Mackenzie Deltabeaufort Sea Region Canada

The Mackenzie Delta-Beaufort Sea region, as described by Procter et al. (1984), is composed in part of modern deltaic sediments and older fluvial deposits of Richards Island, the Tuktoyaktuk Peninsula and offshore areas extending out onto the continental shelf to a water depth of about 200 m. The post Paleozoic sedimentary rocks of the Beaufort Sea continental shelf are subdivided into two major sections: pre-Upper Cretaceous and Upper Cretaceous to Quaternary strata. A major regional unconformity marks the boundary between the Upper Cretaceous and older strata. Above this regional unconformity, sedimentation was dominated by deltaic processes, resulting in a series of thick, generally northward prograding delta complexes (reviewed by Dixon and Dietrich, 1990).

Tuktoyaktuk Exploration Pictures
Figure 3. Map showing the thickness (in meters) of the methane hydrate stability zone in northern Alaska (modified from Collett, 1993).
Prograding Delta Complexes
Figure 4. Map showing the depth (in meters) to the base of the methane hydrate stability zone in the Mackenzie Delta-Beaufort Sea region of northern Canada (modified from Judge and Majorowicz, 1992).

3.1. Gas Hydrate Stability Conditions

In the Mackenzie Delta area, subsurface temperature data come from industry acquired production drill stem tests, bottom hole well log surveys, and long term precise temperature studies undertaken in approximately 50 instrumented exploration wells (Judge et al., 1981; Taylor et al., 1982). The thickness of ice-bearing permafrost and related permafrost temperatures are known to vary considerably over relatively short distances in the Mackenzie Delta (Taylor et al., 1996). Beneath the permafrost interval the geothermal gradients in the Mackenzie Delta-Beaufort Sea region are relatively uniform, ranging from about 3.0°C/100m to 4.0°C/100m (Majorowicz et al., 1990; Majorowicz et al., 1995).

Pore-pressure information from beneath the permafrost in the Mackenzie Delta region suggests a variable stress regime. Data from four wells drilled offshore on the continental shelf indicate that pore-pressures are abnormally high immediately beneath the base of ice-bearing permafrost, possibly as a result of gas hydrate dissociation (Weaver and Stewart, 1982). Limited pore-pressure data from onshore wells suggest near hydrostatic pore-pressures (9.795 kPa/m; 0.433 psi/ft) immediately below the base of permafrost (Hawkings and Hatelid, 1975).

Analyses of gas samples and mud log gas chromatography data from industry wells reveal that the formation gases within the upper 2,000 m of sediment in the Beaufort Sea region consists almost entirely of methane (99.5 %) (Weaver and Stewart, 1982). Four drill stem production tests of suspected gas hydrate occurrences in two wells drilled on Richards Island in the Mackenzie Delta yielded gas composed principally of methane (99.19 to 99.53 %) (Bily and Dick, 1974). These data confirm that Structure-I methane hydrate should be expected as the primary gas hydrate form in the Mackenzie Delta-Beaufort Sea region.

The pore-water salinity of formation waters in the Mackenzie Delta-Beaufort Sea region, within the depth interval from 200 to 2,000 m are low ranging from values near 5 to 35 ppt (Weaver and Stewart, 1982; Hitchon et al., 1990; Dallimore and Matthews, 1997), which would have little effect on gas hydrate stability.

In a review of the geothermal conditions controlling gas hydrate stability, Judge and Majorowicz (1992) mapped the depth to the base of the methanehydrate stability zone in the Mackenzie Delta-Beaufort Sea region. As shown in Figure 4, the zone in which methane hydrate can occur extends to depths greater than 1,200 m on Richards Island and is extensive beneath most of the continental shelf area of the Mackenzie Delta-Beaufort Sea region.

3.2. Gas Hydrate Occurrences

Assessment of gas hydrate occurrences in the Mackenzie Delta-Beaufort Sea area have been made mainly on the basis of data obtained during the course of hydrocarbon exploration conducted over the past three decades (reviewed by Judge et al., 1994). In addition, two dedicated scientific drilling programs (Dallimore and Collett, 1995; Dallimore et al., 1999) have included the collection of gas-hydrate-bearing core samples. A database presented by Smith and Judge (1993) summarizes a series of unpublished consultant studies that investigated well log data from 146 exploration wells in the Mackenzie Delta area. In total, 25 wells (17%) were identified as containing possible or probable gas hydrate (Fig. 4). The frequency of gas hydrate occurrence in offshore wells was greater, with possible or probable gas hydrate identified in 36 out of 55 wells (65%).

Prior to a recently completed gas hydrate research drilling program, the most extensively studied gas hydrate occurrences in the Mackenzie Delta-Beaufort Sea region were those drilled in the onshore Mallik L-38 and Ivik J-26 wells (Bily and Dick, 1974) and those in the offshore Nerlerk M-98, Koakoak 0-22, Ukalerk C-50, and Kopanoar M-13 wells (Weaver and Stewart, 1982). On the bases of open-hole well log evaluation, it is estimated that Mallik L-38 encountered about 100 m of gas-hydrate-bearing sandstone, and Ivik J-26 penetrated about 25 m of gas hydrate. The well-log inferred gas-hydrate-bearing sandstone units in the Mallik L-38 well occur within the depth interval from 820 to 1,103 m, while in Ivik J-26, gas hydrate occupies a series of finegrained sandstone and conglomeratic rock units within the depth interval from 980 to 1,020 meters. Analyses (Weaver and Stewart, 1982) of open-hole well logs and mud-gas logs, indicate that the offshore Nerlerk M-98 well penetrated about 170 m of gas-hydrate-bearing sediments, while the Koakoak 0-22, Ukalerk C-50, and Kopanoar M-13 wells drilled approximately 40 m, 100 m, and 250 m of gas hydrate respectively. In all four cases, the well-log inferred gas hydrate occurs in fine-grained sandstone rock units.

The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, drilled in 1998 near the site of the Mallik L-38 well, included extensive scientific studies designed to investigate the occurrence of in-situ natural gas hydrate in the Mallik field area (Dallimore et al., 1999). Approximately 37 m of core was recovered from the gas hydrate interval (878-944 m) in the Mallik 2L-38 well. Pore-space gas hydrate and several forms of visible gas hydrate were observed in a variety unconsolidated sands and gravels interbedded with non-hydrate bearing silts. The cored and downhole logged gas hydrate occurrences in the Mallik 2L-38 well exhibit both high electrical resistivities and rapid acoustic velocities. In total, the gas hydrate-bearing strata was approximately 150 m thick within the depth interval from 889 to 1,101 m.

