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Gas hydrate quantification at a pockmark offshore Norway from joint effective medium modelling of resistivity and seismic velocity

Gas hydrate quantification at a pockmark offshore Norway from joint effective medium modelling of resistivity and seismic velocity
Gas hydrate quantification at a pockmark offshore Norway from joint effective medium modelling of resistivity and seismic velocity

Methane emissions from gas hydrate deposits along continental margins may alter the biogeophysical properties of marine environments, both on local and regional scales. The saturation of a gas hydrate deposit is commonly calculated using the elastic or electrical properties measured remotely or in-situ at the site of interest. Here, we used a combination of controlled-source electromagnetic (CSEM), seismic and sediment core data obtained in the Nyegga region, offshore Norway, in a joint elastic-electrical approach to quantify marine gas hydrates found within the CNE03 pockmark. Multiscale analysis of two sediment cores reveals significant differences between the CNE03 pockmark and a reference site located approximately 150 m northwest of CNE03. Gas hydrates and chemosynthetic bivalves were observed in the CNE03 sediments collected. The seismic velocity and electrical resistivity measured in the CNE03 sediment core are consistent with the P-wave velocity (V P) and resistivity values derived from seismic and CSEM remote sensing datasets, respectively. The V P gradually increases (~1.75–1.9 km/s) with depth within the CNE03 pipe-like structure, whereas the resistivity anomaly remains ~3 Ωm. A joint interpretation of the collocated seismic and CSEM data using a joint elastic-electrical effective medium model suggests that for the porosity range 0.55–0.65, the gas hydrate saturation within the CNE03 hydrate stability zone varies with depth between ~20 and 48%. At 0.6 porosity, the hydrate saturation within CNE03 varies between ~23 and 37%, whereas the weighted mean saturation is ~30%. Our results demonstrate that a well-constrained gas hydrate quantification can be accomplished by coupling P-wave velocity and CSEM resistivity data through joint elastic-electrical effective medium modelling. The approach applied in this study can be used as a framework to quantify hydrate in various marine sediments.

Effective medium modelling, Gas hydrate, Marine CSEM, Seismic velocity
0264-8172
104151
Attias, Eric
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Amalokwu, Kelvin
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Watts, Millie
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Falcon-suarez, Ismael Himar
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North, Laurence
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Hu, Gao Wei
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Best, Angus I.
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Weitemeyer, Karen
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Minshull, Tim A.
3b2231bc-f741-463f-8b43-76d41b1cbfec
Attias, Eric
abf34bba-f99f-47f9-ba89-92df1c488a5e
Amalokwu, Kelvin
a88bc1e5-5577-49a6-a503-fcd9ea12d8fe
Watts, Millie
74f8c79a-1eee-4337-ae9e-a2681933ecbe
Falcon-suarez, Ismael Himar
f14858f6-d086-4761-9dc5-ba09bd89d95b
North, Laurence
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Hu, Gao Wei
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Best, Angus I.
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Weitemeyer, Karen
22c524f6-b24d-4d2a-a46d-16b06c70a5d1
Minshull, Tim A.
3b2231bc-f741-463f-8b43-76d41b1cbfec

Attias, Eric, Amalokwu, Kelvin, Watts, Millie, Falcon-suarez, Ismael Himar, North, Laurence, Hu, Gao Wei, Best, Angus I., Weitemeyer, Karen and Minshull, Tim A. (2020) Gas hydrate quantification at a pockmark offshore Norway from joint effective medium modelling of resistivity and seismic velocity. Marine and Petroleum Geology, 113, 104151, [104151]. (doi:10.1016/j.marpetgeo.2019.104151).

Record type: Article

Abstract

Methane emissions from gas hydrate deposits along continental margins may alter the biogeophysical properties of marine environments, both on local and regional scales. The saturation of a gas hydrate deposit is commonly calculated using the elastic or electrical properties measured remotely or in-situ at the site of interest. Here, we used a combination of controlled-source electromagnetic (CSEM), seismic and sediment core data obtained in the Nyegga region, offshore Norway, in a joint elastic-electrical approach to quantify marine gas hydrates found within the CNE03 pockmark. Multiscale analysis of two sediment cores reveals significant differences between the CNE03 pockmark and a reference site located approximately 150 m northwest of CNE03. Gas hydrates and chemosynthetic bivalves were observed in the CNE03 sediments collected. The seismic velocity and electrical resistivity measured in the CNE03 sediment core are consistent with the P-wave velocity (V P) and resistivity values derived from seismic and CSEM remote sensing datasets, respectively. The V P gradually increases (~1.75–1.9 km/s) with depth within the CNE03 pipe-like structure, whereas the resistivity anomaly remains ~3 Ωm. A joint interpretation of the collocated seismic and CSEM data using a joint elastic-electrical effective medium model suggests that for the porosity range 0.55–0.65, the gas hydrate saturation within the CNE03 hydrate stability zone varies with depth between ~20 and 48%. At 0.6 porosity, the hydrate saturation within CNE03 varies between ~23 and 37%, whereas the weighted mean saturation is ~30%. Our results demonstrate that a well-constrained gas hydrate quantification can be accomplished by coupling P-wave velocity and CSEM resistivity data through joint elastic-electrical effective medium modelling. The approach applied in this study can be used as a framework to quantify hydrate in various marine sediments.

