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Energy Procedia 97 (2016) 302 - 309
European Geosciences Union General Assembly 2016, EGU Division Energy, Resources & Environment, ERE
Gas hydrate stability zone of the Barents Sea and Kara Sea region
Peter Klitzkeab*, Manja Luzi-Helbinga, Judith M. Schicksa, Mauro Cacacea, Antoine B. Jacqueyab, Judith Sippela, Magdalena Scheck-Wenderotha b, Jan Inge Faleidec d
a GFZ German Research Centre for Geosciences, Potsdam, Germany b RWTH Aachen University, Department of Geology, Geochemistry of Petroleum and Coal, Aachen, Germany c Centre for Earth Evolution and Dynamics (CEED), Department of Geosciences, University of Oslo, Oslo, Norway d Research Centre for Arctic Petroleum Exploration (ARCEx) University of Tromso, Tromso, Norway
Abstract
In this study we assess the present-day gas hydrate stability zone for the Barents Sea and Kara Sea region. Thereby, we make use of a data-based 3D lithosphere-scale pressure and thermal model. The resulting gas hydrate stability zone varies within >1 km across the study area and strongly correlates with the local geological settings and the corresponding geothermal gradient. Gas hydrates containing hydrocarbons from a thermogenic source (CH4+C2H3+C3H8) are potentially more widespread than previously assumed. The corresponding thermogenic feed gas may have derived from leaking petroleum systems during late Cenozoic basin inversion.
© 2016 The Authors.PublishedbyElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of the General Assembly of the European Geosciences Union (EGU)
Keywords: gas hydrate stability zone; 3D modelling, Barents Sea and Kara Sea
1. Introduction
Natural gas hydrates occur widely along continental margins and in permafrost regions where favourable low-temperature and high-pressure conditions exist. Their distribution critically depends on the supply and composition of a feed gas. The most common component is methane, a gas with a global warming potential 25 times stronger than carbon dioxide within 100 years [1]. Generally, the feed gas is considered to derive from shallow microbial sources (methane content: >99%) or as thermogenic gas from deeper leaked petroleum systems (methane content: 25-99%) [2]. Recently, an abiotic methane from mantle serpentinisation has also been proposed as potential source [3,4].
* Corresponding author. Tel.: +49-331-288-2846; fax: +49-331-288-1349. E-mail address: klitzke@gfz-potsdam.de
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of the General Assembly of the European Geosciences Union (EGU) doi: 10.1016/j.egypro.2016.10.005
Temperature (°C) 0 5 10 15 20 25
20° E
30° E
40° E
50° E
-6000 -5000 -4000 -3000 -2000 -1000 0 800
Elevation [m.a.s.l.]
Fig. 1: (a) Bathymetry and topography [57] of the petroliferous Barents Sea and Kara Sea region. Crosses and circles mark the position of gas and oil fields, respectively [58]; (b) Pressure and temperature stability conditions for gas hydrates formed from two different feed gas compositions and 3.5% pore water salinity.
One method to identify submarine gas hydrate occurrences is the interpretation of bottom simulating reflectors in seismic data. [e.g. 5]. A bottom-simulating reflector is the result of an impedance contrast between the base of the hydrate-saturated sediments above and free gas in the sediments below. The absence of a bottom-simulating reflector, however, does not exclude the occurrence of gas hydrates but may be explained by a lower degree of hydrate saturation or the absence of free gas below the gas hydrate stability zone [6,7]. In this sense, geophysical methods may completely underestimate the distribution of gas hydrates.
Bottom-simulating reflectors have been interpreted at several sites west of Svalbard and in the SW Barents Sea [e.g. 8-14]. It is widely discussed that the gas hydrates are fed by leaking petroleum systems in response to late Cenozoic uplift and glacial erosion [15-17]. Potential pathways are well described for the SW Barents Sea where a dense geological and geophysical database exists. Recent findings of gas hydrates and evidences for fluid flow in the eastern Barents Sea [18] and in the southern Kara Sea [19] suggest that gas hydrates may occur more widespread than previously assumed.