During a permafrost-coring program in the Taglu area on Richards Island in the outer Mackenzie Delta, ice-bearing cores containing visible gas hydrate and possible pore-space gas hydrate were recovered (Dallimore and Collett,

1995). The visible gas hydrate occurred at a depth of about 330 to 335 m and appeared as thin ice-like layers that released methane upon recovery. Gas yield calculations suggest that other ice-bearing cores from a corehole in the Niglintgak field area on Richards Island also contained non-visible pore-space gas hydrate.

Estimates of the amount of gas in the gas hydrate accumulations of the Mackenzie Delta-Beaufort Sea region vary from 9.3xl012 to 2.7xl013 m3 (Smith and Judge, 1995; Majorowicz and Osadetz, 1999); however, these estimates are generally poorly constrained. In a recent study by Collett et al. (1999), industry acquired reflection seismic data and available open-hole well logs were used to identify and map the distribution of four distinct gas hydrate accumulations on Richards Island. The total volume of gas trapped as hydrate in the four gas hydrate accumulations on Richards Island is estimated at 90xl09 m3 (Collett et al., 1999).

4. SVERDRUP BASIN, CANADA

The Sverdrup Basin is a structural depression near the northern margin of the North American Craton (Figs. 2 & 5). It is about 1,300 km long and as wide as 400 km in the north-central portion of the basin. The Sverdrup Basin is bordered to the northwest by the Sverdrup Rim and to the south and east by the Franklin Foldbelt; it contains up to 13 km of Lower Carboniferous to upper Tertiary marine and nonmarine terrigenous clastics, carbonates, evaporates, basalt flows, and gabbro dikes and sills. The petroleum geology of the Sverdrup Basin has been described in numerous publications (Smith and Wennekers, 1977; Balkwill, 1978; Nassichuk, 1983, 1987; Procter et al., 1984; and Haimila et al., 1990).

4.1. Gas Hydrate Stability Conditions

Precise temperature surveys have been obtained from 32 petroleum wells drilled in and around the Sverdrup Basin (Taylor, 1988). Temperature logs from the Cape Allison C-47 well drilled in 244 m of water off the southern coast of Ellef Ringnes Island indicates that thick permafrost does not occur beneath the deeper parts of the inter-island channels. In coastal regions, however, permafrost is present, and further inland beyond the marine limit, permafrost has been measured to depths as great as 700 m (Taylor et al., 1982). Thus, gas hydrate may exist on or near the subaeraly exposed islands in the Sverdrup Basin.

Temperature data from five onshore wells drilled on Ellef Ringnes Island show substantial variations due to effects of permafrost dynamics, recent marine regressions, and variable paleoclimatic histories. Geothermal gradients calculated from the temperature surveys in the offshore Cape Allsion C-47 well average about 1.3°C/100m in the Lower Cretaceous Isachesen Formation and about 2.5°C/100m in the Middle to Upper Jurassic Deer Bay Formation (Taylor et al., 1988). Geothermal gradients calculated from the temperature profiles for the five onshore wells drilled on Ellef Ringnes Island range from about

4°C/100m to 8°C/100m within the permafrost sequence and from 3°C/100m to 6°C/100m below permafrost (Taylor et al., 1988).

The review of all known technical sources have yielded no evidence of abnormal formation pore-pressure conditions within the Sverdrup Basin. Due to the lack of data, it is generally assumed that the near-surface sedimentary section in the Sverdrup Basin is characterized by hydrostatic pore-pressure conditions.

All of the known conventional gas fields in the Sverdrup Basin contain dry gas composed almost exclusively of methane (Smith and Wennekers, 1977), suggesting the potential occurrence of only Structure-I methane hydrate.

A review of available data sources uncovered no information on pore-water salinities within the Sverdrup Basin. Again, due to the lack of data, the gas hydrate stability calculations for the Sverdrup Basin have been made in the past assuming no affect from dissolved pore-water salts.

The computer program described in Collett et al. (1993) has been used to calculate the limit of the gas-hydrate stability zone in 30 onshore wells in the Sverdrup Basin (Fig. 5). The gas-hydrate stability program, as described in Collett et al. (1993), requires the following input: (1) Mean annual surface temperature which is assumed to be -20°C in the Sverdrup Basin (Taylor et al., 1988), (2) depth to base of ice-bearing permafrost (modified from Taylor, 1988), (3) temperature at the base of ice-bearing permafrost which in this case is assumed to be 0°C, and (4) the ratio between the geothermal gradient from above to below the base of the ice-bearing permafrost which is assumed to be 1.0 in this study. When present, the thickness of the methane-hydrate stability zone in the Sverdrup Basin (Fig. 5), extrapolated from available permafrost data, ranges from about 36 to 1,138 m. Due to the highly variable nature of the gas hydrate stability zone in the wells assessed, no attempt has been made to contour the stability data in Figure 5.

4.2. Gas Hydrate Occurrences

A study of downhole logs from 138 onshore exploratory wells in the Sverdrup Basin indicate that about 71% of the surveyed wells may have encountered gas hydrate, while about 17 of 30 offshore wells may have penetrated gas hydrate (Smith and Judge, 1993). Most studies dealing with the occurrence of gas hydrate in the Sverdrup Basin have been concerned with gas hydrate induced drilling hazards (Franklin, 1980, 1981). The review of limited information obtained from reports on drilling in the Sverdrup Basin infer the possible occurrence of gas hydrate on or near King Christian, Ellef Ringnes, and Mellville Islands. In 1971, during drilling of the King Christian Island N-06 well, gas leaked into the rig cellar around the outside of the surface casing,

Island Stability Location
Figure 5. Map of the Sverdrup Basin (Canada) showing the location of 30 onshore petroleum wells. Also shown is the thickness of the methane-hydrate stability zone (in meters) in each well.

which was set at 160 m. A well drilled on Ellef Ringnes Island also experienced significant gas flow from behind casing (405 m) while drilling at 2,560 m. Similar gas leaks have been reported throughout the basin, which may be the result of drilling activity thermally disturbing by-passed gas hydrate occurrences. Drilling on Mellville Island has also revealed the possible occurrence of gas hydrate. For example, while drilling a well at Hearne Point several significant gas flows where encountered, one at a depth of 356 m and a second at 895 m. Hydrocarbon production test of both zones yielded classical gas hydrate test results, with low gas flow rates and shut-in pressures that slowly increased beyond hydrostatic during testing. While drilling the Jackson G-16 well off the southwestern coast of Ellef Ringnes Island (approximately 60 m water depth) gas was detected at a depth of 453 m and again at 567 m, indicating possible gas hydrate occurrences. Based on drilling reports, it appears that gas hydrate likely exists in the Sverdrup Basin; however, no direct evidence of gas hydrate has been obtained.

