Short migration of methane into a gas hydrate-bearing sand layer at Walker Ridge, Gulf of Mexico

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1 Article Volume 14, Number 2 21 February 2013 doi: /ggge ISSN: Short migration of methane into a gas hydrate-bearing sand layer at Walker Ridge, Gulf of Mexico Ann E. Cook School of Earth Sciences, Ohio State University, Columbus, OH, USA; Formerly at Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA (cook.1129@osu.edu) Alberto Malinverno Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA [1] The mechanisms governing distribution of natural gas hydrates in marine sediment systems are not well understood. We focus on a hole in the Gulf of Mexico, Walker Ridge Block 313 Hole H, where a 2.5-m-thick gas hydrate-bearing sand occurs within a 152-m-thick fine-grained mud interval containing gas hydrate in fractures. The gas hydrate-bearing sand is surrounded by two distinct hydrate-free zones (10-m-thick above and 3-m-thick below). We hypothesize that microbial methane generated within the hydrate-free zones diffused into the sand and formed gas hydrate. We show that the amount of methane produced in the hydratefree zones is likely enough to explain the gas hydrate content of the sand layer. Additionally, we show that there is enough time for dissolved methane to diffuse from the hydrate-free zones into the sand. We conclude that methane transport over significant distances via fluid flow is not required but that microbial methane could migrate short distances to form gas hydrate in the sand layer. Components: 5,400 words, 4 figures, 1 table. Keywords: methane hydrate; fluid migration; Gulf of Mexico. Index Terms: 3004 Gas and hydrate systems; 3036 Ocean drilling; 3021 Marine hydrogeology. Received 4 September 2012; Revised 7 December 2012; Accepted 13 December 2012; Published 21 February Cook, A. E., and A. Malinverno (2013), Short migration of methane into a gas hydrate-bearing sand layer at Walker Ridge, Gulf of Mexico, Geochem. Geophys. Geosyst., 14, , doi: /ggge Introduction [2] Natural gas hydrate, a solid lattice structure of H 2 O that houses molecules of gas, commonly forms in shallow sediments on continental margins where temperatures are low, pressures are high, and methane is present [Kvenvolden, 1988; Buffett, 2000; Tréhu et al., 2006]. The large amounts of methane hosted in these natural deposits can significantly affect global climate [Dickens et al., 1995; Archer, 2007], can cause submarine slope failure [McIver, 1982; Maslin et al., 2010], and are a potential natural gas resource [Collett, 2002; Boswell and Collett, 2011]. Improving our understanding of how hydrate forms and distributes within continental margin sediments will let us better assess the role played by gas hydrates in climate change and geohazards and better estimate its resource potential. [3] This article focuses on the formation of gas hydrate in a 2.5-m-thick sand layer (termed Unit A sand by Boswelletal. [2012]) buried at ~300 mbsf at Walker Ridge in the Gulf of Mexico. There are two American Geophysical Union. All Rights Reserved. 283

2 possible migration mechanisms to form gas hydrate in a coarse-grained layer: methane could be transported into the permeable sand by fluid flow from a deeper source [Boswell et al., 2012] or dissolved methane could diffuse from adjacent fine-grained marine mud [Malinverno, 2010; Rempel, 2011]. Whereas fluid flow can transport methane from distant sources, migration by diffusion will be effective at short distances and has been aptly named short migration by Boswelletal. [2012]. In this article, we test whether short migration of microbial methane can explain the gas hydrate content of the Unit A sand at Walker Ridge. To be a realistic mechanism, short migration must be able to produce a large enough gas hydrate accumulation and cannot require too much time. 2. Walker Ridge Block 313 [4] Subseafloor gas hydrate has previously been identified in the northern Gulf of Mexico in a variety of different lithologies during Deep Sea Drilling Project Leg 96 [Pflaum et al., 1986], in an industry well in Alaminos Canyon Block 818 [Boswell et al., 2009], and at Keathley