Gas hydrate characterization and grain-scale imaging of recovered cores from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

Marine and Petroleum Geology 28 (2011) 394–403

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Gas hydrate characterization and grain-scale imaging of recovered cores from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope Laura A. Stern*, Thomas D. Lorenson, John C. Pinkston U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 April 2009 Received in revised form 28 July 2009 Accepted 4 August 2009 Available online 12 August 2009

Using cryogenic scanning electron microscopy (CSEM), powder X-ray diffraction, and gas chromatography methods, we investigated the physical states, grain characteristics, gas composition, and methane isotopic composition of two gas-hydrate-bearing sections of core recovered from the BPXA–DOE–USGS Mount Elbert Gas Hydrate Stratigraphic Test Well situated on the Alaska North Slope. The well was continuously cored from 606.5 m to 760.1 m depth, and sections investigated here were retrieved from 619.9 m and 661.0 m depth. X-ray analysis and imaging of the sediment phase in both sections shows it consists of a predominantly fine-grained and well-sorted quartz sand with lesser amounts of feldspar, muscovite, and minor clays. Cryogenic SEM shows the gas-hydrate phase forming primarily as a porefilling material between the sediment grains at approximately 70–75% saturation, and more sporadically as thin veins typically several tens of microns in diameter. Pore throat diameters vary, but commonly range 20–120 microns. Gas chromatography analyses of the hydrate-forming gas show that it is comprised of mainly methane (>99.9%), indicating that the gas hydrate is structure I. Here we report on the distribution and articulation of the gas-hydrate phase within the cores, the grain morphology of the hydrate, the composition of the sediment host, and the composition of the hydrate-forming gas. Ó 2009 Published by Elsevier Ltd.

Keywords: Gas hydrate Mount Elbert Electron microscopy Gas chromatography X-ray diffraction

1. Introduction and background Clathrate hydrates of natural gases, commonly referred to as gas hydrates, are nonstoichiometric crystalline solids consisting of a network of water molecules that are hydrogen-bonded in a manner similar to ice and interstitially encaging gases of small molecular diameter, such as methane, ethane, CO2, hydrogen sulfide, etc. (Sloan and Koh, 2007). Distributed globally and often extensively in shallow marine and permafrost environments (Kvenvolden and Lorenson, 2001), gas hydrates harbor a significant yet virtually untapped hydrocarbon source (e.g., Collett, 2002; Max et al., 2006; Ruppel, 2007). Evaluating the concentration and distribution of natural gas hydrate in nature to assess its resource potential poses challenges however, not only due to the complex arrangements in which it may be distributed in the subsurface, but also due to variations in local saturation level and/or its articulation within the sediment host. On a finer scale, evaluation of gashydrate grain morphologies, microstructures, and the nature of the hydrate grain contacts with sediments also remains an ongoing challenge in the study of natural gas hydrates. The amount, spatial

* Corresponding author. Tel.: þ1 650 329 4811; fax: þ1 650 329 5163. E-mail address: [email protected] (L.A. Stern). 0264-8172/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2009.08.003

distribution, and grain characteristics of natural gas hydrates not only influences the behavior of sediments or formations in which they occur, but also affects physical properties measured on recovered samples. Textural characteristics of such samples can be easily altered by the effects of changes in environmental conditions however, including those incurred during recovery and handling, and hence are not always straightforward to interpret. Cryogenic scanning electron microscopy (CSEM) offers an excellent means for obtaining textural information of gas-hydratebearing samples at the grain-level scale, despite the technical challenges imposed by rapid sublimation of the hydrate phase under the high-vacuum conditions of the SEM column, and minimizing electron beam damage of the imaging area. Distinguishing handling-induced surface artifacts from the intrinsic sample surface morphology can be difficult, as well as distinguishing gas hydrate from ice and/or determining the origin of the ice. Imaging gas hydrates recovered from nature poses additional problems as samples are at least partially decomposed or altered during drilling, transit to the surface, handling, and preservation. Even those samples retrieved by pressure corer may undergo partial alteration when transferred to a different vessel at the surface or when frozen or re-pressurized under a different gas composition than that of in situ conditions. As such, they contain to varying degrees hydrate