5. WEST SIBERIAN BASIN, RUSSIA

The geology and petroleum geochemistry of the West Siberian Basin is described in considerable detail in many English language publications (reviewed by Grace and Hart (1986). Gas production in the northern part of the West Siberian Basin is principally from the Neocomian reservoirs of the Vartov and Megion "Suites" (average depth of 2,800 m) and the Cenomanian reservoirs of the Pokur "Suite" (average depth 1,100 m). The Pokur "Suite" is overlain by the shale sequence of the Kuznetsov "Suite," which forms a regional seal for most of the underlying sandstone reservoirs.

5.1. Gas Hydrate Stability Conditions

In the West Siberian Basin, permafrost thickness increases gradually from areas of discontinuous permafrost in the south to 580 m thick in the northern part of the basin. Measured geothermal gradients range from 4.0°C/100m to 5.0°C/100m in the central and southwest portion of the basin and geothermal gradients as low as 2.0°C/100m to 3.0°C/100m are reported from the northern part of the basin (Cherskiy et al., 1985).

A review of available data uncovered no evidence of significant pore-pressure anomalies in the near-surface (0-1,500 m) sedimentary section of the West Siberian Basin. Therefore, a hydrostatic pore-pressure gradient (9.795 kPa/m; 0.433 psi/ft) can be assumed when considering gas-hydrate stability calculations in the West Siberian Basin.

The Cenomanian reservoirs of the Poker "Suite" in northern West Siberia contain mostly methane (92.5 to 99.0 %) (reviewed by Grace and Hart, 1986). Because methane appears to be the dominant hydrocarbon gas within the Cenomanian reservoirs of the basin, a pure methane gas chemistry can be assumed for gas-hydrate stability calculations in the West Siberian Basin.

Analyses of water samples collected during petroleum formation testing in Cenomanian reservoirs from below the permafrost sequence indicate that the (bulk) pore-water salinities are low (5 to 14 ppt) and would have little effect on gas hydrate stability.

Cherskiy et al. (1985) have calculated the depth to the top and base of the methane-hydrate stability zone at 230 locations in the West Siberian Basin. They determined that the depth to the base of the methane-hydrate stability zone in the West Siberian Basin ranges from zero along the Oba River to the south and reaches a maximum depth of about 1,000 m along the northeastern margin of the basin (Fig. 6).

5.2. Gas Hydrate Occurrences

Production data and other pertinent geologic information have been used to document the presence of gas hydrate in the Messoyakha field, located in the northeastern corner of the West Siberian Basin (Makogon et al., 1972; Makogon, 1981, 1988; Cherskiy et al., 1985; Krason and Ciesnik, 1985). The Messoyakha gas accumulation is confined to the Dolgan Formation of the Pokur "suite," and production has been established from the depth interval between 720 and 820 m. The upper part (about 40 m) of the Messoyakha field lies within the zone of predicted methane-hydrate stability; thus, separating the Messoyakha field into an upper gas-hydrate

West Siberian Basin
Figure 6. Map showing the depth (in meters) to the base of the methane hydrate stability zone in the West Siberian Basin, Russia (modified from Cherskiy et al., 1985).

accumulation and a lower free-gas accumulation. Prior to production, calculated total gas reserves within the gas-hydrate and free-gas parts of the Messoyakha accumulation were estimated to be about 80 x 109 m3, with about one-third of the reserves within the gas hydrate (Krason and Ciesnik, 1985).

Many Russian researchers believe that long-term production from the gashydrate part of the Messoyakha field has been achieved by a simple depressurization scheme (reviewed by Collett and Ginsburg, 1998). When production began from the Messoyakha field in 1969, the reservoir pressure decline curve followed the predicted path; however, in 1971 measured reservoir-pressures began to deviate from predicted values. This deviation has been attributed to the liberation of free-gas from dissociating gas hydrate. Throughout the production history of the Messoyakha field it is estimated that about 36 percent (about 5.17 x 109 m3) of the gas withdrawn from the field has come from gas hydrate (Makogon, 1988). Recently, however, several studies suggest that gas hydrate may not be contributing to gas production in the Messoyakha field and that the potential resource significance of gas hydrate may have been overestimated (reviewed by Collett and Ginsburg, 1998).

6. LENA-TUNGUSKA, RUSSIA

In this paper, the Vilyuy and Anabar-Khatanga basins are included in the Lena-Tunguska province of the eastern Siberia Craton. The geologic setting of the northern oil and gas provinces of Russia indicate that the Vilyuy Basin is the most promising region for the occurrence of gas hydrate. The Vilyuy Basin covers an area of about 250,000 km2 and it is superimposed on the margin of the early Paleozoic Siberian Platform. The Vilyuy Basin opens to the east into the Pre-Verkhoiansk marginal trough, which together with the Vilyuy Basin forms the Lena-Vilyuy Basin.

6.1. Gas Hydrate Stability Conditions

Most of the Lena-Tunguska province is underlain by continuous permafrost, with thicknesses greater than 1,400 m in the north-central portion of the province (Cherskiy et al., 1985). In general, the permafrost thins toward the margins of the province and is absent to the southwest along the Yenisey River. Locally within the Vilyuy Basin permafrost is about 300 to 750 m thick and the geothermal gradient below permafrost averages approximately 2°C/100m.

Formation under-pressuring has been observed within the Lena-Tunguska province, with calculated pore-pressures being 1.5 to 3.0 MPa lower than normal hydrostatic pore-pressures. The origin of the abnormally low formation pore-pressures is unknown. Gas hydrate stability calculations in the Lena-Tunguska province must take into account the apparent affect of low pore-pressure conditions.

