About GANS

Gas hydrates are ice-like substances of water molecules encaging gas molecules (mostly methane) that form under specific pressure and temperature conditions within the upper hundred metres of the sub-seabed sediments. They occur worldwide and are a potential energy resource for the near future (Kvenvolden and Rogers, 2005). Currently it is not known whether the gas hydrate fields are connected to deep hydrocarbon reservoirs, whether they are fed by shallow microbial degradation of organic matter, or whether both these processes are taking place. We are also only at the beginning of understanding how hydrates interact with various types of sediments, how they may shape the seabed and influence the seabed stability, and how they may be utilised as potential resource.

In high-latitude regions as the NBS margin, oceanic methane hydrates occur in two very different sedimentary environments: i) shallow continental shelves, and ii) deep-water continental slopes. So far, only the Storegga gas hydrate system on the mid-Norwegian continental margin has been assessed geologically in terms of hydrate occurrence (Figure 1; Bünz et al., 2003), and ocean-bottom seismometer (OBS) and ocean-bottom cable (OBC) data have provided images of the gas hydrate distribution and yielded a first estimate for hydrate and free gas concentration (Bünz and Mienert, 2004). In addition, modelling show that hydrate stability-zone changes follow the influx of post-glacial warm waters, may have contribute to melting of gas hydrates leading to an increase of the excess pore pressures (Mienert et al., 2005).

a) BSR distribution on the south Vøring and Storegga Slide region (from Bunz et al., 2003), b) Seismic profile showing BSR characteristics on the western Svalbard margin (from Vanneste et al., 2005), c) Seismic and side scan sonar characteristics of a fluid migration pathway on the Mid-Norwegian margin (from Berndt et al., 2003)

A number of key observations at Nyegga, on the northern flank of the Storegga Slide and above the Storegga gas hydrate system, point towards active natural seeping, and these areas deserve further attention in an exploration perspective (Fig. 1). Among the observations are the widespread occurrences and variability of features related to fluid/gas migration, indirect evidence of over-pressured free gas layers underneath the hydrates, fluid-driven gas-hydrate pingos, and the possibility that a significant fraction of the gas is of thermogenic origin (e.g. Hovland et al., 2005, 2006, Bünz and Mienert, 2004). The present knowledge on the identified active seep structures indicates that they have been highly influenced by the dynamic nature of the NBS margin (loading/rebound/build out) and are associated with areas of old or recent large submarine slide activity. So far, very limited research has been carried out on the sediments hosting these seep structures and underlying gas hydrate reservoirs.

In comparison to the Storegga gas hydrate system, relatively little is known about the gas hydrates on the western Svalbard margin and in the Barents Sea. Only a limited number of seismic lines and OBS data sets are available, but the observations so far are striking (Solheim and Elverhøi 1993; Carcione et al, 2005; Vanneste et al., 2005). Bottom-simulating reflections (BSRs) as an indicator for base of the gas hydrate stability zone are much stronger than in the Storegga area (Fig. 1). Their total extent and geological controls, however, have yet to be determined. The heat flow in this setting, near the mid-ocean ridge, is variable; affecting hydrate stability and fluid flow. The seismic data show extensive amplitude anomalies that can be related to the presence of gas in shallow, but also in deeper, strata. Pilot studies show the existence of hydrocarbon injection features such as bacterial mats, pockmarks, and gas anomalies of various sources (biogenic/thermogenic) in the surface sediments (Knies et al., 2004). They are possibly associated with inferred gas hydrate occurrences (BSR mapping) and potential gas hydrate dissociation processes and/or reservoir leakage along faults.

Consequently, it is important to expand our knowledge on the processes that are responsible for the distribution of gas hydrates (meters to hundreds of meters scale) along the NBS margin and to improve our understanding on the associated seepage system including the evolution and the dynamic nature of seep features as well as the secondary responses in sediments hosting these gas hydrate accumulations. This will lead to an improved understanding of gas hydrate dynamics and seafloor stability along the NBS margin, which are of the outmost importance both for safe oil and gas exploitation and for evaluation of the resource potential of gas hydrates.