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that may be partially changed from its original textural and compositional state, or hydrate decomposition products including ice, trapped gas, or frozen pore water. Many of these issues are becoming less problematic however, as we continue to receive increasingly well-preserved samples from new localities and build an increasingly larger archive of images from which to base our interpretations. Comparing natural gas-hydrate samples to those formed in the laboratory under known formation, take-down, and storage conditions yields additional insights. In the present study, we use CSEM to investigate gas-hydrate grain characteristics and distribution of samples retrieved by drill core from the BPXA–DOE–USGS Mount Elbert Gas-Hydrate Stratigraphic Test Well (Mount Elbert well), situated in the Milne Point Unit on the Alaska North Slope (ANS). The full project, in brief, is a cooperative research program established in 2002 involving the U.S. Department of Energy, BP Exploration (Alaska), Inc. (BPXA), and the U.S. Geological Survey, to assess gas-hydrate resource potential on the ANS and to initiate a production test to determine the feasibility for methane production from gas hydrates. The Mount Elbert site within the Milne Point area was selected as the highestranked prospect, with an estimated 60 billion cubic feet (bcf) of gas in-place in two quartz-rich reservoir sands (Hunter et al., 2005). The unit C and D reservoir sands are part of the Sagavanirktok Formation (Collett, 1993; Rose et al., 2011). Pre-drill estimations for the C sand predicted 21 m of gas-hydrate-bearing sand with 89% gas-hydrate saturation, and for the D sand, 14 m with 68% gashydrate saturation (Lee et al., 2011). Drilling proceeded in 2007, and field operations are described in Hunter et al. (2011). The well was continuously cored from 606.5 m (1990 ft) corresponding to a depth just below the base of permafrost, down to 760 m (2494 ft). About 131 m (430 ft) of high-quality 3-inch diameter core were successfully recovered, and two sections, one each from the C and D units, were stored in liquid nitrogen and sent to our laboratory for imaging and analysis. Other relevant physical property information on closely positioned samples to those imaged here can be found in Winters et al. (2011). Here, we build on our previous experience with the CSEM imaging technique (Stern et al., 2004, 2005a,b; Stern and Kirby, 2008) to investigate the physical state, gas-hydrate distribution, and grain contacts within these cored sections. Energy dispersive X-ray spectroscopy (EDS) capabilities of the SEM instrument were also used to help identify the gas-hydrate phase and to differentiate hydrate from ice. To analyze the specific composition of the gashydrate, gas chromatography and methane isotopic analyses were conducted on dissociated gas from bulk samples, building on previous work in this area (Lorenson et al., 2008, 2011). The composition of the sediment host was also determined independently by powder X-ray diffraction methods. 2. Experimental methods 2.1. Sample retrieval and transit Samples from the Mount Elbert test well were retrieved by wireline coring systems as described by (Rose et al., 2011). A lowtemperature oil-based drilling fluid was used to minimize gashydrate dissociation, although cores may still have been outside their pressure–temperature stability conditions for 20–40 min. The samples investigated in this report, both 300 diameter whole-round cores, are designated ‘‘Hyd 1’’ and ‘‘Hyd 700 in accordance with their original shipment labels. Hyd 1 corresponds to core 2, Section 8, subSection 31–36 (unit D), and Hyd 7 corresponds to core 8, Section 5, subSection 31–36 (unit C). Hyd 1 is from a depth of 619.85  05 m top-to-bottom (2033.6  0.2 ft) and Hyd 7 is from 660.96  0.05 m (2168.5  0.2 ft). Both cores were stored in liquid nitrogen (LN2)