Relatively few gas samples have been collected from the Lena-Tunguska province due mainly to the lack of drilling. Analysis of mud-log data from wells drilled in the sandstone units overlying the Lower Jurassic Suntar shale seal indicate that methane is the dominant hydrocarbon gas in the near-surface (0-1,000 m) sedimentary rocks of the Lena-Tunguska province.

Formation pore-waters within the Middle Jurassic-Cretaceous sedimentary section of the Lena-Tunguska province have low dissolved salt content; ranging from 1 to 10 ppt. Therefore, gas hydrate stability is not likely affected by pore-water salts in the Lena-Tunguska province.

Assuming low pore-pressure gradients, methane gas chemistry, and no dissolved pore-water salts; Cherskiy et al. (1985) determined that the base of methane hydrate stability is about 2,000 m deep within the west-central portion of the Lena-Tunguska province and in the Vilyuy Basin about 800 to 1,000 meters deep (Fig. 7).

Figure 7. Map showing the depth (in meters) to the base of the methane hydrate stability zone in the Lena-Tunguska province, Russia (modified from Cherskiy et al., 1985).

6.2. Gas Hydrate Occurrences

Well data from the first 1,000 to 1,200 m of the Vilyuy Basin often show evidence of significant gas flows in the zone of predicted gas-hydrate stability. For example, while drilling at a depth of approximately 700 m in the Badaran field (Badaran Well Number 7) a gas flow of 120,000 m3 per day was encountered. A similar flow of gas (2,000 to 3,000 m3 per day) and water was encountered in the Bogoronts region at a depth of 500 m. Near-surface (0-1,000 m) gas accumulations were also reported from the Mastakh area. The near-surface sedimentary sequence within the Vilyuy Basin is virtually barren of conventional reservoir seals; however, permafrost may be an effective seal which may contribute to the formation of in-situ gas hydrate accumulations. Based on the occurrence of near-surface gas accumulations, it is likely that gas hydrate exists in the Vilyuy Basin, however, no direct evidence of gas hydrate has been obtained.

7. TIMAN-PECHORA BASIN, RUSSIA

The Timan-Pechora Basin occupies an area of about 322,000 km2 in the northwestern portion of Russia (Figs. 2 & 8). The basin is bounded by the Ural Mountains on the east, by the Pay-Khoy Ridge on the northeast, and by the Timan Ridge on the northwest. To the north, the Timan-Pechora Basin opens into the Barents Sea. In the Timan-Pechora Basin the upper Proterozoic basement is overlain by a thick (3 to 4 km) sedimentary sequence of Ordovician through Lower Devonian rocks. The next sedimentary cycle, Late Devonian through Triassic, was characterized by the deposition of deep-water organic rich shales, limestones, and cherts. After a long break in sedimentation, clastic deposition in the Timan-Pechora Basin was renewed in Middle Jurassic time and continued to the end of Early Cretaceous time. Younger sediments in the basin are represented by Late Pliocene and Quaternary marine clastics and glacial deposits.

7.1. Gas Hydrate Stability Conditions

About 40% of the Timan-Pechora Basin is underlain by permafrost, with thicknesses greater than 600 m along the northeastern margin of the basin. For the most part, permafrost does not extend south of the Pechora River. The geothermal gradient in the near-surface (0-1,000 m) stratigraphic section of the Timan-Pechora Basin range from 1.0°C/100m to 3.0°C/100m.

Formation under-pressuring has been observed in the Timan-Pechora Basin (Sergiyenko and Maydak, 1982). Hydrodynamic studies show that the calculated pore-pressures are as much as 1.8 MPa lower than normal hydrostatic pore-pressures, which may significantly affect gas hydrate stability conditions.

Most of the natural gas within the near-surface stratigraphic section of the Timan-Pechora Basin is associated with coals, which yield mostly methane. Therefore, a pure methane gas chemistry can be assumed when considering gashydrate stability conditions in the Timan-Pechora Basin.

Permafrost appears to be absent beneath the Pechora River, which allows the meteoric recharge of low salinity waters into the Timan-Pechora Basin. Thus, the formation pore-waters within the near-surface stratigraphic section of the basin have low dissolved salt contents and would have no affect on gas hydrate stability.

Chersky et al. (1985) have calculated the depth to the top and base of the methane-hydrate stability zone at 114 locations in the Timan-Pechora Basin.

Methane Zone Map
Figure 8. Map showing the depth (in meters) to the base of the methane hydrate stability zone in the Timan-Pechora Basin, Russia (modified from Cherskiy et al., 1985).
Figure 9. Map showing the location of sedimentary basins in the Northeastern Siberia and Kamchatka region of Russia that may contain conditions favorable for the occurrence of gas hydrate (modified from Cherskiy et al., 1985).

Their stability calculations assume a low pore-pressure gradient, methane gas chemistry, and no affect from pore-water salts. The map in Figure 8, of the methane-hydrate stability zone in the Timan-Pechora Basin, reveals two areas in which methane hydrate may occur. In the area east of the Pechora River the methane hydrate stability zone reaches a maximum depth of about 800 m, while to the northwest of the Pechora River a maximum depth of 600 m was calculated.

7.2. Gas Hydrate Occurrences

Little to no hydrocarbons have been discovered within the post-Permian stratigraphic section of the Timan-Pechora Basin. Small amounts of gas have been discovered with coals in the near-surface Cretaceous section, which may indicate the presence of gas hydrate. However, there is no other evidence of gas hydrate in the Timan-Pechora Basin.

8. NORTHEASTERN SIBERIA AND KAMCHATKA, RUSSIA

This region of northeastern Russia (Fig. 9), extends from the Lena and Aldan Rivers on the west to the Pacific Ocean on the east. More than 70 large to small intermountain basins have been mapped in eastern Russia. Most of these basins appear to be filled with thick (5 to 10 km) sections of upper Mesozoic and Cenozoic clastic sediments. In general, the geology and hydrocarbon potential of the basins (Fig. 9) are poorly known.