The potential gas hydrate stability field of the entire Barents Sea and Kara Sea region has been investigated only by a few studies so far [e.g. 20]. These studies cover the entire Arctic or even the global scale and simplify the local geological complexity and the corresponding physical parameterisation of their models. Due to phases of Cenozoic uplift and erosion the sedimentary units of the Barents Sea and Kara Sea region have significantly lower porosities than expected according to their present-day burial depth [15-17]. Laterally varying porosities directly influence the thermal conductivity, the thermal field and hence should be considered when assessing the thickness of the gas hydrate stability zone.
In the frame of this study, we make use of an existing 3D structural-thermal model which reproduces the lithosphere-scale geological and physical complexity of the Barents Sea and Kara Sea region (Fig. la) [21,22]. We want to address in particular two leading questions: Where are gas hydrates of a defined composition potentially stable at present-day? How does the thermal state, as derived from the geological setting, control the gas hydrate stability?
2. Model and methods
Gas hydrates are inclusion compounds commonly composed of water and gas. Natural gas hydrates contain predominantly methane but they may also encase other gases resulting in a mixed gas hydrate. Higher amounts of e.g. ethane, propane, carbon dioxide and hydrogen sulphide besides methane increase the gas hydrate stability with respect to temperature and pressure conditions [23]. To predict the gas hydrate phase boundaries the CSMGem software [24] is employed which considers the feed gas composition, the pressure and temperature conditions and the pore water salinity. Figure lb shows the calculated stability fields for two different feed gas compositions: a biogenic pure
methane feed gas and a thermogenic feed gas (90% methane; 7% ethane, 3% propane) which was recently described for the SW Barents Sea [13].
We obtain the 3D temperature and pressure configuration of the lithosphere below the Barents Sea and Kara Sea region based on a 3D structural model [21,22]. The model has a horizontal resolution of 10 km and resolves vertically five sedimentary units, the continental upper and lower crust, the oceanic crust and the lithospheric mantle. Each geological unit is characterised by lithology-dependent mechanical and thermal parameters such as porosity, density, radiogenic heat production and the matrix thermal conductivity (Table 1). For the sedimentary units, usual porosity-depth curves are inapplicable due to late Cenozoic episodes of uplift and erosion. Therefore, we account for the additional load from the eroded geological units by introducing an artificial layer on top of the model to calculate the maximum-burial dependent porosity (Oz) with the lithology-dependent surface porosity (O0) and the compaction coefficient (c) (Table 1).
(1) Oz = O0*e"cz
The resulting porosity configuration is used to determine the spatial distribution of sedimentary bulk densities (p&) using the weighted geometric mean equation with the depth-dependent porosity (^z), the density of liquid water (1024 kg/m3; pw) assumed to fill the pore space and the matrix density of the solid rock components (p s).
(2) pb = pw*z * ps(l-*z)
Subsequently, the pressure is calculated at each grid node with pb the bulk density, g the gravitational acceleration and z the depth. Thereby, the hydrostatic as well as the lithostatic pressures are considered.
(3) VP=pb*g*Vz
We determine the 3D thermal field by assuming heat conduction as the dominant heat transport mechanism on the scale of the lithosphere. This is expressed by the steady-state equation with Xb the bulk thermal conductivity and H the radiogenic heat generation.
(4) H=V-(AbVT)
20° E
30° E
40° E
50° E 60° E
■ Present-day " Temperature i°Cl
-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10
Fig. 2. (a) Present-day temperature distribution at the seafloor [28]; (b) Geothermal gradient of the uppermost 1 km below the topography.
Governing equations for the mechanical load (3) and conductive 3D thermal field (4) have been integrated in a massively parallel finite-element-method based framework (MOOSE). The MOOSE framework provides a powerful and flexible platform to solve multiphysics problems implicitly on unstructured meshes [25].