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upon retrieval and first shipped to Lawrence Berkeley National Lab for non-invasive X-ray CT imaging (Kneafsey et al., 2011), a procedure that we previously demonstrated imparts no additional damage if performed at sufficiently low temperature (Stern and Kirby, 2008). Samples were then transferred to our laboratory in a LN2 ‘‘dry’’ vapor shipper. Sufficient free LN2 remained in the base of the dewar to ensure thermal stability during transfer. The duration of time that the sections experienced in continual LN2 (or near LN2 temperature) storage was 16 months between initial recovery and SEM (and XRD) analysis, and 23 months between initial recovery and dissociation for gas analysis. 2.2. SEM sample imaging procedures Samples were first broken apart at LN2 temperature to expose the central region of the core that presumably contained the bestpreserved gas hydrate and least-disturbed textural arrangement. Smaller sections (w0.5 cm  0.5 cm  0.75 cm) were cleaved under LN2 from the bulk samples, transferred to a sample stage within an evacuated and pre-chilled (<100 K) cryo-preparation and coating station (Gatan Alto Model 2100), which in turn attached to a LEO 982 field emission SEM. In the preparation chamber, samples were again cleaved by cold knife to produce a fresh fracture surface uncontaminated by surface-water condensation. While still under vacuum, samples were inserted directly through the back of the preparation chamber and on to an auxiliary cryo-imaging stage in the SEM column. Temperature was monitored by a thermocouple embedded in the stage just below the sample, and imaging was conducted at temperature below 102 K and vacuum below 106 kPa (105 mbar). Low accelerating voltage (1.7–2 kV) was used to minimize sample alteration or beam damage to the sample surface, increasing to 10 kV only for energy dispersive X-ray spectroscopy (EDS) elemental analysis. Further details of gas-hydrate imaging procedures are described elsewhere (Stern et al., 2004). 2.3. Gas chromatography and isotopic analyses For analysis of the hydrate-forming gas of Hyd 1 and 7, small sections of each sample were first removed from LN2 and allowed to warm briefly in a dry environment before inserting them into a syringe. Samples typically released a small amount of additional N2 gas that was pushed from the syringe before the onset of the main dissociation event. After the section had completely dissociated, the syringe gas was admitted to pre-evacuated receptacles that were then shipped to Isotech Laboratories, Champaign, Illinois, for full gas compositional analyses and isotopic analyses of methane and carbon dioxide. Compositional analyses of gas samples were measured on a custom-configured Shimadzu 2040 gas chromatography (GC) system. The system operates isothermally, utilizing valve-switching and multiple columns to separate the various components found in natural gas samples. The major fixed gases and high concentration methane were quantified using a thermal conductivity detector (TCD), while low concentration methane and other hydrocarbons down to about 1 ppm were measured using a flame ionization detector (FID). Peak integration and quantification was accomplished utilizing EZChrom software. A gas chromatography–combustion–isotope ratio mass spectrometer (GC–C–IRMS) system, also referred to as ‘‘online’’ or ‘‘continuous flow’’, consisting of an Agilent 6890 GC combustion unit and Finnigan GCCIII interfaced with a mass spectrometer (Delta V Plus or Delta Plus Advantage) was used to analyze the carbon isotopic value of methane and carbon dioxide. Samples were injected into the HP6890 split/splitless injector either manually or using an auto-sampler. The hydrocarbon components were separated by the

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GC column in the HP6890, and each individual component slated for isotopic analysis was burned in a combustion furnace supplied by the instrument manufacturer. The resultant CO2 was introduced directly into the mass spectrometer, and Thermo’s Isodat software was utilized for peak detection and quantification. Hydrogen isotopic values for methane were completed using the same system, but the gas was channeled through a high-temperature pyrolysis furnace instead of through the combustion furnace. The pyrolysis furnace converts methane into H2 and carbon, and the H2 gas was introduced directly into the mass spectrometer. Reference gases were analyzed at the start of each analysis sequence, and 10% of the samples were analyzed in duplicate as check samples. 2.4. X-ray powder diffraction (XRD) procedures X-ray diffraction was used to analyze and identify the sediment fraction of both cores. For all tests, sections of sample were first

warmed and then vacuum dried overnight to remove the ice and gas-hydrate phases. The dry sample material was gently ground by mortar and pestal, then packed into X-ray sample holders. Scans were run at room temperature using a Rigaku MultiFlex X-ray diffractometer with a 2 kW power rating and using a copper source at 1600 W power level (40 kV/40 mA). Continuous scans were run at 0.50 /min from 5 to 80 2-theta. The MultiFlex has a fixed monochromator and was run with a 0.3 mm receiving slit and 1 divergence and scatter slits. No filters were used. An additional portion of sample Hyd 7 (core 2, Section 8) was warmed, dried, and separated using a 75-micron-mesh sieve to create a coarse and fine fraction, and analyzed separately for better identification specifically of the fine-grained sediments and/or clays. Attempts were also made to conduct low-temperature XRD on the samples to investigate the gas-hydrate phase, but were met with only limited success due to the low volume area of the pore-filling gas hydrate and problems with surface ice condensation during analysis.

Fig. 1. Representative low- to high-magnification images of Hyd 1 (core 2, Section 8, unit D). Image A shows an overview of the sample almost immediately after entry into the SEM column, showing the well-sorted fine-grained quartz sand with gas hydrate densely filling the pore space. B shows the overall appearance after a significant fraction of the hydrate has sublimated, showing a clearer view of the sand packing. The box outlined in A is expanded in C, which in turn is expanded in D, showing increasingly closer views of the individual grain contacts and the texture of the pore-filling gas hydrate. E shows a different section of sample, again with abundant gas hydrate filling the pore space. Macropores or gas bubbles are also incorporated in the hydrate section, as seen in D and E. Image F shows a close-up view of a quartz grain and gas-hydrate contact, and the mesoporous surface appearance of the hydrate. See text for additional discussion of mesoporosity.