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Accepted/In Press date: 26 November 2019
e-pub ahead of print date: 3 December 2019
Published date: 1 March 2020
Additional Information: Funding Information: This paper forms part of the PhD studies of Eric Attias, funded by Rock Solid Images Ltd., the University of Southampton, and the National Oceanography Centre Southampton ( NOCS ). A Wolfson Research Merit Award supported TAM. The authors are grateful to Peter Telling for the acquisition of the cores used in this study, as part of the UK Natural Environment Research Council ( NERC ) Arctic Research Programme (project: landslide-tsunami risk to the UK; NERC Grant NE/K00008X/1 ). We also thank the British Ocean Sediment Core Research Facility at NOCS, and the curators S. MacLachan and M. Edwards for their services in maintaining the sediment cores and aiding with the analytical techniques. Additionally, we thank Tongcheng Han and Héctor Marín-Moreno for productive discussions. We thank the captain, crew and scientific party of R/V Pelagia. The data used in this paper are available via https://pangaea.de/10.1594/PANGAEA.876610 . Appendix A The SCA equation for the elastic moduli and electrical conductivity for ellipsoidal inclusions are given by Mavko et al. (1998) : (1) ∑ j = 1 m x j ( K j − K S C ) P j = 0 , ∑ j = 1 m x j ( μ j − μ S C ) Q j = 0 , and (2) ∑ j = 1 m x j ( σ j − σ S C ) C j = 0 , Where j denotes the j t h material, with volume fraction x , bulk modulus K , shear modulus μ, and electrical conductivity σ. The coefficients P , Q and C are geometric factors for ellipsoidal inclusions of arbitrary aspect ratios in a background medium with self-consistent effective bulk and shear modulus, and electrical conductivity of K S C , μ S C , and σ S C , respectively. The general DEM expressions for the elastic moduli and electrical conductivity are given by Mavko et al. (1998) : (3) d K D E M ( x j ) = ( K j − K D E M ) P j ( x j ) 1 − x j d x j , d K D E M ( x j ) = ( μ j − μ D E M ) Q j ( x j ) 1 − x j d x j , and (4) d σ D E M ( x j ) = ( σ j − σ D E M ) C j ( x j ) 1 − x j d x j , Where the terms are as described for the SCA equations above. Appendix B Sensitivity analysis using CQ mixes that contain (a) 65% clay and 35% quartz, and (b) 45% clay and 55% quartz, thus, ± 10% clay content then the content obtained from the XRD analysis. This analysis indicates subtle changes (<2%) in gas hydrate saturation, where lower clay content (higher quartz content) leads to a moderate decrease in gas hydrate content and vice versa ( Fig. A1 ). This is due to the higher values of the physical parameters (moduli, resistivity, density) of quartz in comparison to those of clay. Fig. A1 Model sensitivity analysis. Comparison of the joint elastic-electrical properties obtained from the SCA/DEM model with CSEM and seismic remote sensing data, with three different CQ mixes. (a) The CQ mix contains 65% clay and 35% quartz. (b) The CQ mix contains 55% clay and 45% quartz, corresponding to the values derived from the XRD analysis ( Table 2 ) and used in the manuscript ( Fig. 9 ). (c) The CQ mix contains 45% clay and 55% quartz. White circles denotes depth interval 1 : 80–180 mbsf, V P 1 ~ 1.75 km/s; White-black gradient circles denotes depth interval 2 : 180–200 mbsf, V P 2 ~ 1.83 km/s; Black circles denotes depth interval 3 : 200–280 mbsf, V P 3 ~ 1.90 km/s. The resistivity for the three depth intervals is ~ 3 Ω m (see section 3.1 ). The dashed contour represent 35% of gas hydrate saturation. Note the subtle decrease in the gas hydrate saturation as indicated by superimposing the remotely sensed data (best visible in Figure insets). This decrease results from increasing the quartz content in the CQ mix. Publisher Copyright: © 2019 Elsevier Ltd
Keywords: Effective medium modelling, Gas hydrate, Marine CSEM, Seismic velocity

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Local EPrints ID: 444250
URI: http://eprints.soton.ac.uk/id/eprint/444250
ISSN: 0264-8172
PURE UUID: 3c93a747-e423-4f96-9a37-d42a45fbbe0a

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Date deposited: 06 Oct 2020 18:31
Last modified: 26 Aug 2022 04:01

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Contributors

Author: Eric Attias
Author: Kelvin Amalokwu
Author: Millie Watts
Author: Ismael Himar Falcon-suarez
Author: Laurence North
Author: Gao Wei Hu
Author: Angus I. Best
Author: Karen Weitemeyer
Author: Tim A. Minshull

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