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

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

Local EPrints ID: 444250

URI: http://eprints.soton.ac.uk/id/eprint/444250

DOI: doi:10.1016/j.marpetgeo.2019.104151

ISSN: 0264-8172

PURE UUID: 3c93a747-e423-4f96-9a37-d42a45fbbe0a

Date deposited: 06 Oct 2020 18:31

Last modified: 17 Mar 2024 05:14