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2.5. Laboratory-synthesized methane hydrate We also compare the appearance and fabric of Mount Elbert hydrate-bearing core to material formed in the laboratory with known composition, controlled pressure–temperature (P–T) history, and in some cases known extent of dissociation. The labsynthesized material consisted of either pure polycrystalline methane hydrate produced by warming fine-grained ice (or in some cases ice mixed with quartz sand) in an atmosphere pressurized by methane and then held under specific P–T conditions to convert all H2O to clathrate (Stern et al., 1996, 2005b). Samples were fully instrumented to monitor P and T throughout all synthesis and take-down procedures, and in some cases samples

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were also fully compacted (Durham et al., 2003, 2005) or partially dissociated using a custom gas collection and flow-meter apparatus (Circone et al., 2001) to determine stoichiometry and/or extent of decomposition. 3. Results and interpretation 3.1. Whole-rock SEM imagery and X-ray identification A selection of representative SEM images from the two cores Hyd 1 and Hyd 7 are shown in Figs. 1 and 2, and X-ray analysis of the sediment phase is shown in Fig. 3. The lower magnification SEM images (Figs. 1A,B and 2A), coupled with the XRD scans,

Fig. 2. SEM images of Hyd 7 (core 8, Section 5, unit C) showing low- to high-magnification images of the overall sample appearance and geometrical arrangement of the sediment fraction. Image A shows an overview of the fine-grained quartz that dominates the reservoir sand, with gas hydrate filling most of the pore space. The central outlined region in A is expanded in panel B, and the upper-left outlined region in A is expanded in C, both showing closer views of the gas-hydrate phase. The outlined section in C is expanded in D, showing the mesoporous texture of the gas hydrate that results after partial sublimation in the SEM column (see Figs. 4 and 5 and text.) With further sublimation of the gas hydrate (image E) it becomes easier to assess the framework of the sediment grains and the geometrical configuration of the pore volume between them. Silt and clay fragments (muscovite, chlorite) are also included in the sediment fraction, and the clay-rich section outlined in panel B is expanded in F. Image F shows this section after the hydrate has completely sublimated, revealing the now-empty thin veins around which the clay platelets are commonly oriented sub-parallel.

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show the reservoir sandstone to be comprised of well-sorted and fine-grained sand grains that are dominantly quartz (Fig. 3A), with minor amounts of feldspar, micas, and clays (Fig. 3B). The peaks of the minor phases correspond to those of albite, muscovite, and chlorite. Kaolinite may be present but in such minor amount that it is difficult to determine with certainty (Fig. 3B, ‘‘K’’ peak). Cryogenic SEM shows the gas-hydrate phase forming primarily as a pore-filling material between the sediment grains at approximately 70–75% saturation (Figs. 1 and 2), and only sporadically as thin veins typically several tens of microns in diameter (not shown.) Sediment particles within the samples can be clearly identified by their shape and stand in raised relief in the SEM images, while the pore-filling material is a mixture of sI hydrate and ice. While it is difficult to assess with certainty the hydrate saturation based on the two-dimensional nature of the images and the relatively rapid hydrate sublimation rate, our assessment is based on comparisons of early images taken before significant sublimation has occurred with the pore geometry as assessed after the hydrate has fully sublimated. Our estimates of gashydrate saturation are in relatively good agreement with those based on interpretations of well logs that indicate 60 to upward of 80% (Boswell et al., 2011; see also Winters et al., 2011; Lee and Collett, 2011; Lee and Waite, 2008). The geometry of the pore volume varies considerably due to the nature of the sediment contacts and the distribution of silt and clay-sized particles, but commonly displays diameters of several hundred microns necking down to several tens of microns. Closer views of the gas hydrate and ice phases and interactions in the Mount Elbert samples are shown in Fig. 4. Where both phases are present (Fig. 4A and B), ice is easily distinguished by its smooth surface topology that changes little with time, and that is less prone to alteration during beam focusing procedures. Gas hydrate also often appears as smooth and dense material upon immediate entry into the SEM column, but is easily distinguished by the development of surface damage and sublimation effects that soon gives it