The steady-state heat equation (4) is sensitive to the assigned thermal properties (thermal conductivity, radiogenic heat production; Table 1) as well as to the choice of boundary conditions. The reduced present-day porosities of shallow sediments as a consequence of Cenozoic uplift and erosion result in higher thermal conductivities of the sediments than would be expected given their actual burial depth (Table 1).
For the lower thermal boundary we assign 1300°C at the base of the lithosphere. This interface corresponds to a zone where shear wave velocities are decreasing with increasing depth [21,26,27] which is interpreted as first partial melting of peridotite at a temperature of ~1300°C and hence, as transition between lithospheric and asthenospheric mantle. The depth of the lithosphere-asthenosphere boundary is shallowest in the oceanic domain and increasing below the continental domain from the western Barents Sea (—110 km) to the central and eastern Barents Sea (~150 km) and to the Kara Sea (-200 km) [22].
The upper boundary is defined by seafloor temperatures extracted from the MPI-ESM-MR climate model [28] (Fig. 2a). Highest present-day seafloor temperatures occur in the western Barents Sea (~8°C) due to the inflow of warm water masses from the North Atlantic. Towards the east, temperatures are decreasing gradually down to -2°C in the southern Kara Sea.
Finally, we extract the potential gas hydrate stability zone by comparing the pressure and temperature at each grid node with the gas hydrates stability conditions obtained from the CSMGem software.
Table 1. Mechanical and thermal properties of geological units. BARMOD represents a 3D density dataset was assigned to the upper mantle.
Megasequences Modelled lithology* Surface porosity ®o Compaction coefficient k (km1) Calculated average bulk density (kg m3) Avg. bulk thermal conductivity X (W m1 K"1) Radiogen. heat production S (^W m3)
Earliest Eocene -Present 25% SS 75% SH 0.63 0.7 2200 1.48 1.3
M.Cret -Paleocene 10% SS 90% SH 0.67 0.8 2500 1.62 1.4
M. Jurassic -M.Cret 10% SS 90% SH 0.67 0.8 2600 1.71 1.4
M.Permian -M.Jurassic 50% SS 40% SH 10% LS 0.54 0.5 2650 2.54 1.1
40% SS
Pre-mid-Permian 10% SH 25% LS 25% DOL 0.46 0.4 2650 2.95 0.7
*SH-shale, SS - sandstone, LS - limestone, DOL - dolomite
Subsedimentary Geological Units Density (kg m3) Thermal conductivity (W m"1 K"1) Radiogenic heat prod. (^W m"3)
Cont. Upper Crust 2750 2.7 1.7
Crystalline Crust Cont. Lower Crust 3020 2.5 0.5
High-density body 3300 2.6 0.3
Oceanic Crust 2920 2.3 0.3
Lithospheric Mantle Continent BARMOD (beyond: 3320) 4 0.01
3. Results
The thickness of the potential gas hydrate stability zone varies within >1 km across the Barents Sea and Kara Sea region. In the oceanic domain, methane hydrates reach an average thickness of ~200-300 m (Fig. 3a). Thereby, the methane hydrate thickness distribution reveals a significant correlation with the age and the thickness of the oceanic
lithosphere. Thickness maxima of up to 600 m are restricted to the thick sedimentary fans along the continental margin. These fans formed in response to late Cenozoic uplift and glacial erosion and represent the oldest part of the oceanic domain. Accordingly, they are underlain by the thickest oceanic lithosphere (50 km) and characterised by a rather low geothermal gradient (~45-50°C/km; Fig. 2b). Towards the Knipovich and Gakkel mid oceanic ridges, the gas hydrate stability zone pinches out due to gradually increasing geothermal gradient (>120°C/km) and the absence of sediments. The thickness of the gas hydrate stability field increases by ~130 m along the continental margin for a thermogenic feed gas (Fig. 3b).