Canyon Block 151, drilled during the Gulf of Mexico Gas Joint Industry Project (JIP) Leg 1 [Ruppel et al., 2008]. [5] In spring 2009, the JIP Leg 2 drilled two holes ~280 km south of Louisiana in Walker Ridge Block 313 (Figure 1). Walker Ridge Block 313 Hole G and Hole H targeted natural gas hydrate in sand formations near the base of gas hydrate stability (BGHS) at 900 mbsf [Boswell et al., 2012]. Natural gas hydrate was also unexpectedly found in many shallower intervals. For example, many thin sands ( 3 m thick) were found at a variety of depths in both holes; when these layers were above the BGHS, they were typically saturated with gas hydrate [Boswell et al. 2012; Frye et al., 2012]. [6] In Hole H, the Unit A sand occurs near the base of a 152-m-thick interval of fine-grained mud that contains gas hydrate as fill in near-vertical fractures (Figure 2). The interval has a slightly higher bulk density and compressional velocity than surrounding layers, which makes it easy to identify on seismic data. This strata-bound interval follows the regional stratigraphy and dips gently across the basin at ~8.Itcan be correlated from Hole H to Hole G (781 m west of Hole H) and to an industry hole (714 m southeast of Hole H). Like Hole H, Hole G and the industry hole contain gas hydrate-filled fractures in the strata-bound interval [Boswell et al, 2012; Frye et al, 2012], and in all probability, the strata-bound interval contains gas hydrate in fractures throughout. Further seismic mapping of the strata-bound interval by Frye et al. [2012] shows it to be regionally extensive, covering an area more than 50 km 2. [7] In Hole H, the gas hydrate-filled fractures are visible in logging-while-drilling resistivity images dipping at angles between 71 and 87. Additionally, propagation resistivity logs in the strata-bound interval in Hole H (Figure 2), Hole G, and a neighboring industry hole exhibit characteristic curve separations that indicate near-vertical gas hydrate-filled fractures [Cook et al., 2010]. For example, Figure 2 displays a selection of the 2 MHz, deep-penetrating propagation G H industry 2050 well Figure 1. A map showing the relative seafloor placement of the gas hydrate JIP Leg 2 Holes G and H (red) and an industry hole (black) drilled in Walker Ridge Block 313 in the Gulf of Mexico, ~280 km south of Louisiana. 284

3 150 a16l p16l a40h p40h R o Strata-bound interval 200 Depth(mbsf) 250 within fractures in mud Unit A sand layer 300 Figure Gamma Ray (API) Resistivity (ohm*m) Figure 2. The strata-bound interval, marked with thick dashed lines, in Walker Ridge Hole 313 Hole H. The gamma-ray log (left) indicates that most of the sediment is fine-grained mud, except for the 2.5 m Unit A sand layer. On the right, selected propagation resistivity logs (A16L, P16L, A40H, and P40H) are shown that are collected with different frequencies and different source and receiver spacing. The propagation resistivity log curves separate due to the resistivity anisotropy caused by the near-vertical gas hydrate-filled fractures. These resistivity logs are plotted with R 0, the water-saturated resistivity calculated from the measured formation porosity. When the measured resistivity is greater than R 0,itindicates the presence of gas hydrate. Additionally, depths where fractures were identified on logging-while-drilling resistivity images are highlighted in yellow. Directly above and below the sand layer, the propagation logs overlay with R 0, indicating no gas hydrate was detected. Seismic sections of the strata-bound layer in Walker Ridge can be viewed in Boswell et al. [2012] and Frye et al. [2012]. resistivity logs (A40H and P40H) and the 400 khz, shallow-penetrating propagation resistivity logs (A16L and P16L) from Hole H. Above and below the strata-bound interval, the propagation resistivity logs generally overlay. Within the strata-bound interval, however, the A40H and P40H curves show different resistivities that are distinctly higher than the A16L and P16L curves. This curve separation is a result of the resistivity anisotropy caused by nearvertical gas hydrate-filled fractures [Cook et al., 2010]. [8] Gas hydrate-filled fractures and veins