a characteristic ‘‘spongy’’ or mesoporous surface texture (described below). Gas hydrate is also distinguished from ice in the SEM column using the instrument’s energy dispersive X-ray spectroscopy (EDS) capabilities, which identifies a small but distinct carbon peak (at w0.28 keV) when focused on flat, dense, non-dissociated hydrate surfaces (Fig. 4B, inset; see also Stern et al., 2004 for further discussion of phase identification). Macropores or gas bubbles are commonly found in all sections of both samples examined, occurring both in the gas-hydrate phase (Fig. 1D and E for example, or Fig. 2C and D) as well as associated with the ice phase (Fig. 4A and E). We presume that where the bubbles occur in association with ice, they are related to dissociation or recovery processes, since ice should not be present in the samples at in situ pressure–temperature conditions. It is difficult to ascertain if the bubbles present in the gas-hydrate phase are related to partial dissociation or are in fact free gas incorporated into the hydrate phase during original formation. On a finer scale, the gas-hydrate phase also commonly exhibits a highly mesoporous texture, that is, it appears almost ‘‘sponge-like’’ and contains many closely spaced pores along the exposed surface with pore diameters ranging from roughly 50 to 200 nm. The mesoporous texture can be difficult to identify with certainty as being original microstructure or the result of subsequent processing or time under vacuum in the SEM chamber. Fig. 4C and D, for example, shows two views of the same section gas hydrate within Hyd 1, with image D showing the rapid development of surface porosity and additional damage during just a short amount of time (w15 min). Ice, in comparison (Fig. 4E and F) shows only minor surface damage when the SEM beam is focused on it. For additional comparison, Fig. 4G and H shows a lab-made methane hydrate þ quartz sand sample used for testing, that had a known pressure–temperature history and known initial grain texture (Durham et al., 2005; Stern et al., 2005a). Methane hydrate is situated at the left in both photos, with quartz at the right. In this sample, the hydrate phase was initially fully dense after lab synthesis. With time in the SEM column however, the dense hydrate substrate began to exhibit surface deterioration and develop initial porosity (Fig. 4G, lower central portion of image), which with time further developed to the spongy appearance (Fig. 4H) that closely resembles the hydrate texture in the Mount Elbert samples. Hence there is no direct evidence that the meso- or nano-porous texture of the gas-hydrate phase in the Mount Elbert samples is original microstructure. This issue has implications for physical property measurements and interpretations, and is discussed below in Section 4.1.

3.2. Gas composition

Fig. 3. Powder X-ray diffraction scan of sample Hyd 7 (core 8, Section 5) shows quartz dominating the pattern (panel A). The outlined section in A is expanded in B, with peaks identified as follows: quartz (Q), plagioclase (mostly albite) (P), mica (predominantly muscovite (M), and minor chlorite (Cl)). Scans were also run on the fine fraction only (<75 micron) of Hyd 7 as well as the full sediment portion from Hyd 1, to confirm the uniformity of the reservoir sands.

The gas collected from the dissociated gas hydrate is composed mainly of methane, with minor components of carbon dioxide and ethane. The concentrations of both carbon dioxide and ethane are higher within the deeper unit C hydrate (Table 1), consistent with other gas measurements reported by Lorenson et al. (2011). The isotopic analyses demonstrate that the methane carbon isotopic composition ranges between 48.8 to 49 and 243 to 251, d13C1 and dDC1, respectively. These results are also consistent with gas hydrate sampled by direct dissociation in plastic syringes on the drill site (Lorenson et al., 2011). The carbon isotopic composition of the carbon dioxide, however, ranges from 15.6 in unit D hydrate to 24.5 in the deeper unit C. These results differ significantly from those obtained during directdissociation of hydrate retrieved from similar depths during initial operations, where the carbon isotopic composition ranges from 3.3 in unit D to 18.4 in unit C (Lorenson et al., 2011). The concentration of carbon dioxide is also enhanced in the LN2-stored hydrate samples

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Fig. 4. Close views of gas hydrate and ice in Mount Elbert sample Hyd 1 compared to lab samples. Image A shows a section from Hyd 1 with side-by-side gas hydrate (upper right section of A) and ice (lower left). Where gas hydrate and ice are both present, the large, isolated macropores or bubbles are usually incorporated within the ice phase, as seen in the lower left of image A. Outlined box in A is expanded in B. Here the hydrate phase shows the characteristic porous surface texture that develops quickly in the SEM chamber. EDS scans of this material (B, inset) show the characteristic carbon peak (labeled ‘‘C’’) that also distinguishes gas hydrate from ice. Scans of the adjacent smooth material show no carbon peak (not shown). Panels C and D show two views of a different section of gas hydrate that was observed to rapidly develop surface porosity and additional damage during a relatively short time (15 min) in the SEM column. In comparison, images E and F show a section that is ice, with holes or macropores in the ice exposing the sediment grain below (E). A close-up view of the ice shows only minor surface damage when the SEM beam is focused on it (F, center), compared to the rapid and severe surface damage that the hydrate phase undergoes (B, outlined box). For comparison, images G and H show a lab-made methane hydrate þ quartz sand sample used for testing, that had a known pressure– temperature history and known initial grain texture. The hydrate phase is at the left in both photos (G and H), with quartz at the right. See text for further comparison and discussion.

