The mid-Norwegian margin gas hydrate province: trace of slope stability and geo-hazard through time Nicolas Waldmann, Haflidi Haflidason, Berit Oline Hjelstuen and Hans Petter Sejrup Department of Earth Science, University of Bergen, Allègaten 41, N-5007 Bergen, Norway
Depth / Hydrostatic pressure
-
+
Gas Hydrate Phase boundary
Gas Hydrate
Seafloor Gas Hydrate Stability Zone (GHSZ) BSR Free gas zone Geotherm
Free gas
Submarine slope failure Gas hydrates
W
GHSZ
Chimneys
E
BSR
Free gas Double BSR
NW
SE BSR
Base Pleistocene
In marine sediments along continental margins, methane hydrate formation is bound by temperature and pressure (figure on the left). The depth of the gas hydrate stability zone (GHSZ) is limited by these factors and by sufficient methane concentration to precipitate hydrate. Studying the spatial distribution and dynamics of gas hydrates is crucial in order to evaluate the geo-hazard potential arising from dissociation and migration of hydrates due to thermodynamic changes through time.
Opal A/CT
Trænadjupet slide
1000 m
Trænadjupet slide Vigrid slide
Storegga slide
N 0°
8
°N 0 7
Vøring basin
N 0°
6
°E 0 4
Profile above on the right BSR’s Sklinnadjupet slide Profile above on the left
Norway
The Norwegian margin was influenced by the Fennoscandian ice sheet during the Pleistocene. Increase input of fine sediments with average sand content of 5% ocurred during glacial cycles (figure on the right). Recent studies show that the favorable host sedimentary facies for hydrate development is coarse grained deposits due to greater permeability. Yet, the Vøring Plateau mainly consists of fine grained hemipelagic deposits.
Sand fractions (wt. %) 0
0
20
NW
x1000 Milliseconds
0
Trøndelag Platform
Vøring Basin
480 460 440 420 400 380 360 340 320 300 28 260 240 220 200 180 160 140 120 100 Vøring Escarpment
80
60
40
20 km
Helland-Hansen Arch
4 6
A
1 : 500.000
Nodules
SEABED III (Norwegian Deepwater Programme)
?
A’
0.8
0 10 20
5
30 2-4
10
40
80
0 0.2
90
20
5
100 110
25
6 100
200
0
40
Clay (wt. %)
80
0
0.8
150
1.2
Calculated BSR
Base Pleistocene
Opal A/CT
Nyegga Slide R
Storegga headwall
Chimneys
S
N
Tempen
Storegga
0.4
0.8 1.0
Vigrid Sklinnadjupet
0.6
120 140
BSR
1 km
50 70
MIS 8-10
Dissociation of hydrates may have triggered slope failure in the Vøring region, such as the sequence of slides in the Nyegga region (slides R & S, figure above) or the Sklinnadjupet slide. Interestingly, while methane-related BSR’s are absent beneath the Sklinnadjupet slide, the opal A/CT diagenetic-related BSR is continuous and is not affected by the event (figure on the left).
Slope stability of the Vøring Plateau
60 15
NE
Slide R Møre Slide S
modified from Solheim et al., 2004
BSR’ Nyegga area A
Storegga slide Norway
B
150m 150m
At least seven slides occurred in the southern margins of the Vøring Plateau (figure on the left, map in the left panel). Most of the slides are found within the glacigenic sediment sequence. A thorough understanding of the Pleistocene depositional history is, thus, necessary to assess the sliding.
Ti/K
1cm
2 Opal A/CT
0.4
1
Mag. susc. (10-5 SI)
ESE
0
Slide R
The Vøring Plataeau is one of the most unstable continental margins in the world. Thermodynamic changes at depth might have triggered hydrate to become unstable. This situation probably generated overpressure in the sedimentary column eventually releasing gas to the ocean leaving behind Nyegga pockmarks as fingerprints of the process pockmark area Fig. B (figures on the right). Storegga Studying the spatial distribution and wall dynamics of hydrates in relation to slope stabilty is crucial in order to evaluate the from Hovland and Svendsen, 2006 geo-hazard potential through time.
MIS
Ca/Fe
0 20 40 60 80
40
0
Vøring Marginal High
Silt (wt. %)
30
1000 km
0°
Slide S
Opal A/CT
On the Vøring Plateau, BSR’s are recognized cross cutting different seismic facies: whether deep hemipelagic marine deposits (in the Nyegga region, profile above) or shallower strata consisting of mixed glacial and redeposited material with decrease permeability (in the Trænadjupet slide area, figure above on the right).
MIS 5e
Storegga
BSR
Base Eocene
130
Fractures
20°E
SW
10 km
Slide R Møre slide
Sklinnadjupet slide
Base Miocene
Sedimentary environments of the Vøring Plateau
The Mid-Norwegian continental margin The mid-Norwegian continental margin (figure on the right) presents a unique situation where thick glacigenic units were deposited during past glacial intervals covering older, highly faulted, finegrained hemipelagic siliceous ooze sequences (figure below). These stratigraphic circumstances, combined with features indicating large amount of biogenic methane and probably deep thermogenic methane reservoirs, provide a natural 40 °W laboratory where to study the development and dynamics of 20°W methane hydrates and other diagenesis processes in relation to slope stability through time.
Base Pleistocene
meters
Tempen slide
Previous studies link cyclic loading from earthquakes, excess pore pressure and hydrate dissociation and migration with ocean floor destabilization at different temporal scales. The presence of a diagenetic-related BSR deeper in the stratigraphical sequence and chimneys reaching different depths (figure below) may indicate contribution of deep thermogenic methane to the shallower (mostly biogenic) methane system.
Age (ka)
Sonic velocity
Sea level change
400 m
Temperature
CH4
Depth (m) core MD99-2289/88
Mass flows often evolve from mass-failure processes along continental margins that may originate from various processes including gas hydrate dissociation, release and migration.
Tsunami hazard
A second, deeper BSR is also occasionally recognized on the Vøring Plateau and has been interpreted as a fossil base of the GHSZ caused by hydrate dissociation during postglacial sea level rise and increase bottom water temperature (Berndt et al., 2004). Yet, its occurrence is patchy and discontinuous.
400 m
Hydrate dissociation seafloor instability Hydrate formation in well bores
Linking gas hydrates and slope stability
Hydrate bearing sediments are commonly detected in seismic profiles by cross-cutting bottom simulating reflectors (BSR’s) (profiles below), which commonly represent the GHSZ base. Yet, hydrates have been retrieved as well in regions where BSR’s were acoustically not identified.
200 m
Subaqueous mass movements are important players for sediment transport and redeposition in marine environments (figure on the right). They also pose societal and environmental risks to offshore infrastructures (e.g. pipelines, cables and platforms) as well as to coastal areas due to shore collapses and landslide induced tsunamis.
Geophysical signatures of gas hydrates
Age (ma)
From stability to instability: gas hydrates in continental margins
1cm
Gas hydrates on the Nyegga area (the southern margins of the Vøring Plateau) were retrieved as both nodule concentrations and filling fractures (figures on the right) within fine grained hemipelagic deposits.
The GANS project (Gas hydrates on the Norway-Barents Sea -Svalbard margin) is an initiative leaded by the Department of Earth Science, University of Bergen and in collaboration with five research institutions and the Norwegian Deepwater Program SEABED III. The contract number is 175969/S30.
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