3D thermobaric modelling of the gas hydrate stability zone onshore central Spitsbergen, Arctic Norway

Highlights

• A new workflow is presented to calculate the gas hydrate stability zone in 3D is.

• The workflow is applied onshore central Spitsbergen, Arctic Norway.

• Natural gas hydrates may occur naturally throughout the study area.

• Thermogenic gas discovered in Adventdalen is potentially in hydrate form.

Abstract

Dissociation of onshore natural gas hydrates (NGHs) could lead to the release of methane directly to the atmosphere, especially in Arctic regions such as Svalbard, where enhanced climate warming has the capacity to promote rapid methane evasion to the atmosphere following the decay of permafrost and glacier ice. Here we present the first assessment of the NGH stability zone (GHSZ) in central Spitsbergen, a climate-sensitive part of Svalbard where thermobaric conditions appear favourable for onshore NGH formation. We developed an approach incorporating regionally constrained 3-dimensional parameterisation of temperature, pressure and phase boundary (93% methane, 7% ethane, 35 ppt salinity) to define the GHSZ. This resulted in an up to 650 m thick (mean: 308 m) GHSZ covering 74.8% of the study area, thickening significantly in the east where the climate is colder. Perturbation of the base case parameters was undertaken to quantify the sensitivity of the GHSZ to the variation in environmental conditions across the study area. We present 26 examples of these deterministic scenarios and show that the largest changes in the GHSZ were observed when either the ethane content (to 20%) or the regional pore water pressure (to 125% hydrostatic) were increased. The GHSZ also increased markedly when the geothermal gradient was reduced from 33 to 26 °C km⁻¹, but was almost completely inhibited by a dry gas (100% methane), greater salinity (50 ppt), or exposure to an increase in surface temperatures relative to the mean annual air temperature (e.g., by 2 °C). Most parameters affected both the upper and the lower stability boundary of the GHSZ, with the exception of the geothermal gradient, which impacted primarily upon the latter. Given that Svalbard is host to a proven petroleum system, we conclude that NGHs almost certainly exist onshore Svalbard.

Introduction

Natural gas hydrates (NGHs) form as natural gas migrates through the natural gas hydrate stability zone (GHSZ), the presence of which is favoured by cold temperatures and/or high pressures. Suitable conditions occur in a number of general settings, namely deep water (i.e., marine hydrates), the polar onshore regions, and other permafrost-affected environments (e.g., Lu et al., 2011; Kvenvolden, 1993). Exploring for and quantifying NGHs is important for the potential extraction of this unconventional gas resource (Max et al., 2006), for mitigating hydrate-related geohazards (McConnell et al., 2012; Yakushev and Collett, 1992), including greenhouse gas emissions (Ruppel and Kessler, 2017), and for the exploitation of hydrate reservoirs as sequestration sites for CO₂ (e.g., Ersland et al., 2009; Heeschen et al., 2017; Kvamme et al., 2007). This is especially important in the Arctic, where observations and forecasts suggest significant warming is ongoing and air temperature increase is high compared to lower latitudes (e.g., Hinzman et al., 2005; Marshall et al., 2014; Petrie et al., 2015). Although significant conventional hydrocarbon accumulations have been reported for the Arctic (e.g., Gautier et al., 2009; Persits and Ulmishek, 2003), a quantitative assessment of the state of circum-Arctic onshore NGH and unconventional natural gas accumulations is currently lacking. This has major implications on robust forecasting of the fate of permafrost-associated organic carbon, which has the capacity to form several orders of magnitude more methane than the global atmospheric methane reservoir (e.g., Anthony et al., 2012; Ruppel, 2015; Schuur et al., 2015).

The formation and dissociation of NGHs is governed by a composition-dependent thermodynamic phase boundary. Hydrate-related operational hazards have therefore led to efforts that numerically describe and predict the thermodynamic equilibria of a range of NGH compositions (e.g., Hammerschmidt, 1934; Lee and Kang, 2011; Østergaard et al., 2005; Sloan and Koh, 2007). Even though initially intended to understand and limit hydrate formation in pipelines, these calibrated phase equilibria curves are nowadays commonly used to predict the local, regional and even global thickness and extent of the GHSZ by incorporating appropriate and site-specific thermobaric conditions (e.g., Gorman and Senger, 2010; León et al., 2009; Senger et al., 2010; Kretschmer et al., 2015).