Literature reviews yield almost no information on the geologic parameters that control gas hydrate stability within the unexplored sedimentary basins of eastern Russia. Cherskiy et al. (1985) report that the temperature and pressure conditions conducive to gas hydrate formation are present in only the northwestern portion of the study area (Fig. 9); elsewhere subsurface temperatures appear to be high. Cherskiy et al. (1985) indicate that within basins with subsurface data, the base of the predicted gas hydrate stability zone ranges from a depth of about 500 to 1,000 m. In these frontier basins, however, there is no data available to assess the actual occurrence of gas hydrate.

9. SVALBARD, NORWAY

The Svalbard archipelago is located in the Norwegian Arctic between the cold Barents Sea and the relatively warm Atlantic Ocean (Figs. 1 & 2). The geology of the Svalbard archipelago is dominated by the Spitsbergen Basin, a very pronounced synclinal feature that covers most of central Svalbard. A 5-km-thick late Paleozoic through Tertiary sedimentary section has been preserved in the Spitsbergen Basin (Nottvedt et al., 1992).

Approximately 60% of the land area of Svalbard is covered by glaciers. Information from scientific and industry exploratory boreholes indicate that permafrost may cover the entire land area Svalbard with known depths ranging between 100 and 460 m (Landvik et al., 1988). From studies in Alaska and the Sverdrup Basin, it is known that in areas with permafrost depths greater than about 200 m, in-situ thermal conditions may be favorable for the occurrence of gas hydrate. Therefore, pressure and temperature conditions conducive to the formation of gas hydrate does exist in at least some portion of Svalbard.

The only direct evidence for gas hydrate on Svalbard also comes from scientific and industry drilling projects. Government and industry operators have reported significant shallow gas flows while drilling the permafrost and sub-permafrost section on Svalbard. Gas shows during drilling are often the first and only evidence for gas hydrate in many frontier regions. However, there is no data that confirms the occurrence of gas hydrates on Svalbard.

10. GREENLAND, DENMARK

A vast ice cap covers most of Greenland, and about a third of this area is underlain by sedimentary basins. Data from climate research coreholes suggest that temperatures near the base of the ice cap on Greenland are very low and these low in-situ temperatures likely extend into the underlying sedimentary basins where gas hydrate may exists. Geologic studies of regions covered by glaciers during the Pleistocene, such as the Mackenzie Delta area (Dallimore and Matthews, 1997), suggests that thick ice masses elevate pore-pressures within the underlying sedimentary basins. Thus, it is likely that the pressure and temperature conditions conducive to the formation of gas hydrate are prevalent beneath most of Greenland. However, there is no evidence of gas hydrate beneath the Greenland ice cap.

11. CONCLUSIONS

The primary objectives of this paper were to document the potential distribution of permafrost-associated gas hydrates within the circumarctic of the northern hemisphere and to assess the geologic parameters that control the stability of in-situ natural gas hydrate accumulations. Two primary factors affect the distribution of the gas-hydrate stability zone—geothermal gradient and gas composition. Other factors, which are difficult to quantify and often have little affect, are pore-fluid salinity and formation pore-pressures. Geologic studies and thermal modeling indicate that permafrost and gas hydrate may exist in all of the sedimentary basins examined in this study. However, gas hydrate has only been conclusively identified in the Mackenzie Delta-Beaufort Sea region and on the North Slope of Alaska.

Chapter 6

Oceanic Gas Hydrate

William P. Dillon

U.S. Geological Survey Woods Hole, MA, 02543, USA

Michael D. Max

Marine Desalination Systems, L.L.C. Suite 461, 1120 Connecticut Ave. NW. Washington DC, U.S.A.

1. INTRODUCTION

Many gas hydrates are stable in deep-ocean conditions, but methane hydrate is by far the dominant type, making up >99% of hydrate in the ocean floor (Chapter 2). The methane is almost entirely derived from bacterial methanogenesis, predominantly through the process of carbon dioxide reduction. In some areas, such as the Gulf of Mexico, gas hydrates are created by thermogenically-formed hydrocarbon gases, and other clathrate-forming gases such as hydrogen sulfide and carbon dioxide. Such gases escape from sediments at depth, rise along faults, and form gas hydrate at or just below the seafloor, but on a worldwide basis these are of minor volumetric importance compared to microbial and thermogenic methane. Methane hydrate exists in several forms in marine sediments. In coarse grained sediments it often forms as disseminated grains and pore fillings, whereas in finer silt/clay deposits it commonly appears as nodules and veins. Gas hydrate also is observed as surface crusts on the sea floor. Methane hydrate samples have been obtained by drilling (Fig. 1).

2. THE GAS HYDRATE STABILITY ZONE IN OCEANIC SEDIMENT

Gas hydrate forms wherever appropriate physical conditions exist - moderately low temperature and moderately high pressure - and the materials are present -gas near saturation and water. These conditions are found in the deep sea commonly at depths greater than about 500 m (shallower in the Arctic, where water temperature is colder). The physical conditions that control the presence of methane hydrate are usually diagrammed in terms of the temperature/depth field (Fig. 2). The phase boundary (heavy line) separates colder, higher pressure conditions where methane hydrate is stable to the left of the curve from

conditions to the right where it is not. The dashed line shows how temperature conditions typically vary with depth in the deep ocean and underlying sediments.

Figure 1: Photograph of a gas hydrate sample drilled in the Atlantic Ocean on the Blake Ridge, 500 km east of Savannah, GA (Ocean Drilling Project Leg 164, hole 997A, water depth 2770 m, core depth 327-337 m below the sea floor). Photograph courtesy of William Winters, U.S. Geological Survey.

We chose typical western North Atlantic Ocean thermal conditions and imagine a sea floor at 2 km water depth (Fig. 2). Near the ocean surface, temperatures are too warm and pressures too low for methane hydrate to be stable. Moving down through the water column, temperature drops and an inflection in the temperature curve is reached, known as the main thermocline, which separates the warm surface waters, in which "geostrophic" currents are driven by winds, from the deeper cold waters, in which "thermohaline" currents are driven by density variations that are caused by temperature and salinity differences. At about 500 m, the temperature and phase boundary curves cross; from there downward temperatures are cold enough and pressures high enough for methane hydrate to be stable in the ocean.

If methane is sufficiently concentrated (near saturation), gas hydrate will form. However, like ice, the density of crystalline methane hydrate is less than that of water (about 0.9), so if such hydrate formed in the water (e.g. at methane seeps) it would float upward and would dissociate when it crossed the depth where the curves intersect. However, if the gas hydrate forms within sediments, it will be bound in place. Minimum temperature occurs at the sea floor (Fig. 2).