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LN LN

SD SD

Methoda

by a factor of about 10 times. The cause of these disparities is unclear and is discussed below in Section 4.2.

243 251 0.0 0.0

993,819 941,525

39 80

0.0 0.0

0.0 0.3

0.0 0.0

0.0 0.0

0.0 0.0

0.0 0.0

0.0 0.0

0.0 1.0

0 0

15.6 24.5

49.0 48.8

4.1. Gas-hydrate morphology and microstructures

a

Method: syringe dissociation (S.D.) or liquid nitrogen (LN2).

6142 58,395 8 5 2 8 619.9 661.0 Hyd 1 2-8-31-36 Hyd 7 8-5-31-36

2033.63 2168.50

235 260 619.9 661.1 2-8-20-21 8-6-2-3

2033.17 2168.54

2 8

8 6

643 3971

0.0 0.0

999,357 996,029

0.0 0.0

0.0 0.0

0.0 0.0

0. 0.0

0.0 0.0

0.0 0.0

0.0 0.0

0.0 0.0

17.9 16.9

413 696

3.3 18.4

49.6 48.6

(per mil) (per mil)

H2 C6þ (ppm) nC5 (ppm) iC5 (ppm) nC4 (ppm) iC4 (ppm) C3H6 (ppm) C3 (ppm) C2H4 (ppm) C2 (ppm) C1 (ppm) CO (ppm) CO2 (ppm) Section Core Depth (decimal ft.) Depth (m) Interval

Table 1 Gas compositional and isotopic composition from gas hydrate, comparing samples dissociated onsite (Lorenson et al., 2011) and those stored in liquid nitrogen for this study. Results are normalized to methane.

d13CO2

d13C1 (ppm) dDC1

4. Discussion

Using cryo-SEM techniques to assess original sample textures and grain characteristics is not always straightforward. Without detailed understanding of the complete history of the field samples, it can be difficult to distinguish between textures or structures produced in situ and those produced during core recovery. In the case of samples or cores subjected to 1-atmosphere transfer followed by repressurization into the methane hydrate stability field, for instance, Waite et al. (2008) showed that both the gas-hydrate distribution and texture within samples can be at least partially altered, resulting in a sample with new physical properties that may differ significantly from the original state. Here, while the question of redistribution is of somewhat less concern due to the rapid quenching process in LN2 and the lack of any secondary hydrate formation that might accompany repressurization, the issue of surface morphology and texture of the hydrate phase is still difficult to interpret with confidence. As we have seen in previous investigations (Stern et al., 2004, 2005a,b) SEM imaging reveals that the hydrate (ice) phase in sub-permafrost gas hydrates often exhibits two general physical states: as a dense material surrounding large macropores, and as a spongy-appearing mesoporous material. Both of these habits appear in most sections of samples imaged here as well as from other cores retrieved from sub-permafrost settings (Stern et al., 2005a), and the two habits are often closely juxtaposed or interconnected. Ice, where it occurs in pure form in the samples, may also appear as a dense substrate surrounding large pores, although we have never observed it to occur as a mesoporous material. Understanding the mechanisms or processes that promote development of mesoporosity in gas hydrates, and establishing whether it is original microstructure or instead an artifact of recovery procedures, has important consequences. Such texture can have measurable effects, for instance, on thermal, electrical, and acoustic properties. Alternatively, if the texture is largely due to partial dissociation, then the additional ‘‘contaminant’’ ice phase can further complicate interpretations of measurements, since ice and hydrate properties are in some cases significantly different – most notably in mechanical strength, whether in pure form or as part of a sediment assemblage (Durham et al., 2003, 2005; see also Section 4.2 below). Growth of such an apparently non-equilibrium texture is curious, as the porous microstructure develops at the free-energy cost of additional hydrate surface energy. We have discussed this topic previously so only briefly review it here. Mesoporous growth was first reported by Kuhs et al. (2000) in a variety of gas hydrates grown from ice in the presence of excess gas. The development of the subporous microstructure was investigated further by Staykova et al. (2003), Klapproth et al. (2003), and Genov et al. (2004), all of which provide excellent treatment and quantitative assessment of this growth process and its role and relevance to growth kinetic models at temperatures primarily below the ice point, or to its possible influence on macroscopic properties (Kuhs et al., 2004). More recently, Klapp et al. (in press) provided images and discussion of mesoporous gas hydrate recovered from the Gulf of Mexico, i.e. from a warmer-temperature (above the ice point) setting, and likewise concluded that it is original microstructure. In our own work, we have also imaged and reported on initial growth of hydrate from ice as a mesoporous growth front advancing into the dense ice reactant, at temperatures exclusively