in mud have been found in many locations worldwide [e.g., Boswell and Collett, 2011]. Fracture formation and gas hydrate accumulation in a relatively flat lying, laterally extensive subseafloor interval, however, is not well understood. In Walker Ridge, the gentle dip of the strata-bound interval (8 ) contradicts the steep dip of the observed fractures (71 87 ), indicating that the fractures themselves are not facilitating fluid flow updip. The strata-bound interval is chiefly composed of slightly overcompacted finegrained sediments, which are generally not conducive to appreciable fluid flow. Furthermore, the Unit A sand is clearly not supplying the gas for the entire strata-bound interval as it only occurs in Hole H. Thus, it seems most likely that the methane that fills the fractures in the fine-grained strata-bound interval is formed in place via microbial methanogenesis [Kvenvolden, 1995; Waseda, 1998; Claypool et al., 2006; Pohlman et al., 2009; Malinverno, 2010]. [9] Generally, sands do not contain significant organic matter so they are not locations of significant 285

4 methane generation [e.g., Shum and Sundby, 1996; Burdige, 2007]. Here, we assume that no methane is generated in the sand; consequently, a methane migration mechanism is required. If fluid flow transports methane, the lateral continuity of sand bodies and their connection to deep methane sources is critical. At Walker Ridge, the Unit A sand was mapped over a large area (although this area was considerably smaller than that covered by the strata-bound interval) [Boswell et al., 2012; Frye et al., 2012]. When Unit A intersected the BGHS, it exhibited a reflection phase reversal, indicating that free gas occurred deeper in the layer and may be a source of gas for Unit A [Boswell et al., 2012; Frye et al., 2012]. Alternatively, in short migration, microbial methane generated within organic-rich, fine-grained sediments diffuses into coarse-grained layers where it forms gas hydrate. Figure 3 shows that the Unit A sand is surrounded by hydrate-free zones (a 10.3-m-thick interval above the sand layer and a 3.2-m-thick interval below the sand layer). This pattern is expected if diffusive methane flux from fine-grained sediments BCAV UCAV GR Walker Ridge Hole 313-H: Measurements & Interpretation leads to gas hydrate accumulations in coarsegrained layers [Malinverno, 2010; Rempel, 2011]. 3. Short Migration of Methane Into the Unit A Sand [10] Figure 4 illustrates the short migration mechanism for the Walker Ridge Unit A sand layer. Gas hydrate forms in coarse-grained layers once the concentration of dissolved methane exceeds its solubility in free space. In contrast, the formation of gas hydrate is inhibited in the small pores of fine-grained mud, and a methane concentration greater than the solubility is necessary for hydrates to form [Clennell et al., 1999]. For typical pore sizes in clay-silt marine sediments of nm [Henry et al., 1999; Day-Stirrat et al., 2012], the increase in methane concentration should be 3 6% of the free space solubility [Clennell et al., 1999, equation (11); Liu and Flemings, 2011, equation (3)]. Similarly, the inhibition of gas hydrate formation in small pores can be quantified by a lower required temperature, and the predicted undercooling A B C D E Ring R o 270 in fractures in mud Depth (mbsf) Ring (n=1.5) Ring (n=2.5) free zone (HFZ) 295 in sand HFZ Fracture in fractures in mud Caliper (in) Gamma Ray (API) Resistivity (ohm*m) N E S W Resistivity N Saturation (fraction of pore space) Interpretation Figure 3. Well log measurements, calculations, and interpretation near the 2.5-m-thick sand in Walker Ridge Hole 313-H. Track A, gamma ray; Track B, ring resistivity and R 0 ; Track C, resistivity image log. The hydrate saturation (Track D) is calculated using Archie s equation and is grayed out in fracture intervals, where it is invalid. Track E shows the interpretation. GR, gamma ray; BCAV, best density caliper; UCAV, ultrasonic caliper; Ring, ring resistivity; R 0, water-saturated resistivity predicted from the measured formation porosity; n, Archie s saturation exponent. 286

