Search

Gas seeps and fluid-flow related seabed features are found over the entire Norwegian exclusive economic zone (EEZ). Multibeam water-column data from c. 136 000 km2 has revealed more than 5 000 gas seeps. Most of the gas seeps seem to have biogenic, thermogenic or mixed origin; some may be of abiotic origin. The spatial distribution of the gas seeps appears to correlate with: 1 - structural highs with associated faulting, exposing hydrocarbon reservoir rocks at or near the seabed; 2 - faults serving as conduits for fluid flow; 3 - settings where reservoir rocks overlain by less permeable cap rocks sub-crop at the seabed. Other mechanisms involve seepage around abandoned exploration wells, and possible abiotic gas generation from serpentinisation of ultramafic rocks near mid-oceanic ridges. The gas seeping from the Norwegian cold seeps is mostly methane and has, in many places, led to the formation of methane-derived authigenic carbonate crusts, which give evidence for either extensive gas seepage in the past or long-lived seepage. Chemosynthetic communities are commonly associated with cold seeps and may form special habitats together with the carbonate crusts. Methane seepage has been proposed to contribute significantly to the global carbon budget and may be associated with gas hydrates giving rise to potential geohazards. Gas seeps have been identified and spatially mapped as acoustic gas flares, using multibeam echosounder systems, which have the ability to record reflections from both the water column and the seabed. Water-column data have been recorded in the MAREANO seabed mapping program since 2010, covering an area of c. 262 000 km2, with a data volume in the order of 210 Tb. The observations of extensive gas flares in the Norwegian EEZ are available to the scientific community and other users through a dedicated MAREANO data and web access system. [ABSTRACT FROM AUTHOR]

Hundreds of newly discovered gas seeps, craters, and mounds are presented from the central Barents Sea, Norway. Where they occur together, the crater-mound pairs have a preferred orientation towards the southeast. According to complex cross-cutting relationships with iceberg ploughmarks, both the craters and mounds formed during deglaciation (ca. 11–13 ka BP). Free gas, trapped beneath shallow gas hydrates in discrete areas, deformed or displaced overlying bedrock and sediment, forming mounds. Over-steepened slopes, weak layers, fractured bedrock, and unstable sediment resulted in small landslides towards the southeast: the direction of regional slope. In some cases, the highly buoyant, hydrate-cored sediment mounds may have instead, detached and floated with bottom currents before being re-deposited, either locally or distally. In this scenario, southeasterly flowing bottom currents would also explain the preferred orientation of the observed crater-mound pairs. Crater collapse would have occurred simultaneously or subsequently to landslide or detachment and floating due to the rapid dissociation of any remaining gas hydrates; a result of attendant regional changes in temperature and pressure following deglaciation and local removal of overlying sediment. The model of formation developed in this study explains the southeasterly trend in crater-mound orientation, but differs from other studies that interpret modern mounds, located elsewhere in the Barents Sea, as gas hydrate-cored. [ABSTRACT FROM AUTHOR]

Sibelco Nordic's mine on Stjernøya, North Norway, disposes mine tailings into the fjord Stjernsundet. The tailings, discharged at the shoreline in the bay of Lillebukt, comprise c. 85% fine silt to medium-grained sand (0.01-0.5 mm). Upon discharge of the mine tailings into the fjord, they are redistributed by slides and density currents along major channels with pronounced levees. Multibeam echosounder data show sand waves in the channels, while seabed samples and cores document sand ripples and layers of mud between the sand layers. Mud accumulates outside of the channels on the seabed in Lillebukt and as a thin veneer along the shores to the east and west. The bathymetry data show partly buried slide escarpments and slide deposits, while smaller slide scars are evident in the levees. Three slide events in the tailings are documented of which the most recent occurred 9th October 2017. Comparison of bathymetry data collected in 2016 and 2018 show changes in bathymetry in the channels of ± 3 m. Slides are partly initiated along fine-grained layers and caused by hypersedimentation. From 60 m depth, one single channel continues down to 100 m, where the sediment transport is along a gully in the steep (45°) bedrock slope down to c. 400 m depth. A sand-dominated submarine fan has accumulated at the foot of the slope, extending to c. 463 m depth. Bathymetric data, seismic data and core analysis show that the fan has a radius of up to 1500 m and covers an area of c. 1.5 km2. Comparison of bathymetry data collected in 1998 and 2016 shows that a large part of the tailings disposed of in that 18-year period (4 million tons) have accumulated on the submarine fan. [ABSTRACT FROM AUTHOR]

The presence of gas hydrate in oceanic sediments is mostly identified by bottom-simulating reflectors (BSRs), reflection events with reversed polarity following the trend of the seafloor. Attempts to quantify the amount of gas hydrate present in oceanic sediments have been based mainly on the presence or absence of a BSR and its relative amplitude. Recent studies have shown that a BSR is not a necessary criterion for the presence of gas hydrates, but rather its presence depends on the type of sediments and thein situconditions. The influence of hydrate on the physical properties of sediments overlying the BSR is determined by the elastic properties of their constituents and on sediment microstructure. In this context several approaches have been developed to predict the physical properties of sediments, and thereby quantify the amount of gas/gas hydrate present from observed deviations of these properties from those predicted for sediments without gas hydrate.We tested four models: the empirical weighted equation (WE); the three-phase effective-medium theory (TPEM); the three-phase Biot theory (TPB) and the differential effective-medium theory (DEM). We compared these models for a range of variables (porosity and clay content) using standard values for physical parameters. The comparison shows that all the models predict sediment properties comparable to field values except for the WE model at lower porosities and the TPB model at higher porosities. The models differ in the variation of velocity with porosity and clay content. The variation of velocity with hydrate saturation is also different, although the range is similar. We have used these models to predict velocities for field data sets from sediment sections with and without gas hydrates. The first is from the Mallik 2L–38 well, Mackenzie Delta, Canada, and the second is from Ocean Drilling Program (ODP) Leg 164 on Blake Ridge. Both data sets haveVp andVs information along with the composition and porosity of the matrix. Models are considered successful if predictions from bothVp andVs match hydrate saturations inferred from other data. Three of the models predict consistent hydrate saturations of 60–80 per cent from bothVp andVs from log and vertical seismic profiling data for the Mallik 2L-38 well data set, but the TPEM model predicts 20 per cent higher saturations, as does the DEM model with a clay–water starting medium. For the clay-rich sediments of Blake Ridge, the DEM, TPEM and WE models predict 10–20 per cent hydrate saturation fromVp data, comparable to that inferred from resistivity data. The hydrate saturation predicted by the TPB model fromVp is higher. UsingVs data, the DEM and TPEM models predict very low or zero hydrate saturation while the TPB and WE models predict hydrate saturation very much higher than those predicted fromVp data. Low hydrate saturations are observed to have little effect onVs. The hydrate phase appears to be connected within the sediment microstructure even at low saturations. [ABSTRACT FROM AUTHOR]