Article pubs.acs.org/jced
Permafrost-Associated Gas Hydrate: Is It Really Approximately 1 % of the Global System? C. Ruppel* U.S. Geological Survey, Woods Hole, Massachusetts 02543, United States ABSTRACT: Permafrost-associated gas hydrates are often assumed to contain ∼1 % of the global gas-in-place in gas hydrates based on a study26 published over three decades ago. As knowledge of permafrost-associated gas hydrates has grown, it has become clear that many permafrost-associated gas hydrates are inextricably linked to an associated conventional petroleum system, and that their formation history (trapping of migrated gas in situ during Pleistocene cooling) is consistent with having been sourced at least partially in nearby thermogenic gas deposits. Using modern data sets that constrain the distribution of continuous permafrost onshore5 and subsea permafrost on circum-Arctic Ocean continental shelves offshore and that estimate undiscovered conventional gas within arctic assessment units,16 the analysis done here reveals where permafrost-associated gas hydrates are most likely to occur, concluding that Arctic Alaska and the West Siberian Basin are the best prospects. A conservative estimate is that 20 Gt C (2.7·1013 kg CH4) may be sequestered in permafrost-associated gas hydrates if methane were the only hydrate-former. This value is slightly more than 1 % of modern estimates (corresponding to 1600 Gt C to 1800 Gt C2,22) for global gas-in-place in methane hydrates and about double the absolute estimate (11.2 Gt C) made in 1981.26
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drilling.32 They had also begun to map bottom simulating reflectors (BSRs),34 which mark the base of the gas hydrate stability zone in some marine sediments on global continental margins. In contrast, few researchers6,14 were studying permafrost-associated gas hydrates, despite a clear recognition that they should exist and in theory be widespread. BSRs do not occur in permafrost settings, and gas hydrate and ice are largely indistinguishable based on standard geophysical measurements. The challenges associated with locating permafrost-associated gas hydrates and studying them in harsh, remote environments certainly contributed to the reliance on a rough estimate for the amount of permafrost-associated gas hydrate. There are two ways in which the perception of global gas hydrate distributions as ∼ 99 % marine and < 1 % permafrostassociated could be misleading. First, it is possible that the global gas-in-place values, which have generally dropped in the last three decades,2 are flawed, but that the estimated low proportion of gas hydrate that occurs in permafrost settings is essentially correct. In this case, there could be far more gas sequestered in permafrost-associated gas hydrates than currently believed if the global gas-in-place estimates are now too low. Second, the percentage of global gas hydrate deposits that occur in permafrost settings could exceed the often-cited 1
INTRODUCTION Even the most modern assessments of gas-in-place in global gas hydrates2,22 have not derived an independent estimate for the relative proportion of gas hydrate found in marine settings versus permafrost areas. Over 30 years ago, a seminal study26 proposed that less than 1 % of global gas hydrates occurred in permafrost areas, a figure that has been widely adopted in the literature. Modern estimates of global gas-in-place range from 2.13·1015 kg CH4 (1600 Gt C)22 to a maximum of 16.5·1015 kg CH4 (12 400 Gt C).15 A careful review of many published assessments2 settled on a value at the lower end of this range (1800 Gt C). If the global assessments are reliable, then permafrost-associated gas hydrate would sequester a maximum of 16 Gt C to 124 Gt C according to the 1 % rule of thumb, with the true value likely being closer to the lower end. Despite the very large global gas-in-place estimates37 that were common at the time of the original study, the absolute estimates published there26 (1.1·106 trillion cubic feet or tcf; 1.5·1013 kg CH4; 1672 Gt C) are in good agreement with current best estimates for global gas-in-place in gas hydrates.2,22 For high northern latitude permafrost-associated deposits, the absolute estimate26 was 735 trillion cubic feet (tcf; 2.2·1015 kg CH4; 11.2 Gt C), which was derived by extrapolating Soviet observations to the Western Arctic and making assumptions about permafrost and sedimentary basin distributions and methane production. At the time of the original study,26 the understanding of marine gas hydrates was far more advanced than knowledge of permafrost-associated gas hydrates. In offshore settings, researchers had by 1981 recovered gas hydrate during ocean This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: August 18, 2014 Accepted: October 9, 2014