5 Depth h sand in mud h above h below in mud free zone (HFZ) in sand HFZ CH 4 concentration c CH solubility in free space CH concentration for hydrate to form in mud CH concentration in pore water Figure 4. Pore water methane concentration profiles for the case of short migration into the Unit A sand layer. The pore water methane concentration equals the freespace solubility in the gas hydrate-bearing sand but is higher by an amount Δc in hydrate-bearing fine-grained mud. This concentration difference drives a diffusive flux of methane from the mud intervals into the sand layer. The pore water concentration profiles are assumed for simplicity to be locally linear; microbial methane generation in the hydrate-free zones will result in concave concentration profiles (dotted lines). for the same pore sizes gives a similar result [Rempel, 2011, equation (9); Daigle and Dugan, 2011, equation (1)]. The concentration difference Δc listed in Table 1 is 3 6% of the methane solubility [Davie et al., 2004] at 300 mbsf for the temperature gradient at the Walker Ridge location [McConnell and Kendall, 2002]. [11] Figure 4 shows a simple scenario where the increase in methane solubility with depth and the methane concentration profile are assumed to be locally linear. If methane concentration was everywhere equal to the free space solubility (i.e., Δc = 0), the diffusive fluxes at the top and bottom of the sand would be the same. As Δc > 0, however, there is a net flux of methane into the sand J sand that is, from Fick s first law of diffusion, 1 J sand ¼ Φ mud DΔc þ 1 ; (1) h above h below where Φ mud is the fine-grained mud porosity, D is the diffusion coefficient in bulk sediment, and h above and h below are the thicknesses of the hydrate-free zones above and below the Unit A sand (Table 1). We note that the diffusive flux in equation (1) is a minimum value; if methane is generated in the mud intervals, the steady-state concentration profile will not be linear but concave downward [Schulz, 2006; Malinverno, 2010], leading to a steeper concentration gradient in the hydrate-free zones adjacent to the sand layer (Figure 4). [12] Over time, methane transported by diffusive flux will form gas hydrate in the sand layer, where there is no inhibition. This short migration mechanism is constrained by two factors: the amount of methane locally available and by the relatively slow process of diffusion Amount of Gas in the Sand Layer [13] If short migration was responsible for the gas hydrate accumulation in the Unit A sand, the total amount of gas hydrate in the sand layer should be comparable with the amount that would have been expected in the hydrate-free zones above and below [Malinverno, 2010]. The bulk volume fraction of gas hydrate in the sand layer is v sand ¼ s sand Φ sand ; (2) Table 1. List of Parameters Symbol Definition Units Value Range c hydr Methane concentration in gas hydrate a kg/m D Diffusion coefficient of methane in sediment b m 2 /ka 12.3 h above Thickness of hydrate-free zone above sand layer m 10.3 h below Thickness of hydrate-free zone below sand layer m 3.2 h sand Thickness of hydrate-bearing sand layer m 2.5 s sand Gas hydrate saturation in sand layer 33 45% Δc Difference in methane solubility between mud and sand layers kg/m Φ mud Porosity of mud layers 36% Φ sand Porosity of sand layer 25 35% a c hydr = r hydr 16 / (16 + n18), where the gas hydrate density r hydr = 925 kg/m 3 [Helgerud et al., 2009] and the number of water molecules per methane molecule n = 6 [Circone et al., 2005]. b D = D 0 / θ 2, where the diffusion coefficient of methane in water D 0 is after Schulz [2006], the tortuosity θ 2 (1 = Φ m) mud [McDuff and Ellis, 1979], and the cementation exponent m = 1.9 is from an analysis of downhole porosity and resistivity logs at Hole H. 287

