Thermal Characterization of Pockmarks Across Vestnesa and Svyatogor Ridges, Offshore Svalbard

M Riedel,T Pape,G Bohrmann,S Bünz,H Villinger,T Freudenthal

doi:10.1029/2020jb019468

Abstract

AbstractThe Svalbard margin represents one of the northernmost gas hydrate provinces worldwide. Vestnesa Ridge (VR) and Svyatogor Ridge (SR) west of Svalbard are two prominent sediment drifts showing abundant pockmarks and sites of seismic chimney structures. Some of these sites at VR are associated with active gas venting and were the focus of drilling and coring with the seafloor‐deployed MARUM‐MeBo70 rig. Understanding the nature of fluid migration and gas hydrate distribution requires (among other parameters) knowledge of the thermal regime and in situ gas and pore fluid composition. In situ temperature data were obtained downhole at a reference site at VR defining a geothermal gradient of ~78 mK m−1 (heat flow ~95 mW m−2). Additional heat probe data were obtained to describe the thermal regime of the pockmarks. The highest heat flow values were systematically seen within pockmark depressions and were uncorrelated to gas venting occurrences. Heat flow within pockmarks is typically ~20 mW m−2 higher than outside pockmarks. Using the downhole temperature data and gas compositions from drilling we model the regional base of the gas hydrate stability zone (BGHSZ). Thermal modeling including topographic effects suggest a BGHSZ up to 40 m deeper than estimated from seismic data. Uncertainties in sediment properties (velocity and thermal conductivity) are only partially explaining the mismatch. Capillary effects due to small sediment grain sizes may shift the free gas occurrence above the equilibrium BGHSZ. Changes in gas composition or pore fluid salinity at greater depth may also explain the discrepancy in observed and modeled BGHSZ.

Highlights

The Svalbard continental margin is one of the most studied Arctic gas hydrate provinces since it is a highly dynamic area with vigorous active free gas and aqueous fluid seepage, gas hydrate formation, and submarine slope failures (Hustoft et al, 2009; Posewang & Mienert, 1999; Vanneste et al, 2005; Vogt, Gardner, & Crane, 1999; Vorren et al, 1998)
Using the downhole temperature data and gas compositions from drilling we model the regional base of the gas hydrate stability zone (BGHSZ)
Across Svyatogor Ridge, seismic data show a bottom simulating reflection (BSR) (Figure 12) indicating existence of a gas hydrate system with free gas accumulating beneath the phase boundary (Johnson et al, 2015)

Summary

Introduction

The Svalbard continental margin is one of the most studied Arctic gas hydrate provinces since it is a highly dynamic area with vigorous active free gas and aqueous fluid seepage, gas hydrate formation, and submarine slope failures (Hustoft et al, 2009; Posewang & Mienert, 1999; Vanneste et al, 2005; Vogt, Gardner, & Crane, 1999; Vorren et al, 1998). Gas hydrate research in deeper water (>600 m water depth) has focused on Vestnesa Ridge (e.g., Bünz et al, 2012) and Svyatogor Ridge (e.g., Johnson et al, 2015) west of Svalbard These topographically prominent ridges are underlain by young oceanic crust that results in a highly variable heat flow regime (Figure 1, Crane et al, 1991). The northern sediment drift of the Vestnesa Ridge is separated by the Molloy Fracture Zone from the southern equivalent sediment drift of the Svyatogor Ridge (Johnson et al, 2015) These ridges consist of mostly fine‐grained sediments, which are pierced by subvertical vent features ( referred to as acoustic chimneys) forming pockmarks at the seafloor (Goswami et al, 2015; Johnson et al, 2015; Petersen et al, 2010). During an initial study, Vogt et al (1994) mapped the crest of Vestnesa Ridge showing abundant pockmarks, which was followed by sediment coring and some heat flow measurements (Vogt, Gardner, Crane, Sundvor, et al, 1999). Hustoft et al (2009) discovered active seepage of free gas into the water column from one pockmark at the RIEDEL ET AL

Results

Discussion

Conclusion