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although gas hydrate can form beneath ice sheets under certain circumstances.39 The genetic relationship between permafrostassociated gas hydrates and free gas means that the reservoir rocks for these gas hydrates at some point must have had a physical connection to conventional gas deposits. Just because gas hydrate is stable at certain depths within the sedimentary section does not mean that it occurs there. The limited available data, which are based mostly on logging, seismic, and drilling programs in the Mackenzie Delta12,13 and on the Alaskan North Slope,3,10,23 imply that permafrostassociated gas hydrate occurs mostly in high-permeability lithologies within the lower reaches of the permafrost zone or beneath the base of permafrost.11 In the locations studied to date, the hydrate-bearing zones are not thick, do not continue laterally for tens of kilometers, and are typically associated with petroleum system features, including a gas source at depth (although sometimes displaced laterally from the gas hydrates), migration pathways for this gas to have reached the hydrate stability zone, reservoir permeability, and sometimes structural traps. A microbial methane component has been identified in gas mixtures from some permafrost wells,24 but the mixture of gases and the isotopic composition of the methane are consistent with a largely thermogenic origin for these gases. This observation implies that most permafrost-associated gas hydrates formed as already-migrated, but still deeply buried, gas was trapped in place during extreme cooling events. However, the present-day permafrost-associated gas hydrate deposit need not be directly connected to a known conventional gas deposit in the contemporary geologic record. On the other hand, within the context of a single petroleum basin or assessment unit, a first-order analysis of locations that simultaneously host continuous permafrost and that are underlain by conventional gas resources should highlight areas that have the potential to host permafrost-associated gas hydrates. A special category of gas hydrates is associated with subsea permafrost.29 Subsea permafrost is Arctic Ocean continental shelf permafrost that was previously exposed subaerially and that is now inundated with up to 125 m of water due to sea level rise since the disappearance of ice sheets at approximately 11 700 years ago. The inundation process has dramatically raised temperatures in what is now the continental shelf seafloor (e.g., by 15 °C32). This has in turn led to the degradation of the subsea permafrost and the gas hydrate within the permafrost.28,30,35,36 In some cases, gas hydrate below the base of ice-bearing permafrost may survive today despite thawing of the overlying permafrost, but this deeper gas hydrate will eventually dissociate as well as geotherms continue to equilibrate to thermal conditions at the seafloor.30 Although gas hydrates associated with subsea permafrost formed by the same processes as those presently located within or beneath terrestrial permafrost, the offshore gas hydrates are subject to more rapid degradation and have therefore become a focus for climate researchers.21,27,28,30,32
%. In this case, it may be necessary to reassess how the climate system will interact with these gas hydrates21,29 as permafrost continues to thaw. This paper provides a quantitative estimate of the gas-inplace in permafrost-associated gas hydrates in the arctic region based on simple assumptions. Since such an evaluation has not been previously undertaken, this study should be viewed as a first step that will be subject to substantial further refinements as better quality information becomes available.
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BACKGROUND Permafrost-associated gas hydrates can form at the pressure− temperature conditions that characterize the lowermost parts of terrestrial continuous permafrost and the uppermost part of the sedimentary section beneath the base of ice-bearing permafrost (Figure 1). Within permafrost, gas hydrate is stable at subzero
Figure 1. Approximate geotherm measured near Prudhoe Bay, Alaska (dashed line) in an area of continuous permafrost within the Arctic Alaska assessment unit and gas hydrate stability conditions calculated for three gas mixtures: pure methane (black) and the principal gases measured at 180 m depth (77 % CO2 and 21 % CH4) and 240 m (12 % CO2 and 87 % CH4) depth in the Iapetus well.24 Gas hydrate is stable both in the lowermost part of the permafrost zone and for hundreds of meters below the base of permafrost.
