of variable continuity that mimics the seafloor and cross-cut stratigraphy is interpreted as a Bottom Simulating Reflector (BSR). The BSR is observed in several areas that are predominantly located in the E-SE of the study area, in water depths of 1300–1900 m and at depths below seafloor of 250–440 m. Sediments above BSR locations generally show higher seismic interval velocities reaching values of ∼1920–1940 m/s, which are consistent with the presence of gas hydrate in shallow marine sediments. Furthermore, the BSR lies at approximately the same depth as the theoretical base of the gas (methane) hydrate stability zone (BGHSZ), calculated assuming a 3.5 % wt pore water salinity and using existing geothermal gradient and seafloor temperature data from the study region. However, in places, the BSR lies deeper or shallower than the base of the modelled BGHSZ. These discrepancies include areas where faults/fractures and seismic evidence linked to fluid flow from deeper reservoirs reach the GHSZ disrupting its stratigraphic continuity. At these locations, we suggest that faults/fractures act as fluid migration pathways causing localized heat-flow perturbations and/or changes in the hydrate-forming gas composition both likely affecting the depth of the GHSZ. Our results provide the first evidence of the gas hydrate potential in the offshore Bangladesh and should drive future research and data acquisition aiming to understand the composition, saturation and thickness of the gas hydrate-bearing sediments in this region.

Bangladesh, BSR, Gas hydrate, Gas hydrate stability modelling, Gas migration, Geothermal gradient, Seismic reflection data
0264-8172
Monteleone, Vanessa
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Marín-Moreno, Héctor
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Bayrakci, Gaye
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Best, Angus
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Shaon, Farhana
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Hossain, Mohammad Moinul
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Al Karim, Ahmad
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Alam, Md Khurshed
a765f7af-13c6-4376-8774-3360afcc43f0
Monteleone, Vanessa
8edcf456-5bfa-44cf-99b0-a504c1c2cc0d
Marín-Moreno, Héctor
e466cafd-bd5c-47a1-8522-e6938e7086a4
Bayrakci, Gaye
e0b89aa5-d514-4ecb-91b1-8ed8bd472eda
Best, Angus
f962ede8-2ff2-42b6-baa1-88d93dfb08dd
Shaon, Farhana
c1bab130-6f11-4ad2-85ac-3ac2cc451406
Hossain, Mohammad Moinul
3470645e-f560-486e-b0e2-67e7998a2e11
Al Karim, Ahmad
2a648c9a-d709-4592-b02e-1141e2d6c0cc
Alam, Md Khurshed
a765f7af-13c6-4376-8774-3360afcc43f0

Monteleone, Vanessa, Marín-Moreno, Héctor, Bayrakci, Gaye, Best, Angus, Shaon, Farhana, Hossain, Mohammad Moinul, Al Karim, Ahmad and Alam, Md Khurshed (2022) Seismic characterization and modelling of the gas hydrate system in the northern Bay of Bengal, offshore Bangladesh. Marine and Petroleum Geology, 141, [105690]. (doi:10.1016/j.marpetgeo.2022.105690).

Record type: Article

Abstract

The offshore Bangladesh includes the northern Bengal fan, where sediment supply from the Ganges and Brahmaputra rivers has resulted in the accumulation of up to 20 km of shallow-marine, fluvio-deltaic and slope sediments that have accumulated during rapid tectonic subsidence since the late Miocene. The high sedimentation rates, along with high organic matter content, make this area favorable for the formation of natural gas from both microbial and thermogenic sources. Here we use multichannel seismic reflection profiles and modelling of the gas hydrate stability zone (GHSZ) to present the first evidence for the occurrence of natural gas hydrate in the offshore Bangladesh. First, we analyze the sediments of the shelf and slope areas, which are characterized by downslope sediment transport features and by the presence, in places, of faults/fractures as well as widely distributed amplitude anomalies and seismic facies that we relate to the presence of gas. A high-amplitude reversed polarity reflection of variable continuity that mimics the seafloor and cross-cut stratigraphy is interpreted as a Bottom Simulating Reflector (BSR). The BSR is observed in several areas that are predominantly located in the E-SE of the study area, in water depths of 1300–1900 m and at depths below seafloor of 250–440 m. Sediments above BSR locations generally show higher seismic interval velocities reaching values of ∼1920–1940 m/s, which are consistent with the presence of gas hydrate in shallow marine sediments. Furthermore, the BSR lies at approximately the same depth as the theoretical base of the gas (methane) hydrate stability zone (BGHSZ), calculated assuming a 3.5 % wt pore water salinity and using existing geothermal gradient and seafloor temperature data from the study region. However, in places, the BSR lies deeper or shallower than the base of the modelled BGHSZ. These discrepancies include areas where faults/fractures and seismic evidence linked to fluid flow from deeper reservoirs reach the GHSZ disrupting its stratigraphic continuity. At these locations, we suggest that faults/fractures act as fluid migration pathways causing localized heat-flow perturbations and/or changes in the hydrate-forming gas composition both likely affecting the depth of the GHSZ. Our results provide the first evidence of the gas hydrate potential in the offshore Bangladesh and should drive future research and data acquisition aiming to understand the composition, saturation and thickness of the gas hydrate-bearing sediments in this region.

