Gas Hydrate Technical Review

Under appropriate conditions of temperature and pressure (Figure 2), gas hydrates usually form one of two basic crystal structures known as Structure-I and Structure-II (Figure 3).  Each unit cell of Structure-I gas hydrate consists of 46 water molecules that form two small dodecahedral voids and six large tetradecahedral voids.  Structure-I gas hydrates can only hold small gas molecules such as methane and ethane, with molecular diameters not exceeding 5.2 angstroms.  The chemical composition of a Structure-I gas hydrate can be expressed as 8(Ar,CH₄,H₂S,CO₂)46H₂O or (Ar,CH₄,H₂S,CO₂)5.7H₂O (Makogon, 1981).  The unit cell of Structure-II gas hydrate consists of 16 small dodecahedral and 8 large hexakaidecahedral voids formed by 136 water molecules.  Structure-II gas hydrates may contain gases with molecular dimensions in the range of 5.9 to 6.9 angstroms, such as propane and isobutane.  The chemical composition of a Structure-II gas hydrate can be expressed as 8(C₃H₈,C₄H₁₀,CH₂Cl_2,CHCL₃)136H₂O or (C₃H₈,C₄H₁₀,CH₂Cl_2,CHCL₃)17H₂O (Makogon, 1981).  At conditions of standard temperature and pressure (STP), one volume of saturated methane hydrate (Structure-I) will contain as much as 189 volumes of methane gas -- because of this large gas-storage capacity, gas hydrates are thought to represent an important source of natural gas.

An overview of gas hydrate structures would not be complete without mentioning the newly discovered hydrate structure, Structure H.  The existence of this structure was determined by laboratory nuclear magnetic resonance studies of Ripmeester et al. (1987), and is characterized by three types of cages.  Structure H hydrates have been shown to be unique, with a number of large molecules able to fit into the largest cage of these newly discovered gas hydrate.  Structure H guest molecules include numerous naturally occurring substances, including adamantane, gasoline range hydrocarbons, and napthalene ingredients.  For a complete description of the structure and properties of gas hydrates see the summary by Sloan (1998).

Gas hydrates have been inferred to occur at about 50 locations throughout the world (Figure 4, modified from Kvenvolden, 1988).  However, only a limited number of gas hydrate accumulations have been examined in any detail.  In the following section of this paper, four of the best known marine and onshore permafrost-associated gas accumulations are introduced and described.  Discussions pertaining to the volume of gas within each of the gas hydrate accumulations described in the following section are included later in the energy resource assessment section of this paper.  The four gas hydrate accumulations considered include those (1) on the Blake Ridge along the southeastern continental margin of the United States, (2) along the Cascadia continental margin off the Pacific coast of the United States, (3) on the North Slope of Alaska, and (4) in the Mackenzie River Delta of northern Canada.

Blake Ridge Gas Hydrate Occurrence

Seismic profiles along the Atlantic margin of the United States are often marked by large-amplitude bottom simulating reflectors (BSRs) (Dillon et al., 1993; Lee et al., 1993), which in this region are believed to be caused by large acoustic impedance contrasts at the base of the gas-hydrate stability zone that juxtaposes sediments containing gas hydrates with sediments containing free-gas.  BSRs have been extensively mapped at two locations off the east coast of the United States -- along the crest of the Blake Ridge and beneath the upper continental rise of New Jersey and Delaware (Tucholke et al., 1977; Dillon et al., 1993).

The Blake Ridge is a positive topographic sedimentary feature on the continental slope and rise of the United States (Figure 5).  The crest of the ridge extends approximately perpendicular to the general trend of the continental rise for more than 500 km to the southwest from water depths of 2,000 to 4,800 m.  The Blake Ridge is thought to be a large sediment drift that was built upon transitional continental to oceanic crust by the complex accretion of marine sediments deposited by longitudinal drift currents (Tucholke et al., 1977).  The Blake Ridge consists of Tertiary to Quaternary sediments of hemipelagic muds and silty clay (Shipboard Scientific Party, 1996).  The thickness of the methane-hydrate stability zone in this region ranges from zero along the northwestern edge of the continental shelf to a maximum thickness of about 700 m along the eastern edge of the Blake Ridge (Collett, 1995).  The occurrence of gas hydrates on the Blake Ridge was confirmed during Leg 76 of the Deep Sea Drilling Project (DSDP) when a sample of gas hydrate was recovered from a sub-bottom depth of 238 m at Site 533 (Shipboard Scientific Party, 1980).