temperatures and to depths as shallow as ∼200 m (depending on the geotherm) for pure methane trapped in the gas hydrate cages. For gas mixtures including certain thermogenic components (e.g., C2−C6 gasesethane to hexane derived by heating of the formation), the top of gas hydrate stability can be significantly shallower (e.g., 75−100 m; Figure 1). At depth, the gas hydrate stability zone may extend for hundreds of meters below the base of permafrost at temperatures above the ice point. Because gas hydrate can occur both within and beneath permafrost, these deposits are properly referred to as permafrost-associated gas hydrates. In contrast to marine settings, where most researchers have gained their experience focusing on gas hydrates that formed from dissolved phase gas,40 permafrost-associated gas hydrates formed from gas that combined with water to form clathrates as the climate system descended into major glacial episodes.25 Most of these gas hydrates probably formed in areas that were unglaciated at the very end of the Late Pleistocene (in the millennia leading up to approximately 11 700 years ago),
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DATA To place constraints on the amount of gas-in-place in permafrost-associated gas hydrate deposits, two primary data sets are used. The first is the distribution of arctic permafrost.5,18 Although permafrost distribution models17 are constantly improving, this paper relies on data-driven maps.5 Because of recent evidence that these maps significantly overestimate the extent of subsea permafrost,4,27 only the B
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terrestrial portion of the data set5 was used. Permafrost also occurs in some intermontane and plateau areas south of the Arctic Circle, and gas hydrate has been investigated in locations such as the Tibetan Plateau.41,42 Gas hydrates likely also occur on the Antarctic continent, mostly owing to the pressure associated with the thick ice sheet that loads organic-rich, methane-laden sediments in buried basins.39 Because of the lack of reliable data in these other areas, this study only uses northern high-latitude areas with continuous permafrost identified as having high or intermediate ice content and significant overburden.5 The overburden constraint means that these locations have relatively unconsolidated sediment cover, which makes the sites more conducive to hosting gas hydrate. The second data set comprises the locations and estimated volume of undiscovered gas within the Arctic energy assessment units.16 That study16 split the Arctic region into petroleum basins and used geologic and structural data, thermal histories, and other information to constrain the amounts of undiscovered oil, gas, and gas liquids within these basins. For this analysis, only the undiscovered gas component is used. As noted, the permafrost data set5 could not be adopted to estimate the extent of subsea permafrost, yet some researchers imply that vast quantities of gas hydrate could be present under the Arctic Ocean continental shelves.31 It is therefore important to include the possible distribution of permafrost-associated gas hydrates on the continental shelves in these calculations. There is presently no modern circum-Arctic Ocean map of subsea permafrost distributions based on auditable data. To date, only the Canadian and US Beaufort margins4,19,20 and the Kara Sea27 have been subject to extensive assessments of their subsea permafrost distribution using modern techniques. The observations on the US Beaufort margin and at the edge of the Kara Sea are similar, namely that the 20 m isobath marks the minimum offshore extent of subsea permafrost. East of the Mackenzie River in the Canadian Beaufort Sea, subsea permafrost extends to the 100 m isobath,19,20 but conditions there may be unique owing to the area’s morphology, degree of freshwater input, deglacial history at the edge of the Laurentide ice sheet, and other factors. For the calculations here, it is assumed that subsea permafrost extends to the 30 m isobath on the Arctic Ocean continental margins. In the area east of the Mackenzie River, the 100 m isobath is adopted as the seaward extent of subsea permafrost.
intersection of the undiscovered gas units and the inner continental shelf polygons constitutes areas that are assumed to have the potential for gas hydrate associated with subsea permafrost. Based on simple assumptions, the total amount of gas-in-place in gas hydrate in the resulting area can be calculated by determining the area of these intersections for the onshore and offshore components. In addition, the assessment units16 can be ranked in terms of those most likely to contain gas hydrate deposits based on a combination of the percentage of their areas containing permafrost and the scaled amount of undiscovered gas in that area.
