POCKMARKS IN THE FJORDS OF WESTERN SVALBARD AND THEIR IMPLICATIONS ON GAS HYDRATE DISSOCIATION 1,2 1,2,3 1 2,4 Srikumar Roy , Kim Senger , Riko Noormets , Martin Hovland 1 Department of Arctic Geology, University Centre in Svalbard, PO Box 156, N-9171 Longyearbyen, Svalbard, Norway 2Department of Earth Science, University of Bergen, Allégaten 41, N-5007 Bergen, Norway 3 4 Centre for Integrated Petroleum Research (CIPR), Uni Research, Allégaten 41, N-5007 Bergen, Norway Statoil ASA, Forusbeen 50, 4035 Stavanger, Norway * corresponding author: [email protected], +4746235159 INTRODUCTION STUDY AREA - ISFJORDEN (WESTERN SVALBARD) Pockmarks are circular to elliptical in planform depression commonly occurring on muddy sea bottom. Their formation is mostly related to the seepage of gaseous or fluid hydrocarbons through the seabed. The pockmark-forming gas/fluid may originate either directly from the biogenic or thermogenic source or from the dissociating gas hydrates that may locally be associated with the subsea permafrost. In the Arctic, gas hydrate is widespread, trapped within marine sediments and permafrost. The gas hydrate stability zone (GHSZ) for a specific gas to form hydrate beneath the sea floor is controlled by temperature (dependent on bottom water temperature and geothermal gradient) and pressure (dependent on water depth and depth beneath seabed), as well as pore water chemistry. Dissociation of gas hydrate in response to changed temperature and/or pressure can produce a rapid release of free methane through the sediments creating pockmarks on the seafloor (Judd & Hovland, 2007; Mienert et al, 2010). The presence of active hydrocarbon source rocks along the western Svalbard continental margin (Knies et al., 2004), the Central Spitsbergen Basin (Nøttvedt et al., 1993) and reactivated fault systems like the Billefjorden Fault Zone form favourable preconditions for hydrocarbon accumulation and migration through the seabed and hence formation of pockmarks. Dewatering of marine sediments due to sediment overloading during glacial readvances may also result in the formation of pockmarks. Study area is located in the Isfjorden fjord system in central Spitsbergen, Svalbard. During the last glacial maximum c. 18-20ky BP, Svalbard and its surrounding continental shelf was covered by an ice sheet. Retreat of the ice sheet margin from the Isfjorden Trough begun c. 15ky BP, by 12ky BP the shelf was ice free and by c. 11ky BP the the ice margin had retreated well into the fjords (Ingólfsson, 2011). High resolution swath bathymetric data were used to systematically map the submarine glacial landforms and pockmarks in the Isfjorden fjord system in central Svalbard. Pockmarks recorded in this study occur exclusively in the post glacial marine muds. SPATIAL DISTRIBUTION OF POCKMARKS IN ISFJORDEN a) Isfjorden b) Figure1 : Geographic location of Isfjorden in western Svalbard (Source: Norwegian Polar Institute) MORPHOLOGY OF POCKMARKS The Isfjorden area comprises circular, V to U shaped and elliptical pockmarks. They appear as either single features or string/composite pock marks. They vary from symmetrical to asymmetrical on the slopes in vertical cross sections. A total of 844 pockmarks have been identified in the bathymetry imagery. i) Diameter ranges from 15m to 265 m ii) Maximum vertical depth of 11 m Locally raised rims occur. The appearance of the pockmarks varies from sharply outlined, with well-defined edges and steeper slopes, to less sharply outlined, with smooth edges and gentler slopes. Figure 2 : Map showing the distribution of pockmarks on the bathymetry data in (a) Billefjorden, (b)Sassenfjorden and (c) Isfjordbanken respectively. Elevated rims Figure 3: Cross-section of a pockmark in Billefjorden illustrating the V-shaped structure (extreme left). Composite pockmarks associated with raised rims (left). Figure 4: Closer look at the glacial lineations and associated pockmarks in Billefjorden ( right). Note the colour-coding for exact location from figure 2(a). Figure 5: Distribution of pockmark diameter (extreme left) and relief (left) in the Isfjorden. MODELING THE DISTRIBUTION, DEPTH AND THICKNESS OF THE HYDRATE STABILITY ZONE IN ISFJORDEN -2-C 5-C oC Temperature -2 0.5 3 Depth to top of HSZ (Pure 224m 286m 366m methane