Evidence from three‐dimensional seismic tomography for a substantial accumulation of gas hydrate in a fluid‐escape chimney in the Nyegga pockmark field, offshore Norway

Abstract

[1] In recent years, it has become evident that features commonly called gas chimneys provide major routes for methane to pass through the methane-hydrate stability zone in continental margins and escape to the ocean. One of many such chimneys lying beneath pockmarks in the southeastern Vøring Plateau off Norway was investigated with a high-resolution seismic experiment employing a 2-D array of sixteen 4-component ocean bottom seismic recorders at approximately 100 m separation and a dense network of shots to define the 3-D variation of the chimney's structure and seismic properties. The tomographic model derived from P wave travel times shows that P wave velocity inside the chimney is up to 300 m/s higher than in the surrounding strata within the methanehydrate stability zone. The zone of anomalously high velocity, about 500 m wide near its base, narrowing to about 200 m near the seabed, extends to a depth of 250 m below the seafloor. The depth extent of this zone and absence of high velocity beneath the base of the methane-hydrate stability field make it more likely that it contains hydrate rather than carbonate. If a predominantly fracture-filling model is appropriate for the formation of hydrate in low-permeability sediment, the maximum hydrate concentration in the chimney is estimated to be 14%-27% by total volume, depending on how host-sediment properties are affected by hydrate formation. Doming of the strata penetrated by the chimney appears to be associated with the emplacement of hydrate, accompanying the invasion of the gas hydrate stability zone by free gas.Citation: Plaza-Faverola, A., G. K. Westbrook, S. Ker, R. J. K. Exley, A. Gailler, T. A. Minshull, and K. Broto (2010), Evidence from three-dimensional seismic tomography for a substantial accumulation of gas hydrate in a fluid-escape chimney in the Nyegga pockmark field, offshore Norway,

Controlled-source electromagnetic and seismic delineation of subseafloor fluid flow structures in a gas hydrate province, offshore Norway

“…Seismic anisotropy may also exist in regions where vertical to subvertical, fluid-filled fractures and micro-cracks are present . The Nyegga pockmarks region exhibits a sub-horizontal stratigraphy (Kjeldstad et al 2003) as seen from extensive seismic reflection data Berndt et al 2003;Westbrook et al 2008;Plaza-Faverola et al 2010. The underlying chimney-like and pipe structures cut vertically through the horizontal layers Berndt et al 2003;Plaza-Faverola et al 2011).…”

“…Both Vp representations start at the seafloor (∼727 m below sea-surface) and extend to a depth of 1000 m (line 1, S to N) and 1050 m (line 2, NW to SE), below sea-surface. The velocities in the first and the second layer (727-750 m, and 750-800 m) are poorly constrained at the chimney interior and the flanks, due to a low density of seismic-ray impact points in this region (Plaza-Faverola et al 2010). The model shows an increased seismic velocity along the pipe structure compared with the surrounding strata, most probably resulting from the presence of hydrate in the pores of the host sediments within the GHSZ (Figs 3a and c).…”

“…Therefore, the high velocities observed at the pipe centre (∼820-1050 m) that coincide with relatively high resistivities, are indicative of high hydrate saturation at these depths. Westbrook et al (2008) suggested hydrate saturation of up to 35 per cent within the CNE03 pipe structure, while based on a more detailed analysis, Plaza-Faverola et al (2010) proposed a saturation range between 14 to 27 per cent.…”

“…Travel time tomography seeks to resolve the subsurface velocity model that best satisfies the travel times of seismic waves that propagate through the subsurface (Plaza-Faverola et al 2010). The layered Vp model for the CNE03 pockmark (Plaza-Faverola et al 2010) was created using TomoInv, a pre-stack travel time tomography software, developed at Institut Français du Pétrole (Delbos et al 2001(Delbos et al , 2006.…”

“…At CNE03, free gas from deep reservoirs is inferred to migrate vertically into the gas hydrate stability zone (GHSZ) via polygonal faults and fractures (Plaza-Faverola et al 2010) and forms hydrate in veins (Hornbach et al 2004;Jain & Juanes 2009). Additionally, water with methane in solution continues to propagate upward, forming small amounts of hydrate and authigenic carbonate near the seafloor (Foucher et al 2009), both of which are electrically resistive.…”