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Accepted/In Press date: 11 April 2022
e-pub ahead of print date: 15 April 2022
Published date: 22 April 2022
Additional Information: Funding Information: the authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Vanessa Monteleone reports financial support was provided by Government of Bangladesh, Ministry of Foreign Affairs (MOFA). Gaye Bayrakci reports financial support was provided by Government of Bangladesh, Ministry of Foreign Affairs (MOFA). Angus Best reports financial support was provided by Government of Bangladesh, Ministry of Foreign Affairs (MOFA). Hector Marin-Moreno reports financial support was provided by Government of Bangladesh, Ministry of Foreign Affairs (MOFA).The authors are thankful to the Bangladesh Ministry of Foreign Affairs (MOFA) for sponsoring and contributing to this study, and to Petrobangla and Bangladesh Petroleum Exploration & Production Company (BAPEX) for making available the seismic reflection data. We are grateful to an anonymous reviewer, to Alexey Portnov and to the editor Daniel Praeg for the useful comments that helped strengthening the results of this study. We also thank Schlumberger for providing the Petrel software that was used for seismic display, interpretation and depth conversion. The open source QGIS software was used for bathymetric map and seismic profiles plotting, and MATLAB was used to calculate and plot the BGHSZ. The seafloor temperature data used in this study are from NOAA database and are available at https://www.nodc.noaa.gov/OC5/woa18f/. This project has been supported by the ACCORD funding scheme provided by the Natural Environment Research Council (NERC) as part of a National Capability, Official Development Assistance award (NC-ODA), NE/R000123/1. Funding Information: the authors are thankful to the Bangladesh Ministry of Foreign Affairs (MOFA) for sponsoring and contributing to this study, and to Petrobangla and Bangladesh Petroleum Exploration & Production Company (BAPEX) for making available the seismic reflection data. We are grateful to an anonymous reviewer, to Alexey Portnov and to the editor Daniel Praeg for the useful comments that helped strengthening the results of this study. We also thank Schlumberger for providing the Petrel software that was used for seismic display, interpretation and depth conversion. The open source QGIS software was used for bathymetric map and seismic profiles plotting, and MATLAB was used to calculate and plot the BGHSZ. The seafloor temperature data used in this study are from NOAA database and are available at https://www.nodc.noaa.gov/OC5/woa18f/ . This project has been supported by the ACCORD funding scheme provided by the Natural Environment Research Council (NERC) as part of a National Capability, Official Development Assistance award (NC-ODA) , NE/R000123/1 . Funding Information: the authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Vanessa Monteleone reports financial support was provided by Government of Bangladesh, Ministry of Foreign Affairs (MOFA). Gaye Bayrakci reports financial support was provided by Government of Bangladesh, Ministry of Foreign Affairs (MOFA). Angus Best reports financial support was provided by Government of Bangladesh, Ministry of Foreign Affairs (MOFA). Hector Marin-Moreno reports financial support was provided by Government of Bangladesh, Ministry of Foreign Affairs (MOFA).
Keywords: Bangladesh, BSR, Gas hydrate, Gas hydrate stability modelling, Gas migration, Geothermal gradient, Seismic reflection data

Identifiers

Local EPrints ID: 483975
URI: http://eprints.soton.ac.uk/id/eprint/483975
ISSN: 0264-8172
PURE UUID: ac1a6df9-864c-47fd-9c89-7b4c9fc9a18c
ORCID for Vanessa Monteleone: ORCID iD orcid.org/0000-0002-5188-5879
ORCID for Héctor Marín-Moreno: ORCID iD orcid.org/0000-0002-3412-1359

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Date deposited: 08 Nov 2023 18:03
Last modified: 11 Nov 2023 03:08

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Contributors

Author: Vanessa Monteleone ORCID iD
Author: Héctor Marín-Moreno ORCID iD
Author: Gaye Bayrakci
Author: Angus Best
Author: Farhana Shaon
Author: Mohammad Moinul Hossain
Author: Ahmad Al Karim
Author: Md Khurshed Alam

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