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RESULTS The individual permafrost and undiscovered gas data sets and the resulting map of areas with the potential to host permafrostassociated gas hydrates are shown in Figures 2, 3, and 4. Tables 1 and 2 summarize these results and provide other analyses. In the Eastern Arctic, the West Siberian, Yenisey-Khatanga, and Lena-Vilyui Basins have a combined 5.75·105 km2 where continuous terrestrial permafrost intersects with undiscovered gas deposits. Among these three basins, the West Siberian Basin has the largest undiscovered gas resources, making this the best
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METHODS Shapefiles, which store the geometry and attribute information on geographic features, were downloaded from the U.S. National Sea and Ice Data Center for the permafrost map.5 Shapefiles for arctic assessment data set16 were obtained from the U.S. Geological Survey. The files were imported into Environmental Systems Research Institute (ESRI) ArcGIS geographic information systems (GIS) software in a polar projection. The ETOPO 1 min bathymetric database1 was used to identify the 30 m isobath in the Arctic region. To represent the inner shelf between the coastline and the 30 m isobath, polygons were made in the ESRI ArcGIS software. Only areas where the 30 m isobath was more than a few kilometers offshore were included in the subsea permafrost calculation. The intersection of terrestrial areas characterized by continuous, high to intermediate ice content permafrost and having significant overburden and those areas with undiscovered gas should be the locations with the greatest potential for hosting permafrost-associated gas hydrates on land. The
Figure 2. Index map showing areas of terrestrial continuous permafrost with high (dark blue) and intermediate (royal blue) ice content. The extent of subsea permafrost (pale blue) is assumed to be the 30 m isobath except east of the Mackenzie River, where it was taken as the 100 m isobath. Only large expanses of potential subsea permafrost were considered, but additional subsea permafrost may ring many circum-Arctic Ocean land masses. The additional amount is probably not significant in terms of determining potential gas hydrate distributions. C
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Figure 3. Arctic resource assessment units16 color coded by the amount of undiscovered gas. Hot colors correspond to more gas. The amounts are given in Table 1. Only those units relevant for this study are shown. Note that some units are entirely onshore or offshore, while others, most notably the Arctic Alaska and West Siberian Basins, have both onshore and offshore components.
Figure 4. Areas most likely to host permafrost-associated gas hydrates based on combining permafrost maps with information on undiscovered circum-Arctic Ocean gas resources. Only the areas of intersection between the two databases are shown. Assessment units are color-coded by how much gas might be expected to be beneath permafrost areas in each unit assuming a homogeneous distribution of undiscovered gas. Arctic Alaska and the West Siberian Basin dominate.
target for surveys seeking onshore permafrost-associated gas hydrate, if appropriate permeable lithologies (resource rocks) are present there. In the Western Arctic, the Arctic Alaska and Northwest Canadian Interior basin have 3.85·105 km2 with coincident continuous permafrost and undiscovered gas. However, the Arctic Alaska basin has 725 times more undiscovered gas than the Northwest Canadian Interior basin despite being only two-thirds its size. Thus, the Arctic Alaska basin remains the best prospect for onshore prospecting for permafrost-associated gas hydrates in the Western Arctic. On Arctic Ocean continental shelves, the results are more complicated, and it should be noted that the calculations have only first-order value