Leg 164 of the Ocean Drilling Program (ODP) (Shipboard Scientific Party, 1996) was designed to investigate the occurrence of gas hydrate in the sedimentary section beneath the Blake Ridge (Figure 5).  Sites 994, 995, and 997 comprise a transect of holes that penetrate below the base of gas hydrate stability within the same stratigraphic interval over a relatively short distance (Figure 6).  This transect of holes on the southern flank of the Blake Ridge extends from an area where a BSR is not detectable to an area where an extremely well-developed and distinct BSR exists (Figure 6).  The presence of gas hydrates at Sites 994 and 997 was documented by direct sampling; however, no gas hydrates were conclusively identified at Site 995 (Shipboard Scientific Party, 1996).  Although a BSR does not occur in the seismic reflection profiles that cross Site 994, several pieces of gas hydrate were recovered from 259.90 mbsf (mbsf = meters below sea floor) in Hole 994C and disseminated gas hydrate was observed at almost the same depth in Hole 994D.  One large, solid piece (about 15 cm long) of gas hydrate was also recovered from about 331 mbsf at Site 997 (Hole 997A).  Despite these limited occurrences of gas hydrates, it was inferred, based on geochemical core analyses and downhole logging data, that disseminated gas hydrates occur within the stratigraphic interval from about 190 to 450 mbsf in all the holes drilled on the Blake Ridge (Figure 7).

The depths to the top and the base of the zone of gas hydrate occurrence at Sites 994, 995, and 997 were determined using interstitial water chloride concentrations and downhole log data (Figure 7).  Interstitial water chloride concentrations were used to establish whether gas hydrate occurred within a given core sample, based on the observation that gas hydrate decomposition during core recovery releases water and methane into the interstitial pores, resulting in a freshening of the pore-waters.  The observed chloride concentrations also enable the amount of gas hydrate that occurs on the Blake Ridge to be established by calculating the amount of interstitial water freshening that can be attributed to gas hydrate dissociation.  The estimated gas-hydrate saturations in the recovered cores had a skewed distribution, ranging from a maximum of about 7% and 8.4% at Sites 994 and 995 to a maximum of about 13.6% at Site 997.  For a more complete discussion on the chlorinity calculated gas hydrate contents see Shipboard Scientific Party (1996).

Natural gas hydrate occurrences are generally characterized by the release of unusually large amounts of methane during drilling and an increase in downhole log-measured acoustic velocities and electrical resistivities.  The well log inferred gas-hydrate-bearing stratigraphic interval on the Blake Ridge (190-450 mbsf; Figure 7) is characterized by a distinct stepwise increase in both electrical resistivity (increase of about 0.1-0.3 ohm-m) and acoustic velocity (increase of about 0.1-0.2 km/sec).  The depth of the lower boundary of the log inferred gas-hydrate-bearing interval on the Blake Ridge is in rough accord with the predicted base of the methane hydrate stability zone and it is near the lowest depth of the observed interstitial-water chlorinity anomaly (Figure 7).

Cascadia Continental Margin Gas Hydrate Occurrence

BSRs have been extensively mapped on the inner continental margin of northern California (Field and Kvenvolden, 1985).  These constitute a single, inferred, gas-hydrate accumulation that covers an area of at least 3,000 km² on the Klamath Plateau and the upper continental slope at water depths ranging from 800 to 1,200 m.  Limited seismic data show that this regionally extensive inferred gas-hydrate occurrence extends northward to offshore Canada (Hyndman et al., 1996) and seaward at least to the base of the slope (3,000 m water depth).  The occurrence of gas hydrates on the Pacific margin of the United States was confirmed in 1989 when numerous gas-hydrate samples were obtained during seabed (0-6 mbsf) sediment coring operations (water depths ranging between 510 and 642 m) in the Eel River Basin (Brooks et al., 1991).  Recovered gas-hydrate samples consisted of dispersed crystals, small nodules, and layered bands.  The location of these gas hydrates coincides nearly, but not exactly, with the area of BSR-inferred gas hydrates described by Field and Kvenvolden (1985) along the northern California coast.  Gas hydrates have also been recovered along the Cascadia margin from a relatively restricted zone within 17 m of the sea floor in three research coreholes drilled during Leg 146 of the Ocean Drilling Program: Holes 892A, 892D, and 892E (Figure 8) (Shipboard Scientific Party, 1994).  All of these coreholes are located on the Oregon continental slope in about 675 m of water.