Permafrost-associated NGH deposits have been identified in a range of circum-Arctic onshore settings including parts of Siberia (e.g., Makogon, 2010; Makogon and Omelchenko, 2013), northern Alaska (e.g., Anderson et al., 2011; Schoderbek et al., 2013), the Canadian Arctic (e.g., Dallimore et al., 1999; Kurihara et al., 2010) and west Greenland (e.g., Nielsen et al., 2014). The Norwegian-governed Arctic archipelago of Svalbard, comprising all islands between 74 and 81°N and 10–35°E including the largest island of Spitsbergen (Fig. 1), fulfils the main criteria for the existence of natural NGH from deep water to onshore settings (Senger et al., 2017, Fig. 2).

Significant quantities of hydrate-bound gas are present in the deep water Vestnesa gas hydrate province on the continental slope west of Svalbard (e.g., Hustoft et al., 2009; Knies et al., 2015; Plaza-Faverola et al., 2017, Fig. 2A). In addition, the GHSZ may exist in parts of the fjords around Svalbard, especially where thermogenic gas dominated by higher-order hydrocarbons is present (Roy et al., 2012, Fig. 2C). Several studies of the fjord systems (Forwick et al., 2009; Roy et al., 2016, 2015; Liira et al., In Review) have found evidence of fluid seepage, but so far no clear hydrate-related signatures have been identified in spite of good 2D seismic coverage (Bælum and Braathen, 2012; Blinova et al., 2013). The identification of bottom-simulating reflections is difficult onshore Svalbard due to the low signal-to-noise ratio and low vertical resolution of the seismic data (Bælum and Braathen, 2012; Elverhoi and Gronlie, 1981). In addition, there is likely very little acoustic impedance between potential NGH accumulations and the well cemented, permafrost-bearing, and deeply buried and uplifted sedimentary successions.

Even though central Spitsbergen experiences a significantly milder climate than other parts of the High Arctic (Eckerstorfer and Christiansen, 2011), the temperature range allows for the occurrence of gas in hydrate form either in, above, or below the permanently frozen soil layers (Fig. 2D–G), but to what extent is currently unknown. The recent discovery of a wet gas accumulation in Adventdalen (Olaussen et al., 2016), a large valley in central Spitsbergen, has yielded direct evidence of a petroleum system. From the context of geosphere-atmosphere interactions and given sufficient time, the degradation of onshore NGH accumulations is likely to contribute to the atmospheric carbon pool either directly (e.g., through seeps and pingos) or indirectly (e.g., microbiota-based metabolisation to CO₂), with only limited amounts of carbon being absorbed or oxidised as opposed to full uptake by the water column in deep marine settings (e.g., Myhre et al., 2016; Hodson et al., In Review). Onshore, there exists the potential for methane oxidation and uptake in either active layer soils or lakes (e.g., Tveit et al., 2013), although both may be effectively by-passed by springs fed by sub-permafrost ground water (Hodson et al., In Review).

In this contribution we first present a semi-automated workflow to calculate the 3D extent of the GHSZ, allowing for laterally and vertically changing input parameters. Subsequently we apply this workflow to calculate the GHSZ for onshore Spitsbergen for the first time and discuss its sensitivity to the poorly understood spatial and temporal variability in the environmental conditions that are known to influence hydrate stability. Thereafter, we assess the application and development of the work flow through its integration with existing knowledge of Svalbard's petroleum system. Through our results we aim to bring the European High Arctic on par with onshore exploration efforts in the North American (e.g., Schoderbek et al., 2013; Kurihara et al., 2010) and Russian (e.g., Makogon and Omelchenko, 2013) High Arctic, and our contribution sets a first step to establish Svalbard as an onshore NGH province.