due to the assumptions made about the extent of subsea permafrost. Nonetheless, the analysis provides basic insights into potential gas hydrate distributions on continental shelves. In the Eastern Arctic, the Laptev Sea Shelf and the East Siberian Sea Basin respectively have 48 % and 38 % of their areas likely to be underlain by subsea permafrost. The East Siberian Sea Basin is less than 25 % the size of and hosts less than 20 % as much gas as the Laptev Sea Shelf, rendering the Laptev Sea the best location for seeking relict gas hydrate associated with subsea permafrost over a large area. However, the West Siberian Basin, the largest of the assessment units,16 is a combined onshore/offshore basin. It contains more than 20 times as much undiscovered gas as the Laptev Sea Shelf and could have 1.56·105 km2 of its area
underlain by subsea permafrost. Taken together, the characteristics of the Laptev Sea and East Siberian basins provide confidence that permafrost-associated gas hydrate probably occurs offshore Siberia. One reason for such extensive potential subsea permafrost on the Russian shelves is their morphology. Compared to the Western Arctic, which has narrow continental shelves, the shelves offshore Siberia continue hundreds of kilometers poleward before reaching the shelf-break. Even if subsea permafrost is limited to water depths shallower than 30 m, a vast area of the Russian shelves still remains within this limit. In the Western Arctic, the maximum intersection of subsea permafrost and an assessment unit occurs in Arctic Alaska, which is an onshore/offshore unit. The ∼4·104 km2 of the assessment unit that could be underlain by subsea permafrost is far smaller than the areas calculated on the Russian margin. However, the calculated area is larger than that near the Mackenzie Delta, even though the calculation there was done with the 100 m isobath as the seaward limit of subsea permafrost. The results can be used to estimate the amount of methane sequestered in permafrost-associated gas hydrates. There are five Arctic assessment units where continuous permafrost onshore and possible subsea permafrost offshore occur in areas larger than 1.9·105 km2. In order from highest to lowest these D
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Table 1. Overlap between Arctic Region Resource Assessment Units and Terrestrial and Subsea Permafrosta b
assessment unit
Arctic Alaska West Siberian Basin Yenisey-Khatanga Basin Laptev Sea Shelf Timan-Pechora Basin Amerasia Basin East Barents Zyryanka Basin Lena-Anabar Basin Sverdrup Basin Lena-Vilyui Basin Barents Platform NW Laptev Sea Shelf North Kara Basin and Platforms Norwegian Margin NW Canadian Interior East Siberian Sea Basin Hope Basin total
undiscovered gas (bcf)b
area (km2)
area with continuous land permafrost (km2)
approx. area with subsea permafrost (km2)
percentage of total area as permafrost
gas beneath permafrost (bcf)
gas beneath permafrost (bcm)
221397 651498
358214 2558722
153269 274688
39908 156203
53.9 16.8
119395 109713
3382 3108
99964
396552
181389
13371
49.1
49096
1391
32562 9063
501711 399743
87969 60092
239352 35997
65.2 24.0
21244 2179
602 62
56891 317557 1506 2107 8596 1335 26219 4488
1140124 873799 58944 127439 562331 278608 520724 112733
4648 2192 50585 38729 23460 128270 10045 405
37422 2793 0 3393 19150 0 0 11716
3.7 0.6 85.8 33.1 7.6 46.0 1.9 10.8
2099 1812 1292 696 651 615 506 483
59 51 37 20 18 17 14 14
14974
333493
5559
0
1.7
250
7
32281 305
432198 300117
3282 231817
0 189
0.8 77.3
245 236
7 7
618
120512
0
45512
37.8
233
7
648
213200
11141 1267537
0 605006
5.2
34
1
a
Assessment units are in descending order of potential gas beneath permafrost zones based on the assumption of homogeneous distribution of undiscovered gas within the unit. Figure 3 shows the geographic locations for the units. bAssessment unit designations, amount of undiscovered gas, and areas are from ref 16.