Leg 146 of the Ocean Drilling Program (Shipboard Scientific Party, 1994) was designed to examine fluid movement in the Cascadia continental margin and to provide well-constrained estimates of the volume of fluid associated with accretionary sedimentary wedges.  In addition, the presence of distinct BSRs on the Cascadia margin also provided an opportunity to examine the potential interrelation between the occurrence of natural gas hydrates and BSRs.  Four locations were drilled off the west coast of Vancouver Island and Oregon (Figure 8).  As mentioned above, gas hydrate crystals were recovered in the near-surface (2-17 mbsf) sediments at Site 892.  Downhole logs and a vertical seismic profile (VSP) at Site 892 established that locally the BSR is caused by free-gas below about 71 mbsf; however, the borehole surveys yielded relatively little useful gas hydrate data.

Site 889, located off the west coast of Vancouver Island (Figure 8), yielded a wealth of data pertaining to the in-situ nature of gas hydrates on the Cascadia margin.  Massive accumulations of gas hydrate were not encountered at Site 889.  Rather, indirect evidence from recovered cores and downhole geophysical surveys suggests that most of the gas hydrates at Site 889 occur as finely disseminated pore-filling substances.  Temperature measurements of the recovered cores and the dilution of pore-water salts suggest that about 10 to 40 percent of the pore-space within the sediment is filled with gas hydrate at Site 889 (Shipboard Scientific Party, 1994).

Gas hydrates were not conclusively identified at Site 889 (Shipboard Scientific Party, 1994); however, it’s presence was inferred, based on geochemical analyses of cores and downhole geophysical surveys (VSPs) and borehole logging data within the depth interval from about 127.6 to 228.4 mbsf (Figure 9).  Similar to the observations from the Blake Ridge boreholes, the presence of gas hydrates at Site 889 was inferred on the basis of gas-rich cores, low interstitial water chloride concentrations, and low temperature measurements in the recovered cores (Shipboard Scientific Party, 1994; Spence et al., 1995; Hyndman et al., 1996).  In addition, sediment velocity data from downhole VSP and ocean bottom seismometer (OBS) surveys (Shipboard Scientific Party, 1994; Spence et al., 1995; Hyndman et al., 1996) indicate that gas hydrates occur in the 50- to 80-m-thick interval above the BSR (approximate depth of 230 mbsf) at Site 889.  Observed chloride anomalies were also used to estimate the amount of gas hydrate that occurs at Site 889 by calculating the amount of interstitial water freshening that can be attributed to gas hydrate dissociation.  The estimated volume of sediment porosity occupied by gas hydrate in the recovered cores ranged from a minimum of about 5% immediately below the sea floor to a maximum of about 39% near the bottom of well log inferred gas hydrate occurrence at Site 889 (Hyndman et al., 1996).

North Slope of Alaska Gas Hydrate Occurrence

Previous North Slope studies (Collett, 1983; Collett et al., 1988; Collett, 1993) indicate that the Prudhoe Bay-Kuparuk River gas hydrate accumulation is restricted to Tertiary age sediments of the Sagavanirktok Formation.  The Sagavanirktok Formation consists of shallow-marine shelf and delta-plain deposits composed of sandstone, shale, and conglomerate whose provenance is the Brooks Range, to the south.  The  Sagavanirktok Formation includes the informally named West Sak and Ugnu sands.  These oil-bearing horizons have been extensively described by Werner (1987) and are estimated to contain more than approximately 6 million metric tons of in-place oil.

The occurrence of natural gas hydrate on the North Slope of Alaska was confirmed in 1972 with data from the Northwest Eileen State-2 well located in the northwest part of the Prudhoe Bay Oil Field.  Studies of pressurized core samples, downhole logs, and the results of formation production testing have confirmed the occurrence of three gas-hydrate-bearing stratigraphic units in the Northwest Eileen State-2 well (reviewed by Collett, 1993).  Gas hydrates are also inferred to occur in an additional 50 exploratory and production wells in northern Alaska based on downhole log responses calibrated to the known gas hydrate occurrences in the Northwest Eileen State-2 well.  Many of these wells have multiple gas-hydrate-bearing units, with individual occurrences ranging from 3- to 30-m-thick.  Most of these well-log inferred gas hydrates occur in six laterally continuous sandstone and conglomerate units; all these gas hydrates are geographically restricted to the area overlying the eastern part of the Kuparuk River Oil Field and the western part of the Prudhoe Bay Oil Field (Figures 10 and 11).  The six gas-hydrate-bearing sedimentary units have each been assigned a reference letter (Units A through F); Unit A is stratigraphically the deepest (Figure 10).  Three-dimensional seismic surveys and downhole logs from wells in the western part of the Prudhoe Bay Oil Field indicate the presence of several large free-gas accumulations trapped stratigraphically downdip below four of the log-inferred gas hydrate units (Figures 10 and 11; Units A through D).  The total mapped area of all six gas hydrate occurrences is about 1,643 km²; the areal extent of the individual units range from 3 to 404 km².  The volume of gas within the gas hydrates of the Prudhoe Bay-Kuparuk River area is estimated to be about 1.0 to 1.2 trillion cubic meters, or about twice the volume of conventional gas in the Prudhoe Bay Field (Collett, 1993).