Methane Hydrates Curves

■SEA SURFACE

-TEMPERATURE

PHASE BOUNDARY

SEA FLOOR

i i i i i i i i i 4 O 10 20 30 40 TEMPERATURE (°C)

□ SEDIMENTS GAS HYDRATE PRESENT

Figure 2: Stability region of methane hydrate in sea water as defined by temperature (T) and pressure (P, indicated as water depth). The heavy line defines the limit in P/T of the stability field of gas hydrate, known as the "phase boundary". We indicate the effects of having a sea floor at 2 km water depth in an area with typical temperature distribution. The variation in T with depth is indicated by the dashed line. Gas hydrate can exist where the P/T conditions are to the left of the phase boundary, thus gas hydrate cannot exist in the shallow water, nor at depths in the sediment below 2.5 Km. The gas hydrate stability zone in the sediments, in this example, will extend from the sea floor to about 0.5 km below it.

Downward through the sediments, the temperature rises along the geothermal gradient toward the hot center of the earth. At the point where the curve of conditions in the sediments (dashed line) crosses the phase boundary, we reach the bottom of the zone where methane hydrate is stable.

The precise location of the base of the gas hydrate stability zone (GHSZ) under known pressure/temperature conditions varies somewhat depending on several factors, most important of which is gas chemistry. In places where the gas is not pure methane, for example the Gulf of Mexico, at a pressure equivalent to 2.5 km, the base of GHSZ will occur at about 21° C for pure methane, but at 23° C for a typical mixture of approximately 93% methane 4% ethane, 1% propane and some smaller amounts of higher hydrocarbons. At the same pressure (2.5 km water depth) but for a possible mixture of about 62% methane, 9% ethane, 23% propane, plus some higher hydrocarbons, the phase limit will be at 28°C. These differences will cause major shifts in depth to the base of the GHSZ as you can see from Figure 2. Below the base of the GHSZ (500 m in our example in Fig. 2) methane and water will be stable and methane hydrate will not be found.

The thermal gradient tends to be quite uniform across broad regions where sediments do not vary, so, for a given water depth, the sub bottom depth to the base of the GHSZ will be quite constant. However, because a change in water depth causes change in pressure, we anticipate that the base of GHSZ will extend further below the sea floor as water depth increases (Fig. 3).

SEA LEVEL 500 1000 g 1500

UJ O

3000 3500 4000 4500

Figure 3: Inferred thickness of the GHSZ (dot pattern) in sediments of a continental margin assuming a typical geothermal gradient (from Kvenvolden and Barnard, 1982)

Fortunately, the base of the the GHSZ is often easy to detect by remote sensing acoustic methods. That will be discussed further in Chapter 21 by Peter Miles, but briefly, free gas bubbles commonly accumulate just beneath the base of GHSZ, where free gas is stable and gas hydrate will not exist. Presence of bubbles in intergranular spaces reduces the acoustic velocity of the sediment markedly. Conversely, in the GHSZ the velocity is increased slightly by the presence of gas hydrate, which in the pure state has twice the velocity of typical deep sea sediments. A large velocity contrast generates a strong echo when an acoustic pulse impinges on it. Thus we can create an image of the base of the GHSZ by measuring the return time of echoes along a profile. This approach of using seismic reflection profiles shows that the base of GHSZ generally acts as predicted and that it roughly parallels the sea floor; hence the reflection in seismic profiles has become known as the "Bottom Simulating Reflection" (BSR). The BSR is a sure sign that gas exists trapped beneath the base of GHSZ, and strongly implies that gas hydrate exists, since free gas, which has a tendency to rise, exists just below and in contact with the zone where gas would be converted to gas hydrate.

The discussion so far has implied that the zone where gas hydrate exists forms a more-or-less uniform layer below the sea floor, thickening toward greater depths. This commonly is true, but exceptions exist, generally because

Contlnental Rise

Contlnental Rise

Oil Well Geothermal Gradient

Figure 3: Inferred thickness of the GHSZ (dot pattern) in sediments of a continental margin assuming a typical geothermal gradient (from Kvenvolden and Barnard, 1982)

Assumed geothermal gradient = 27.3° C/ 10001m

Hydrate zone under the ocean

Assumed geothermal gradient = 27.3° C/ 10001m

Hydrate zone under the ocean the thermal structure in the sediments has been disturbed so that it is not uniform. The thermal structure can be disrupted in several ways. Seafloor landslides remove the cooler near-surface sediments, leaving warmer-than-normal materials near the sea floor; that causes local shallowing of the base of the GHSZ. A second common cause of thermal disruption is the presence of salt diapirs, which produce warm spots because the salt has greater thermal conductivity than other sediments; this forces the base of the GHSZ shallower. A secondary affect associated with salt diapirs is that ions dissolved out of the salt act as inhibitors (anti-freeze) to gas hydrate formation, just as salt does to ice. A third way that thermal structure can be disrupted is by circulation of warm fluids up to shallow subbottom regions, using faults as channelways. The region around faults can be warmed enough so that a conduit is created in which temperatures are so high that gas hydrate cannot form. Thus methane and other gases moving with the fluids can reach the sea floor, often in high concentrations. Of course, on reaching the ocean water the fluids are abruptly chilled, and gas hydrate is commonly formed directly on the ocean floor. Such seafloor deposits of gas hydrate often co-exist with the escape of free methane and form distinctive biological environments that are characterized by unique organisms

3. WHERE IS OCEANIC GAS HYDRATE FOUND?

The amount of gas hydrate in the sediments of the world ocean is clearly immense, as discussed in Chapter 2. It has been identified almost anywhere that anyone has looked intensively around the edges of the continents. Methane accumulates in continental margin sediments probably for two reasons. 1. The margins of the oceans are where the flux of organic carbon to the sea floor is greatest. That is because oceanic biological productivity is highest there and organic detritus from the continents also collects to some extent. 2. The continental margins are where sedimentation rates are fastest. The rapid accumulation of sediment covers and seals the organic material before it is oxidized, allowing the bacteria in the sediments to use it as food and form the methane that becomes incorporated into gas hydrate.