Unless a much larger percentage of the area of coincident continuous/subsea permafrost and undiscovered gas assessment units is assumed to host permafrost-associated gas hydrates, the 20 Gt C calculated above does not vary by as much as an order of magnitude. Thus, this figure is probably a reasonable estimate on which to base interpretations of the relative importance of marine and permafrost-associated gas hydrates in the global system.
are the West Siberian Basin, the Laptev Sea Shelf, the Northwest Canadian Interior, the Yenisey-Khatanga Basin, and Arctic Alaska. The total area where basins with significant undiscovered gas overlap with regions of continuous onshore permafrost or assumed subsea permafrost is 1.87·106 km2. Assuming that this entire area hosts gas hydrate would imply that the distribution of undiscovered gas in the assessment units is homogeneous and that gas related to these contemporary undiscovered resources had migrated upward prior to the Late Pleistocene events that trapped the gas in hydrates. Instead, the calculation done here assumes that 10 % of the coincident assessment unit and continuous/subsea permafrost area contains gas hydrate. Adopting conservative assumptions for the cumulative thickness of gas hydrate over the entire thickness of the sedimentary section within the hydrate stability zone (50 m), the porosity of the hydrate-bearing unit (50 %) and the saturation of gas hydrate (5 %) yields a total estimate of 20 Gt C (2.7·1013 kg CH4) in permafrost-associated deposits, assuming methane-only gas hydrates and full cage filling. The calculation is done by multiplying the assumed area of gas hydrate distribution by the assumed thickness, the porosity, and the percentage of the porosity containing gas hydrate. This yields the volume of gas hydrate, which is converted to mass assuming 912 kg/m3 for gas hydrate density. For perfect filling of gas hydrate cages in Structure I methane hydrate, about 13 % of the mass can be attributed to methane. The new estimate of gas-in-place in permafrost-associated gas hydrates scales linearly with changes in any single parameter: percentage of the total area hosting gas hydrate deposits, cumulative thickness of gas hydrate, porosity, or saturation.
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DISCUSSION Of the approximately 2.8·106 km2 of arctic lands that are underlain by thick, continuous permafrost with substantial ice content, only about 45 % coincides with basins identified as having significant undiscovered gas.16 This finding underscores the importance of not assuming that gas hydrates are either ubiquitous in permafrost areas or that all permafrost areas are equally likely to host gas hydrates. If permafrost-associated gas hydrate is genetically related to the presence of deep conventional gas, then targeting areas with well-defined conventional petroleum systems is a reasonable strategy in prospecting for permafrost-associated gas hydrates. The rough estimate (2.7·1013 kg CH4; 20 Gt C) of gas-inplace in permafrost-associated gas hydrates is slightly larger than 1 % of the 1600 to 1800 Gt C estimated for global gas-inplace in hydrates.2,22 This estimate is approximately double the 11 Gt C estimated as gas-in-place in permafrost-associated gas hydrates in 198126 based on extrapolation of Soviet estimates to the Western Arctic and assumptions about methane production in sediments in permafrost areas. Thus, the calculations done here yield a higher absolute estimate of the amount of methane E
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Table 2. Possible Permafrost-Associated Gas Hydrates in Each Assessment Unit Based on Scaling the Calculated 20 Gt C Total Carbon Content in Artic Permafrost-Associated Gas Hydrates Using the Results Shown in Table 1a
Arctic Alaska West Siberian Basin YeniseyKhatanga Basin Laptev Sea Shelf TimanPechora Basin Amerasia Basin East Barents Zyryanka Basin Lena-Anabar Basin Sverdrup Basin Lena-Vilyui Basin Barents Platform NW Laptev Sea Shelf North Kara Basin and Platforms Norwegian Margin NW Canadian Interior East Siberian Sea Basin Hope Basin total
percentage of total overlap between undiscovered gas and permafrostb
scaled carbon content in permafrost-associated gas hydrates (Gt C)c
mass of methane in permafrost-associated gas hydrates (kg CH4)d
volume of methane in permafrost-associated gas hydrates (m3 CH4)e
trillion cubic feet (tcf) of methane in permafrostassociated gas hydratesf
38.4 35.3