A biogenic pure methane hydrate is only locally stable in the continental domain where sufficient water depths or a low geothermal gradients exists such as in the areas next to Novaya Zemlya, the troughs along the northern continental margin and in the northern Kara Sea region (Fig. 3a). In addition, methane hydrates are locally stable in the central, eastern Barents Sea and in the Timan Pechora Basin with thickness of 100 to 300 m (Fig. 3a). In contrast, a mixed gas hydrate formed from thermogenic gas sources (feed gas composition: 90% methane, 7% ethane and 3% propane) is potentially stable across wide parts of the study area (Fig. 3b). Thickness maxima (>1 km) occur in the northern Kara Sea, Franz Josef Land, Svalbard and the troughs west and east of Novaya Zemlya [21,22]. These regions are characterised by low geothermal gradients (~25°C/km; Fig. 2b) due to a thick underlying lithosphere and/or the wide absence of young insulating sediments. In the remaining shelf region the thickness of the stability zone varies between 200 m and 550 m such as the eastern Barents Sea and the southern Kara Sea. These regions are characterised by similar shallow water depths (~200 m to 300 m), so that not surprisingly, the local geothermal gradient (~35-40°C/km) controls local thickness variations of the modelled gas hydrate stability zone (Figs. 2b, 3).
4. Discussions
4.1. Oceanic Domain/ western Svalbard
Seismic profiles reveal numerous bottom-simulating reflectors along the continental margin west of Svalbard [e.g. 8,10,12]. Geissler et al. (2014) mapped a bottom-simulating reflectors across wide parts of the margins between latitude 79° and 82° (Fig. 3c). The interpreted gas hydrates show reveal a patchy distribution with strongly varying thickness of several hundred meters which may point to locally differing gas sources. This is further supported by measurements of gas emissions which suggest microbial [29-31] as well as thermogenic gas origins [29,32]. Interpreted bottom-simulating reflectors at average depths of 200 m to 300 m coincide with modelling results for a pure methane hydrate composition. Locally, bottom-simulating reflectors show depth anomalies of more than 400 m. These depths rather correspond to the modelled thermogenic gas hydrate compositions.
A potential thermogenic feeding system might be an Early to Middle Eocene source rock interpreted recently across the Lomonosov Ridge [33-35]. This source rock is not sufficiently deep across the Lomonosov Ridge to produce hydrocarbons. Nevertheless, seismic lines imply increasing depositional depths towards the Armundsen Basin so that burial depths may be sufficient to generate hydrocarbons in the deeper Arctic Basins. However, according to the inferred oceanic age, Early to Middle Eocene sediments would be restricted to the margins of the Eurasia Basin and to the southern Norwegian Greenland Sea within the study area. The Fram Strait, which characterises the region west of Svalbard, did not open before Miocene times and thus, after the deposition of the potential source rocks [36]. An alternative potential source rock is of Miocene age and was drilled at ODP leg 151 Hole 909c west of Svalbard [37]. Though this source rock is described as immature to marginal mature [38] it has been recently proposed that the rapid burial of the Miocene source rock and elevated heat flow due to seafloor spreading are sufficient to generate hydrocarbons [39]. Another potential gas source in the oceanic domain might be serpentinisation processes typically occurring at mid-oceanic ridges where lithospheric mantle is exposed to sea water [3,4]. Fluid-rock interactions between sea water and the upper mantle result in the metamorphism of peridotite whereby abiogenic methane is formed potentially feeding gas hydrates. Rajan et al. (2012) interpreted a bottom-simulating reflector overlying a serpentinised basement segment which supports this scenario [40,41]. In contrast, geochemical analysis from gas hydrates in vicinity of the Vestnesa Ridge west of Svalbard reveal a light S13C composition which would exclude a mantle source [32].