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below the ice point (Stern et al., 2004, 2005b). Extremely similar textures, however, can be produced by dissociation of originally dense grains of hydrate to ice (Stern et al., 2004, 2005b). Moreover, prolonged exposure of gas hydrate to high-vacuum conditions in the SEM column can also produce surface deterioration or breakdown with near-identical appearance (Fig. 4H, for example). Figs. 4 and 5 illustrate the similarity in some of these textures produced within laboratory-synthesized samples, and how they can closely resemble certain textures observed in Mount Elbert samples as well as other sub-permafrost gas-hydrate material (see Stern et al. 2005a for further discussion). At the present time we have no definitive means of establishing the origin of mesoporosity in the Mount Elbert samples due to the many environmental unknowns involved in their formation and recovery history. However, we note that in our own investigations of growth and recrystallization processes in gas hydrates, mesoporosity was captured only as

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a transient morphology that persisted during early to mid stages of synthesis from ice þ gas mixtures (Stern et al., 2004, 2005b). Original-growth mesoporosity was never observed in samples that were synthesized or subsequently held above the ice point for extended time, or in any samples in contact with an aqueous liquid phase. Instead, the initially porous hydrate was always found to anneal to a dense material. Although it could be argued that the dense hydrate material observed in both Mount Elbert and Mallik samples annealed from initially grown mesoporous hydrate, or resulted from dissociation of mesoporous hydrate followed by melting and refreezing, the positive identification of carbon peaks in the dense material, combined with our observations of known formation textures as outlined above, suggests otherwise. The dense material also exhibits obvious degassing when removed from the SEM and warmed. We surmise therefore that the mesoporosity exhibited by

Fig. 5. Comparison of natural vs lab-made gas hydrate þ sediments. Images A and B show two views of lab-synthesized pure methane hydrate þ quartz sample, made for material properties testing of known mixtures (see text, Section 2.5). The material filling the space between the quartz sand grains is pure methane hydrate with no ice impurities. The gas hydrate used in these tests is fully dense initially, but develops surface damage upon sublimation in the SEM column that causes development of the meso- or nano-porous appearance (see also Fig. 4G and H, and text.) Images C and D show representative images of a sub-permafrost gas hydrate þ sand section from Mallik Well 5L-38 (sample 107358), and images E and F show images from Mount Elbert Hyd 1, all showing good comparison of phase distribution with that of the lab-made samples. The gas hydrate (grey material between the sand grains) in the natural samples shown in C–F has partially decomposed to ice, rendering the samples poorly suited for physical properties testing; the natural samples provide useful information on phase distribution and grain contacts however, allowing close simulation of gas hydrate þ sediment textures in the lab, but without the complications of partial dissociation, frozen pore water, or other secondary ice.

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the Mount Elbert samples is likely, but not conclusively, an artifact of partial dissociation to an ice product, and enhanced by the surface degradation and sublimation incurred under the highvacuum SEM column conditions. Whether mesoporosity can persist stably on the geological timescale, particularly in sub-permafrosttype environments or any setting at temperatures above the ice point, remains an intriguing question for further investigation. Similarly, interpreting the origin of ice in the samples also remains a question for further investigation, due to the difficulty in distinguishing between frozen pore water and ice formed as a dissociation by-product. In summary, we emphasize that while SEM imaging provides an excellent method for assessing and documenting the distribution of phases and their articulation with one another, it is not always well suited for interpreting surface appearance or the origin of phases unless the sample’s history is well understood and/or well constrained. 4.2. Implications for material properties The issues discussed above – i.e., mesoporous microstructure, identifying and quantifying ice in the samples, and determining the origin of that ice – also impact the assessment of material properties of gas-hydrate-bearing material. We noted above the possible gas-hydrate redistribution and associated physical property changes that may accompany depressurization followed by repressurization (Waite et al., 2008), as well as the possible effects on macroscopic properties caused by mesoporous microstructure (Kuhs et al., 2004). For natural samples in which the precise environmental conditions or history of the material is not fully known, including all pressure, temperature, and gas chemistry parameters during retrieval, storage, and handling, some of the most useful information is best gleaned from LN2-quenched samples that best show phase distribution and grain contacts and that can be essentially duplicated in the lab using known materials. Fig. 5A and B, for example, shows two views of lab-synthesized pure methane hydrate þ quartz made expressly for material properties testing of known mixtures. Here, the material filling the space between the quartz sand grains is pure methane hydrate with no ice impurities. The gas hydrate used in these tests was observed to be fully dense initially (i.e. after mechanical testing and upon immediate entry into the SEM column), but developed surface damage upon sublimation in the SEM column that caused development of the meso- or nano-porous appearance (similar to that shown in Fig. 4G vs. H). Mechanical tests on the pure, lab-made material showed that methane hydrate is in fact significantly stronger than water ice, and hydrate/sand mixtures are similarly far stronger than ice/sand mixtures (Durham et al., 2003, 2005). Even relatively small amounts of ice can greatly alter (and weaken) the strength characteristics of the sample material (Stern et al., 1996), reinforcing the importance of determining whether secondary ice in natural samples is original material or an artifact of dissociation (or frozen pore water) during recovery and/or storage processes. Panels 5C and D show representative images of a sub-permafrost gas hydrate þ sand section from Mallik Well 5L-38, and 5E and F show additional images of Mount Elbert Hyd 1 for comparison. In these four images (5C–F), the gas hydrate has partially decomposed to ice, rendering the sample poorly suited for physical properties testing but very useful for information on phase distribution within natural samples.