7.68 7.06
1.02·1013 9.41·1012
1.43·1013 1.31·1013
504 463
15.8
3.16
4.21·1012
5.88·1012
207
6.8
1.37
1.82·1012
2.54·1012
90
0.7
0.14
1.87·1011
2.61·1011
9
0.7
0.14
1.80·1011
2.51·1011
9
0.6 0.4
0.12 0.08
1.55·1011 1.11·1011
2.17·1011 1.55·1011
8 5
0.2
0.04
5.98·1010
8.33·1010
3
0.2
0.04
5.59·10
10
10
3
0.2
0.04
5.27·1010
7.36·1010
3
0.2
0.03
4.34·1010
6.05·1010
2
0.2
0.03
4.14·10
10
10
2
0.1
0.02
2.14·1010
2.99·1010
1
0.1
0.02
2.10·1010
2.93·1010
1
0.1
0.02
2.02·1010
2.82·1010
1
0.1
0.02
2.00·1010
2.79·1010
1
0.0
0.00 20
2.91·1009 2.67·1013
4.05·1009 3.72·1013
0 1313
7.79·10
5.77·10
a
Basins are ranked in the same way as in Table 1. The top four basins constitute more than 96 % of the total permafrost-associated arctic gas hydrates based on the methodology used here. bPercentage calculated as “gas beneath permafrost” (Table 1) for a given assessment unit relative to the total gas beneath permafrost. cScaled amount of carbon in permafrost-associated arctic gas hydrates based on total estimate of 20 Gt C, which is equivalent to 20·1015 g C. dMass of methane in kilogram in permafrost-associated arctic gas hydrates, calculated from the amount of carbon, assuming that carbon constitutes 0.75 by mass of methane. eVolume of methane in m3 in permafrost-associated arctic gas hydrates, assuming density of 717 g/m3 for methane. fVolume of methane in tcf in permafrost-associated arctic gas hydrates based on a conversion factor of 35.3 ft3/m3.
does have a large area that overlaps with subsea permafrost, but the amount of gas in the unit is relatively low compared to some other Siberian basins. While the 35 Gt C value is likely too high, the Laptev Sea and adjacent areas are more likely than some other circum-Arctic Ocean continental shelves to host permafrost-associated gas hydrates based on the arguments in the previous section. An early USGS assessment7 of gas-in-place in gas hydrates on the Alaskan North Slope estimated that 590 tcf (1.2·1013 kg CH4) could be stored in these deposits, while a more recent study determined that 85 tcf (1.7·1012 kg CH4) of gas might be technically recoverable.8,38 The older gas-in-place assessment for the Alaskan North Slope is approximately 40 % of the panArctic estimate for gas-in-place in gas hydrates calculated here. Perhaps coincidentally, if the amount of undiscovered gas in each assessment unit is scaled by the percentage of its area that is underlain by gas hydrate (last column in Table 1), the Arctic Alaska basin leads in absolute terms and comprises approximately 38 % of the total pan-Arctic value.
sequestered in permafrost-associated gas hydrates, but the percentage of these hydrates relative to the global gas-in-place in hydrates remains close to 1 %. The best-studied permafrost-gas hydrates are those in the Prudhoe Bay area of the Alaskan North Slope.10,11 Even there, the cumulative thickness of gas hydrate is less than the 50 m assumed in the calculations done here. On the other hand, saturations vary from 0 % to more than 90 % of pore space in hydrate-bearing units, which are mostly coarse-grained lithologies. The Prudhoe Bay area is just one component of the Arctic Alaska assessment unit, and analyses there and elsewhere in Arctic Alaska imply that far less than the 10 % of the area assumed in the calculations above is associated with gas hydrates.10 A recent study31 estimates 35 Gt C stored in gas hydrates on the Laptev shelf, 75 % greater than the value calculated here for the entire onshore and shallow offshore arctic region and three times the 1981 estimate for gas-in-place in all permafrostassociated gas hydrates.26 The Laptev Sea Shelf assessment unit F
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arctic area with permafrost-associated gas hydrates were underestimated by a factor of 2 or 3, the overall percentage of the global gas-in-place would remain low. Even at 20 Gt C, the value estimated here is almost double that of the 1981 study (approximately 11.2 Gt C),26 which was based on scant observational constraints. While the original study found that less than ∼1 % of global gas-in-place in gas hydrates is probably found in permafrost-associated deposits, the true value is probably closer to 1 %. However, the inclusion of Antarctic gas hydrates39 buried beneath ice sheets would not only increase the global gas-in-place estimate (1600 Gt C)22 by 4−24 %, but would also reduce the relative proportion attributable to arctic region permafrost-associated gas hydrates.
