Correlation between tectonic stress regimes and methane seepage on the western Svalbard margin

Abstract

Abstract. Methane seepage occurs across the western Svalbard margin at water depths ranging from < 300 m, landward from the shelf break, to > 1000 m in regions just a few kilometres from the mid-ocean ridges in the Fram Strait. The mechanisms controlling seepage remain elusive. The Vestnesa sedimentary ridge, located on oceanic crust at a depth of 1000–1700 m, hosts a perennial gas hydrate and associated free gas system. The restriction of the occurrence of acoustic flares to the eastern segment of the sedimentary ridge, despite the presence of pockmarks along the entire ridge, indicates a spatial variation in seepage activity. This variation coincides with a change in the faulting pattern as well as in the characteristics of the fluid flow features. Due to the position of the Vestnesa Ridge with respect to the Molloy and Knipovich mid-ocean ridges, it has been suggested that seepage along the ridge has a tectonic control. We modelled the tectonic stress regime due to oblique spreading along the Molloy and Knipovich ridges to investigate whether spatial variations in the tectonic regime along the Vestnesa Ridge are plausible. The model predicts a zone of tensile stress that extends northward from the Knipovich Ridge and encompasses the zone of acoustic flares on the eastern Vestnesa Ridge. In this zone the orientation of the maximum principal stress is parallel to pre-existing faults. The model predicts a strike-slip stress regime in regions with pockmarks where acoustic flares have not been documented. If a certain degree of coupling is assumed between deep crustal and near-surface deformation, it is possible that ridge-push forces have influenced seepage activity in the region by interacting with the pore-pressure regime at the base of the gas hydrate stability zone. More abundant seepage on the eastern Vestnesa Ridge at present may be facilitated by the dilation of faults and fractures favourably oriented with respect to the stress field. A modified state of stress in the past, due to more significant glacial stress for instance, may explain vigorous seepage activity along the entire Vestnesa Ridge. The contribution of other mechanisms to the state of stress (i.e. sedimentary loading and lithospheric flexure) remain to be investigated. Our study provides a first-order assessment of how tectonic stresses may be influencing the kinematics of near-surface faults and associated seepage activity offshore of the western Svalbard margin.

Detection of Gas Hydrates in Faults Using Azimuthal Seismic Velocity Analysis, Vestnesa Ridge, W‐Svalbard Margin

“…In the GHSZ, faults can act as seals either due to stress or due to the presence of impermeable gas hydrates. The likelihood of faults being sealed in this segment of Vestnesa Ridge is relatively low as methane gas seeps through several pockmarks in this area (Plaza‐Faverola et al, ; Plaza‐Faverola & Keiding, ). If a fault is sealing due to the occurrence of gas hydrates in faults, we expect a relatively uniform free gas saturation below the BSR across the fault, as gas hydrate‐filled sealed faults cannot affect the free gas distribution below.…”

“…Tectonic stresses also play an important role in deciding whether a fault is sealing (blocks the passage of fluids) or nonsealing (allows the passage of fluids). Stress field variations and the kinematics of faults through geological time have been suggested to exert a control on the temporal and spatial distribution of seepage along Vestnesa Ridge (Plaza‐Faverola et al, ; Plaza‐Faverola & Keiding, ). The eastern segment of Vestnesa Ridge has several pockmarks, through which methane seeps, compared to the western segment, where pockmarks are inactive (Bünz et al, ).…”

“…These horizons are shown in the vertical seismic profile in Figure 1d. seepage along Vestnesa Ridge (Plaza-Faverola et al, 2015;Plaza-Faverola & Keiding, 2019). The eastern segment of Vestnesa Ridge has several pockmarks, through which methane seeps, compared to the western segment, where pockmarks are inactive (Bünz et al, 2012).…”

“…The eastern segment of Vestnesa Ridge has several pockmarks, through which methane seeps, compared to the western segment, where pockmarks are inactive (Bünz et al, 2012). This is potentially due to the alignment of tectonic stresses, which govern the opening and sealing of faults in the eastern and western segment of Vestnesa Ridge, respectively (Plaza-Faverola et al, 2015;Plaza-Faverola & Keiding, 2019). P-and S-wave velocity analysis (Singhroha et al, 2019) and Q analysis (Singhroha et al, 2016) suggest that faults control the distribution of gas hydrate and free gas in the eastern segment of Vestnesa Ridge.…”

Mechanisms of Methane Hydrate Formation in Geological Systems

“…Therefore, hydraulic fracturing is expected at the BHSZ and throughout the overlying section (e.g., Liu & Flemings, ; Torres et al, ; Tréhu, Flemings, et al, ) and is a natural explanation for the presence of hydrate‐filled fractures (Daigle et al, ; Daigle & Dugan, , ). In some other cases, faults and fractures may be induced by a modified state of tectonic stress, which facilitates free gas flow into the HSZ to form fracture‐filling hydrates (e.g., Vestnesa Ridge offshore Svalbard; Plaza‐Faverola & Keiding, ).…”