et al. (2011). All analyses are consistent with structure I gas hydrate. Isotopic analyses of the methane carbon isotopic composition are also consistent with gas hydrate sampled at the drill site (Lorenson et al., 2011). A surprising discrepancy exists, however, in the measured carbon isotopic composition of the carbon dioxide compared with those obtained during direct-dissociation of gas hydrate retrieved from similar depths during initial operations. The isotopic composition of the carbon dioxide in this study ranges from 15.6 in unit D to 24.5 in the deeper unit C, differing significantly from those obtained during direct-dissociation of hydrate retrieved from similar depths during initial procedures (Table 1; see also Lorenson et al., 2011). Furthermore, the concentration of carbon dioxide is enhanced in the LN2-stored gas-hydrate samples by a factor of about 10. It is unclear if the disparity is a storage effect in liquid N2, an adsorption effect, or perhaps natural variation. What is certain is that the isotopic signature of the excess carbon dioxide is relatively light in comparison to hydrate samples dissociated onsite (Table 1). One possible source is CO2 scrubbed from the atmosphere by liquid N. The atmospheric composition of CO2 at (relatively nearby) Point Arena, California, for comparison, ranges from w7.9 to 9.2 ppt (NOAA Earth System Research Lab; see references for URL: www.esrl. noaa.gov/gmd/ccgg/iadv/) and some isotopic fractionation of atmospheric CO2 would be expected when condensed by contact with liquid N, resulting in incorporation of much lighter CO2 into the liquid N. It remains unclear, however, how or if this CO2 becomes incorporated into the stored gas-hydrate sample. 5. Summary Cryogenic SEM serves as a powerful tool not only for investigating the progress of gas-hydrate-forming reactions and the consequent development of grain and pore structures, but also for enabling close examination of the morphology and grain contacts in samples from nature, and for making relevant comparisons to hydrates made and tested in the laboratory. Here, using CSEM, powder X-ray diffraction, and gas chromatography methods, we investigated the appearance, grain characteristics, gas composition, and methane isotopic composition of two gas-hydrate-bearing sections of core recovered from the Mount Elbert test well on the Alaska North Slope. Cryogenic SEM shows the gas-hydrate phase forming primarily as a pore-filling material between the sediment grains at approximately 70–75% saturation, and more sporadically as thin veins typically several tens of microns in diameter. X-ray analysis of the sediment portion shows it to be a quartz-rich sand reservoir with minor feldspar, micas, and clays. Gas chromatography analyses of the hydrate-forming gas demonstrate that the gas hydrate is composed primarily of methane (>99.9%), indicating that the gas hydrate is sI. We anticipate that future tests and measurements on hydrate/sediment aggregates with known phase articulation, based on geometrical arrangements displayed by natural samples, should further improve our understanding of the possible effects of gas hydrates on elastic and inelastic properties of formations in which they may occur. Further tests and imaging of laboratory-synthesized samples will also help relate measurements on such materials to those made on either recovered or in situ gas-hydrate-bearing material, and in turn, to appropriate rock-physics models.

4.3. Gas composition

Acknowledgements

As listed in Table 1, the gas released from the hydrate in our cored sections is predominantly methane with only minor components of carbon dioxide and ethane, and with values and trends consistent with gas measurements reported by Lorenson

This work was supported by the USGS Gas Hydrate Project and partially funded by the U.S. Department of Energy. We thank the Mount Elbert scientific parties and field crews for providing us with the cored sections investigated in this study. We thank T. Kneafsey

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