Mackenzie River Delta of Canada Gas Hydrate Occurrence

Assessments 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).  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 hydrates (Figure 12).  All of these inferred gas hydrates occur in clastic sedimentary rocks of the Kugmallit, Mackenzie Bay, and Iperk sequences (Dixon et al., 1992).  Two of the occurrences were associated with ice-bearing permafrost while the remainder were beneath the permafrost interval.  The frequency of gas hydrate occurrence in offshore wells was greater, with possible or probable gas hydrates identified in 35 out of 55 wells (63%).

The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was designed to investigate the occurrence of in-situ natural gas hydrates in the Mallik area of the Mackenzie River Delta of Canada (Figure 12) (Dallimore et al., 1999).  The Mallik 2L-38 gas hydrate research well was drilled near the site of the existing Mallik L-38 well, which was drilled by Imperial Oil in 1972 (Bily and Dick, 1974).  As described in Collett and Dallimore (1998), the Mallik L-38 well is believed to have encountered at least 10 significant gas-hydrate-bearing stratigraphic units within the depth interval from 810.1 to 1,102.3 m.  Bily and Dick (1974) concluded that each of the gas-hydrate-bearing units in the Mallik L-38 well contained substantial amounts of gas hydrate.  However, no attempt was made to quantify the amount of gas hydrate or associated free gas that may have been trapped within the log inferred gas hydrate occurrences.

While drilling the Mallik 2L-38 well, a major emphasis was placed on coring the log inferred gas hydrate intervals identified in the Mallik L-38 well.  A total of 13 coring runs were attempted with a variety of coring systems.  Approximately 37 m of core was recovered from the gas hydrate interval (878-944 m) in the Mallik 2L-38 well (Dallimore et al., 1999).  Pore-space gas hydrate and several forms of visible gas hydrate were observed in a variety of sediment types.

Data from downhole logging in both the Mallik L-38 and 2L-38 (Figure 13) wells and formation production testing in the Mallik L-38 well have been used to assess local geology, permafrost, and gas hydrate conditions.  In the upper 1,500 m, three stratigraphic sequences have been identified using reflection seismic records and well data (Jenner et al., 1999):  These include the Iperk Sequence (0-337.6 m), the Mackenzie Bay Sequence (337.6-918.1 m), and the Kugmallit Sequence (918.1 m-bottom of hole).  The Iperk Sequence appears to be composed almost entirely of coarse grained sandy sediments.  Previous coring experience (Dallimore and Matthews, 1997) indicates that the Iperk sediments are unconsolidated.  The Mackenzie Bay sequence is also sand dominated with a distinct fining upward section near its upper contact with the Iperk Sequence.  The Kugmallit sequence (>918 m) consists of interbedded sandstone and siltstone.  Drill-cuttings and drilling records suggest that the grain cementation in the Mackenzie Bay and Kugmallit Sequences is quite variable.  The base of ice-bearing permafrost in the Mallik 2L-38 well is estimated at about 640 m on the basis of available well log information.

The well-log-inferred gas hydrate occurrence in the Mallik 2L-38 well occupies the depth interval between 888.84 and 1,101.09 m (Figures 13 and 14); however, not all of this interval is occupied by gas hydrate.  The cored and logged gas hydrate occurrences in the Mallik 2L-38 well (Figure 14) exhibit deep electrical resistivity measurements ranging from 10 to 100 ohm-m and compressional-wave acoustic velocities (V_p) ranging from 2.5 to 3.6 km/sec.  In addition, the measured shear-wave acoustic velocities (V_s) of the confirmed gas-hydrate-bearing units in the Mallik 2L-38 well range from 1.1 to 2.0 km/sec.

Bily and Dick (1974) originally interpreted the presence of free-gas in contact with gas hydrate on the basis of spontaneous-potential well log responses within several intervals of the Mallik L-38 well.  They also speculated that rapid pressure responses during a production test (Production Test-1: 1,098-1,101 m) within a suspected free-gas unit are evidence of highly permeable free-gas-bearing sediments.  Acoustic transit-time log data from the Mallik 2L-38 well, confirmed the occurrence of a relatively thin free-gas zone (1,100.0-1,101.9 m) at the base of the deepest downhole log-inferred gas-hydrate.  As shown in Figure 14, the log-measured compressional-shear-wave velocity ratios (V_p / V_s) below 1.8 are indicative of free-gas-bearing sediment.