4. WHERE IS OCEANIC GAS HYDRATE CONCENTRATED?

Most of the reports of gas hydrate in marine sediments, such as those mapped by Kvenvolden (Chapter 2, Fig. 1) are only indications that gas hydrate exists at some place. Almost every natural resource that we extract for human use, including petroleum, is taken from the unusual sites where there are natural high concentrations. Much of the large volume of gas hydrate may be dispersed material, and therefore may have little significance for the extraction of methane from hydrate as an energy resource. However, even if only a small fraction of the estimated gas hydrate exists in extractable concentrations, the resource could be extremely important. Little mapping of the variations in concentration of gas hydrate has been done in the world (see Dillon, Chapter 13 for the Atlantic continental margin of the United States). The important goal for the future use of methane hydrate as an energy resource is to be able to predict where methane hydrate concentrations exist.

Research still is in an early stage, but field studies in marine settings (seismic profiling, velocity studies, and drilling) suggest that the highest concentrations of hydrate commonly exist near the base of the GHSZ. This suggests that methane is being introduced below the base of the GHSZ by some process, and that the gas is probably being trapped there before entering the lower part of the GHSZ. Of course the presence of trapped gas just beneath the base of the GHSZ is demonstrated by the common occurrence of BSR's. Some of the methane may come from bacterial activity at considerable depth below the base of the GHSZ, but most is probably recycled from above. Many continental margin settings have ongoing sediment deposition. As the sea floor builds up, the thermal gradient tends to remain constant, so the isothermal surfaces must rise with the sea floor. A sediment grain or bit of gas hydrate in the shallow sediments effectively sees the GHSZ migrate upward past it as the sea floor builds up. Eventually it ends up sufficiently far below the sea floor that it is beneath the base of the GHSZ, outside the range of gas hydrate stability. Then the hydrate breaks down (dissociates) and releases its methane, which will tend to rise through the sediments because of the low density of gas and ultimately accumulate at the base of the GHSZ (Fig. 4).

Figure 4: Diagram showing the effect of sedimentary accretion of the sea floor, which causes the base of the gas hydrate to migrate upward over time in order to maintain a constant thickness of the GHSZ. This causes continued dissociation of gas hydrate and release of gas at the base of hydrate. The gas becomes available to be recycled back up into the gas hydrate zone.

Another cause for dissociation of gas hydrate at the base of the GHSZ is present in active continental margins where sediments are thrust into a subduction zone. In such situations a tectonic accretionary wedge is built in which new sediment that is brought into the wedge is thrust in along a fault at the base. This has the effect of folding and lifting the previously accreted

Figure 4: Diagram showing the effect of sedimentary accretion of the sea floor, which causes the base of the gas hydrate to migrate upward over time in order to maintain a constant thickness of the GHSZ. This causes continued dissociation of gas hydrate and release of gas at the base of hydrate. The gas becomes available to be recycled back up into the gas hydrate zone.

BASE, OF

L-REGION OF HYDRATE BREAKDOWN

L MIGRATION OF GAS

BASE, OF

L-REGION OF HYDRATE BREAKDOWN

L MIGRATION OF GAS

N TIME ALONG TRACK (HOURS! S

2200 2000 1800 1600 1400

PROFILE 0BC 5A

20 Km

TURBIDITE BASIN

PLATE MOTION

TECTONIC ACCRETIONARY WEDGE

THRUSTING CAUSES UPLIFT AND HYDROSTATIC PRESSURE REDUCTION

N TIME ALONG TRACK (HOURS! S

2200 2000 1800 1600 1400

PROFILE 0BC 5A

Figure 5: Seismic reflection profile (5A) and interpretation (5B)across the northern Haiti margin, where plate motions cause underthrusting of sediment that results in folding and uplift. The uplift reduces pressure, causing dissociation of gas hydrate at the base of the GHSZ and release of gas. The released gas rises, returns to the GHSZ and forms gas hydrate (from Dillon et al., 1992).

sediments. The upward movement transports the thrusted sediments into shallower water, thus reducing the pressure and causing the gas hydrate to dissociate at the base of the GHSZ, where it is at its phase limit. A seismic profile (Fig. 5) indicates the result of this mechanism and shows a BSR that transects reflectors from strata.

The gas that is released can migrate laterally, of course. When it does so, the gas will often reach a site where it becomes trapped at a culmination

(shallow spot) on the sealing surface formed by the base of the GHSZ. Gas trapped at a place will tend to continuously nourish the gas hydrate above that place and produce high concentrations of gas hydrate. Gas traps at the base of the GHSZ can take several forms. The simplest is formed at a hill on the sea floor, where the base of the GHSZ parallels the sea floor and forms a broad arch or dome that acts as a seal to form a gas trap (Fig. 6). Such a hill can be a

Hydrate Layer
Figure 6: Diagrams of situations that can act as gas traps in which the gas hydrate layer serves as a seal.

sedimentary buildup, such as at the Blake Ridge off South Carolina, a well known site of gas hydrate concentration, or it can be a fold in a tectonic setting.

In some cases, a culmination that traps gas at the base of the GHSZ can form independent of the morphology of the sea floor. This happens over salt domes as a result of control by two parameters. First, salt has a higher heat conductivity than sediment, so a warm spot will exist above a salt dome. Secondly, the ions that are dissolved out of the salt act as inhibitors (antifreeze) to gas hydrate, just as salt lowers the freezing temperature of water ice. This double effect of chemical inhibition and disturbance of the thermal structure causes the base of the GHSZ to be warped upward above a salt dome, creating a gas trap (second panel in Fig. 6).

SE LINE USGS 95-18-20 NW

SP 200 too 600

SE LINE USGS 95-18-20 NW

SP 200 too 600

Figure 7: Seismic profile across a salt dome on the Carolina continental margin.