4.2. SW Barents Sea
Though the SW Barents Sea is characterised by a shallower bathymetry (<450 m), interpreted bottom-simulating reflectors are at similar depths (-220 to -345 m) compared to the western Svalbard area (cf. >1000 m) [9,13,4244]. This is partly related to a generally lower geothermal gradient in the SW Barents Sea (~30-60°C/km; cf. 70-120°C western Svalbard; Fig. 2b) and inferred mixed hydrates with a larger stability field [e.g. 42]. This is in line with our modelling results which reveal that a pure methane hydrate is unstable in the SW Barents Sea (Fig. 3a). Instead, the best fit with the interpreted depth to bottom-simulating reflectors is achieved with a mixed feed gas composition of 90% methane, 7% ethane and 3% propane (Fig. 3b) which fits the chemical composition of the Sn0hvit gas field [13]. In contrast, a feed gas with lower amounts of heavy hydrocarbons (3% ethane; 1% propane) reproduces the inferred base of the gas hydrate stability zone at 220 m depth below the central Barents Sea [44,45].
Several gas-prone petroleum systems are inferred beneath the SW Barents Sea providing a potential source for mixed gas hydrates (Fig. la) [46,47]. Oligocene to Miocene episodes of tectonic uplift and glacial erosion resulted in gas expansion, tilting of reservoirs, seal failure and hydrocarbon leakage [47-49]. In addition, Pliocene/Pleistocene ice sheet coverage involved further erosion which likely reinforced hydrocarbon leakage. During the glacial periods, permafrost may have acted as a seal for the rising gases and the additional load of the ice sheets may have triggered the formation of gas hydrates from the trapped gas. Ostanin et al. (2013) estimate thickness of 600 m for thermogenic hydrates during the last glacial maximum. The modelled present-day thickness of -200 m indicate that the stability zone for thermogenic hydrates thinned by -400 m following the retreat of the ice sheet and corresponding pressure release. The released fluids reached the surface and formed the widely observed pockmarks (Fig. 3c) [12,50-52]. At present-day, these pockmarks of the SW Barents Sea are assumed to be widely inactive [50,51]. Gas flares mapped by seismic data suggest that present-day fluid flow is restricted to open fault systems [52].
Fig. 3. (a) Thickness of the gas hydrate stability zone for a pure methane hydrate and (b) a thermogenic feed gas composition composed of 90% methane, 7% ethane and 3% propane. Interpreted gas hydrates are marked with a cross. The distribution of pockmarks and gas flares is marked with striped polygons; (c) A magnification of the Svalbard region. The polygon delineates the distribution of gas hydrates and the red dots characterise regions where pockmarks and gas flares are described (see text for references).
4.3. Remaining continental domain of the study area
Petroleum systems are inferred for all major sedimentary basins beyond the SW Barents Sea such as the Kara Sea and the eastern Barents Sea [34,35,37,47,53,54]. Likewise, these basins experienced late Cenozoic inversion and glacial erosion including most likely hydrocarbon leakage [13,15-17,49]. Our modelling results reveal that released thermogenic fluids would find stable pressure and temperature conditions over wide parts of the study area (Fig. 3b). In fact, there are a few studies which describe evidences for fluid flow such as gas flares and pockmarks in the SE Barents Sea [18], the central Barents Sea [45] and the southern Kara Sea [19,55], though the data coverage is generally very sparse for these remote regions. Recent measurements of gas flares in the southern Kara Sea indicate a biogenic
gas origin released from thawing permafrost [55,56]. The presence of permafrost suggest that some offshore regions may have not reached thermal steady-state at present-day following the last glacial maximum. Accordingly, the shallow subsurface might be locally colder than predicted by the presented thermal model.
5. Conclusion
In summary, we assess the gas hydrate inventory of the Barents Sea and Kara Sea region for two different feed gas compositions. The modelled potential gas hydrate stability zone varies significantly across the study area and strongly depends on the local geological setting, the corresponding geothermal gradient and the feed gas composition. Mixed gas hydrates formed from thermogenic feed gas are potentially stable over wide parts of the study area. The corresponding thermogenic feed gas may have derived from leaking petroleum systems in response to late Cenozoic basin inversion. Accordingly, these mixed gas hydrates may be more widespread than previously assumed. Future studies require more in situ geochemical and geophysical analysis in combination with petroleum system modelling to further assess the formation and dissolution of gas hydrates through time and space. Nevertheless, this study presents a first valuable insights into the potential gas hydrate inventory of a geological complex region.
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