The fact that these calculations consider only arctic regions means that they likely underestimate the absolute amount and the percentage of the global gas-in-place that can be attributed to permafrost-associated gas hydrates. Other permafrost areas (e.g., Tibetan Plateau, intermontane zones) are expected to add negligible amounts to the total gas hydrate budget. On the other hand, it is estimated that (1.3 to 7.3)·1014 m3 ((9.3 to 53)·1013 kg CH4; 70 Gt C to 390 Gt C) occurs as gas-in-place in methane hydrates in Antarctica,39 largely trapped beneath the thick ice sheet. This is substantially larger than the estimates made here for gas-in-place in permafrost-associated arctic gas hydrates. If the Antarctic estimate is valid, then the amount of methane sequestered in permafrost-associated arctic gas hydrates may be largely inconsequential in the global system. While the calculations done here are meant to provide only a first-order constraint on the distribution of permafrostassociated gas hydrates, there are several potential weaknesses to the approach. The key assumption is that undiscovered gas resources in an assessment unit are homogeneously distributed (e.g., last column of Table 1), meaning that spatial correlation between these units and continuous permafrost is enough to predict potential permafrost-associated gas hydrate deposits. The assumption of coincidence between undiscovered gas and overlying thermogenic gas hydrate in permafrost zones may not apply if a structural or lithologic trap prevented gas from migrating upward prior to Late Pleistocene cold events. Gas generally would not have migrated between basins though, so the regional assessment unit approach adopted here probably is the best that can be used without more specific information about the distribution of individual gas deposits. Another concern is the use of methane-only gas hydrate to compute the amount of methane in permafrost-associated gas hydrate deposits whose locations were initially predicted on the basis of the configuration of undiscovered thermogenic gas deposits that likely contain numerous hydrate formers other than methane (e.g., C2−C6 gases). For this first-order calculation, the methane-only approach is probably unavoidable at the level of detail that is possible. If the gas hydrate contains appreciable thermogenic gas, then the amount of methane computed here would be an overestimate. The approach adopted here also ignores details of the geology and permafrost distribution that could preclude the presence of gas hydrates in specific areas. For example, if continuous permafrost is thin (less than several hundred meters), as it is on the western side of the Arctic Alaska assessment unit,9 then gas hydrate will generally not be present. This and other issues related to local geology and cryospheric structure are in part accounted for by assuming that only a fraction of the intersection (10 % in the base case) between the assessment units16 and the continuous/subsea permafrost maps can host gas hydrate.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +1 508-457-2339; E-mail:
[email protected]. Funding
This work was supported by interagency agreement DEFE0002911 between the U.S. Dept. of Energy and the U.S. Geological Survey. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS I am grateful to T. Collett for many discussions of permafrostassociated gas hydrates over several years. P. Overduin provided advice about some issues related to global permafrost distributions. Three anonymous reviewers and especially W. Waite provided helpful comments. Mention of trade names does not imply U.S. Government endorsement of commercial products.
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REFERENCES
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CONCLUSIONS Using the spatial correlation between arctic region resource assessment units containing significant amounts of undiscovered gas,16 areas of continuous permafrost,5 and constraints on the extent of subsea permafrost, the best areas for prospecting for permafrost-associated gas hydrates are identified as Arctic Alaska and the West Siberian Basin. Conservative assumptions yield an estimated 20 Gt C (2.7·1013 kg CH4) sequestered in arctic permafrost-associated gas hydrates. This is slightly more than 1 % of the most recent global gas-in-place assessments,2,22 and less than 0.2 % of the maximum estimate.15 Even if the G
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