Figure 7 is a seismic reflection profile at a site off South Carolina (southeastern United States), where the gas hydrate layer is strongly affected by a rising salt dome (diapir). In a sense any seismic profile represents a cross section through the seabed, but keep in mind that the image is formed by reflections of sound from reflectors formed by density and acoustic velocity changes that are inherent to sedimentary layers and gas hydrate phenomena. The vertical axis is imaged in travel time of sound. The image has been computer processed so that it is essentially stretched such that vertical distances are proportionately about ten times horizontal ones (in a true section, features would appear much flatter). Water depth is about 2200 m. Notice that the deep strata appear to be bent up sharply by the rising salt. The base of the GHSZ, as indicated by the BSR, also rises over the dome and the BSR cuts through reflections that represent sedimentary layers. This does not actually represent a physical bending of the base of the GHSZ, but rather a thermal/chemical inhibition that prevents gas hydrate from forming as deeply as it would if the salt dome were not present. The extremely strong reflections just below the BSR to the left of the diapir indicate that considerable gas is trapped beneath the GHSZ. Notice that the reflections above this trapped gas, in the GHSZ are weak. We infer that the weakening of reflections, known as "blanking", may be caused by the preferential accumulation of (fast) gas hydrate in the more porous (slower) strata, thus reducing velocity contrasts that are required to create strong reflections. The blanking suggests that gas hydrate is concentrated there.

To review, we have considered three situations where a dome-like trap would form at the base of the GHSZ. In the first two (formed at a hill on the sea floor or a fold) the trap is formed because the base of the GHSZ parallels the sea floor. In the third case (at a salt diapir) the base of the GHSZ does not follow the sea floor, but rather the shape that traps gas is formed by thermal and chemical control on gas hydrate phase stability. The three cases also demonstrate the three influences that cause dissociation of gas hydrate and release of methane to the traps. In the first case, where sediments are accumulating, the controlling factor is thermal, in the second case, where the trap is being raised to shallower water depths by tectonic forces, the controlling factor is pressure, and in the third case, at a salt dome, the controlling factors are both thermal and chemical.

Of course, there are innumerable ways in which gas can be trapped beneath the gas hydrate-bearing zone and geologists need to be imaginative in searching for such situations. A simple trap (Fig. 6) in which dipping strata of alternating permeability could be sealed at their updip ends by the gas hydrate-bearing layer, forming traps in the more permeable layers. Such a system might develop in a turbidite fan, for example.

The analysis of gas hydrate concentrations as energy resources may employ an analogy to strata-bound mineral deposits, such as the Upper Mississippi Valley Pb-Zn deposits. Gas hydrate deposits are analogous in a number of important respects..

1. Oceanic hydrates are frequently strata-bound because the GHSZ is parallel not only to the seafloor, but commonly to the bedding of recent marine sediments. Where older sediments have been affected by gas hydrate formation or tectonically disturbed, the seafloor-following base of the GHSZ may pass across strata. The zone of gas hydrate formation is the important feature even though it is diagenetic or secondary to the sediment within which it occurs.

2. Oceanic hydrate may generally be horizontally distributed, rather than vertically distributed along fault wall or fault stockwork. Small-scale faulting may be important to fine-scale gas-hydrate distribution within the GHSZ, though.

3. Oceanic hydrate appears to be deposited slowly from low-temperature groundwater fluids carrying the economic material in small quantities to the GHSZ where hydrate forms.

Following this model, the first part of the process of extracting methane from gas hydrrate may utilize techniques that are adaptive analogs of mining methods that are designed to preserve mine integrity.

5. TRANSFER OF METHANE FROM THE GAS HYDRATE RESERVOIR TO THE ATMOSPHERE; CLIMATE IMPLICATIONS

The gas hydrate reservoir in the ocean sediments has significant implications for climate because of the vast amount of methane situated there and the strong greenhouse warming potential of methane in the atmosphere. Methane absorbs energy at wavelengths that are different from other greenhouse gases, so a small addition of methane can have important effects. The climate issue will be considered more extensively by Haq (Chapter 11), but briefly, if a mass of methane is released into the atmosphere it will have an immediate greenhouse impact that will slowly decrease as the methane is oxidized to carbon dioxide in the air. The global warming potential (GWP) of methane is calculated to be 56 times by weight greater than carbon dioxide over a 20 year period. That is, a unit mass of methane introduced into the atmosphere would have 56 times the warming effect of an identical mass of carbon dioxide, over that time period. Because of chemical reactions in the atmosphere, this factor decreases over time; for example, the GWP factor is 21 for a 100 year time period (Houghton et al., 1995, p. 22). Reasonably conservative estimates (Kvenvolden,1988) suggest that there is roughly 3,000 times as much methane in the gas hydrate reservoir as there is in the present atmosphere.

The methane that reaches the atmosphere can be gas released by dissociation of gas hydrate and/or gas that escapes from traps beneath the gas hydrate seal, but even in the latter case the gas will escape most easily when the seal is disturbed by dissociation of the hydrate. Warming or pressure reduction can accomplish the dissociation.

Obviously, warming will occur if ocean bottom waters warm up. However, gas hydrate will only dissociate at its phase boundary. In Figure 2, for example, the gas hydrate at the sea floor exists well within its zone of stability and a few degrees of warming will not cause it to dissociate. In Figure 2 the phase boundary is at about 500 meters below the sea floor. If bottom water became abruptly warmer the warming front would have to propagate downward through the sediment to the depth where the gas hydrate is at the phase limit, which might take hundreds or thousands of years. The change would have to occur as a conductive heat flow, as downward flow of water that could transfer (advect) heat is extremely limited in ocean sediments. Obviously, the place where warming of bottom water will have a rapid influence is where the base of the GHSZ is very close to the sea floor (see Figure 3). Atmospheric warming is presently occurring. Global surface air temperature has probably increased by roughly 0.8° C over the last century. This warming is probably being transferred to the ocean in a manner comparable to chemical tracers that have been observed, which means that warmed surface water can be expected to circulate down to depths of the shallower gas hydrates in several tens of years. In specific cases in the Gulf of Mexico, where warm bottom currents sometimes sweep through the region, and off northern California, active dissociation of gas hydrate at the seafloor has been observed (evidence is the absence of previously observed gas hydrate and release of methane bubbles). Some of this activity has been related to identifiable water temperature changes and present atmospheric warming may be leading to hydrate dissociation that would reinforce the warming trend.

Another process of gas-hydrate warming has been proposed for Arctic areas, where the ongoing sea level rise is causing ocean waters to spread across the coastal plains. In the Arctic, ground temperatures are cold enough that gas hydrate exists in association with permafrost, (see Chapter 5). Ocean water is warm compared to Arctic ground temperatures, so heat transferred from the ocean would be expected to cause dissociation of gas hydrate (Fig. 8). Attempts to verify this process in the field have not been successful to now, but transfer of methane seems likely by this process.

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