phase boundary) Case Number 01 02 03 Depth to top of HSZ (Methane + 129m 170m 224m 10% ethane phase boundary) Case Number 05 06 07 Bottom water temperature (°C) The presence of higher molecular weight gases in the shallow waters of Svalbard might facilitate formation of gas hydrates as they become more stable with added gas types (e.g. ethane and propane) at much lower pressures and higher temperatures than for pure methane hydrate (Baker, 1972). Taking into account the uncertainty of the unknown gas composition, four cases are also discussed wherein the methane gas is assumed to be contaminated with up to 10% of ethane. Case_05 Case_06 Case_01 Case_07 Case_02 Case_08 Case_03 5 0.5-C 445m Case_04 3-C Figure 7 (top): Map showing an extrapolated seabottom temperature map of Isfjorden. 04 279m 08 Table 1: Temperature and top of HSZ values of the eight case studies discussed in this study. Figure 6: Ocean bottom temperature data from the International Council for the Exploration of the Sea (ICES) demonstrates that the sea-bottom temperature varies from – 2 to 5 °C in the Isfjorden region. Source: Ices.dk, Data bounding box (13.75-17.75E, 78-78.75N), Temperatures within 10m of seabed. Figure 8 (right): Water depth required for each case study considering the corresponding temperatures for methane hydrate stability. (using HWHydrate software). Depth to the top of hydrate stability zone at different ocean bottom temperatures. Case_04 indicates that pure methane does not form hydrate at 5 °C in this region considering maximum water depth of 428m. THICKNESS CALCULATION OF HYDRATE STABILITY ZONE IN ISFJORDEN The gas hydrate stability zone (HSZ) in sediments can be delineated on a temperature versus depth (pressure) profile with respect to the hydrothermal gradient (for subsea gas hydrates), geothermal gradient and clathrate phase boundary. A geothermal gradient of 25 °C/km has been used for the HSZ thickness calculations in this study. The phase boundaries were calculated using HWHydrate software. a) b) c) Figure 10 (a to g): Thickness maps of hypothetical Hydrate Stability Zones for pure methane hydrate and methane + 10% ethane hydrate formation. d) e) f) g) Longyearbyen Figure 9: 25 degrees / km geothermal gradient making a secant with the two phase boundaries to delineate the HSZ a) b) c) Figure 11 :(Above)Distribution of pockmarks in different water depth intervals in (a) Billefjorden, (b) Sassenfjorden and (c) Isfjordbanken. CONCLUSIONS • The preliminary mapping and modelling results suggest that, in addition to the potential deep sources for the seabed seeps, the pockmarks within the mapped GHSZ regions in the Isfjorden could have resulted partly from the dissociation of gas hydrates. The GHSZ is expected to taper out at its landward limit at a depth of about 400m, where water temperature is 3 °C. Moreover the effect of progressive warming of the northward-flowing West Spitsbergen current of about 1 °C over the last 30 years (Schauer et al., 2004) in the area could potentially increase the hydrate instability. • Another potential cause for the formation of the pockmarks may be the release of methane associated with the thawing permafrost. • The production of biogenic sediments in Svalbard fjords is comparatively low (Elverhøi, 1984). Hence, thermogenic gas is more likely a cause for the formation of the pockmarks in the fjords of Svalbard. • Elongated planform of pockmarks on Isfjorbanken could result from the stronger bottom currents as compared to the inner Isfjorden where the pockmarks are of more circular shape. Further investigation of gas hydrates and monitoring of methane release is needed to quantify the likely magnitude of future emissions and their possible implications on the regional and global climate. ACKNOWLEDGEMENTS This work is financed by the Research Council of Norway. The multibeam data from the Norwegian Hydrographic Service is presented in accordance with permission number 08/620. Schlumberger provided academic license of Petrel. HWHydrate was used for the hydrate stability modelling. REFERENCES Knies, J., Damm, E., Gutt, J., Mann, U., Pinturier, L. (2004). Near-surface hydrocarbon anomalies in shelf sediments off Spitsbergen: Evidences for past seepages. 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