6 where s sand is the gas hydrate saturation (gas hydrate as a fraction of pore space) in the sand and Φ sand is the porosity (Table 1). In coarse-grained sediments, gas hydrate saturation can be estimated from Archie s relationship [Archie, 1942; Ellis and Singer, 2007; Goldberg et al., 2010] as s sand ¼ 1 R 1 0 n; (3) R t where R t is the measured electrical resistivity, R o is the water-saturated resistivity, and n the saturation exponent. For R t, we employ the ring resistivity measurement, which has a high vertical resolution of 5 10 cm and lets us obtain a detailed profile of gas hydrate saturation [Cook et al., 2012]. We use a quick-look method [e.g., Collett and Ladd, 2000; Goldberg et al., 2010] to estimate R o (Figure 3). The exponent n is generally observed to be ~2, and we compute a minimum and maximum gas hydrate saturation by taking a range of n between 1.5 and 2.5 (Figure 3). The resulting average gas hydrate saturation s sand is 33 45%. For this saturation and the porosity range in Table 1, v sand =8 16%. If this amount of gas hydrate were distributed in the hydrate-free zones above the sand layer, whose total thickness is 13.5 m (Table 1), the resulting volume fraction of gas hydrate would be 1.5 3%. [14] If the Unit A sand was not present in Hole H, the gas hydrate accumulation in the hydrate-free zones would likely be similar to the gas hydrate volume in the strata-bound interval. Recall that most of the strata-bound mud contains gas hydrate distributed in fractures and Archie s equation is not applicable to fractured systems [Archie, 1942]. Instead, using the modeling methods of Cook et al. [2010], we estimate the volume fraction of gas hydrate from the propagation resistivity logs. We divide the propagation response into two intervals, a top interval from 191 to 224 mbsf with lower resistivity and minor curve separation and a bottom interval from 229 to 270 mbsf with higher resistivity and increased curve separation (Figure 2). The volume fraction of gas hydrate in the Hole H strata-bound interval was found to be 2 4% for the top interval and to 4 8% for the bottom interval. [15] The overall amount of gas hydrate in the Unit A sand layer does not exceed and is quite similar to the amount that would have been expected in the surrounding hydrate-free zones. Hence, diffusive transport of methane from the hydrate-free zones as in short migration could have resulted in the gas hydrate accumulation observed in the Unit A sand Time Required by Short Migration [16] Solute diffusion in porous media is a slow process. For example, the characteristic diffusion timescale for a distance Δz equal to the thickness of the gas hydrate stability zone at Hole H (~900 m) [Frye et al., 2012] is Δz 2 /D = 65.8 Ma. In the case we examine, short migration is transporting methane over shorter distances of ~10 m, but we still need to check whether the time duration required to transport enough methane into the Unit A sand is not unreasonably large. [17] In the hypothesis proposed here, because sediments are buried below the seafloor, the concentration of microbial methane will increase until the local methane solubility is reached within the sand layer. Hence, the amount of time that short migration requires cannot exceed the time that the sand layer was buried beneath the seafloor, that is, the sediment age ~300 mbsf. The sediment age can be estimated from sedimentation rates at the industry well drilled ~500 m from WR313H in the Terrebonne Basin [Boswell et al., 2012]. The most recent biostratigraphic data reported at this site give ages of Ma at a depth of ~1000 mbsf. An assumed constant sedimentation rate in the top km of sediment gives an age of ka at 300 mbsf. [18] Assuming that the methane concentration profile shown in Figure 4 has persisted over time, we estimate the time needed by short migration from the mass per unit area M sand of methane contained in the sand layer divided by the diffusive flux J sand of Equation (1): M sand ¼ c hydr v sand h sand ; (4) where c hydr is the methane concentration in gas hydrate, v sand is the volume fraction of gas hydrate in the sand (Equation (2)), and h sand is the thickness of the sand layer. Taking the mean values of the parameter ranges in Table 1, the time needed by short migration is 306 ka, which is less than the sediment age of ka estimated above. If the ranges of the uncertain parameters in Table 1 are fully taken in account, the corresponding range of the calculated time needed by short migration is ka. The high end of this range is somewhat greater than the estimated sediment age. However, as noted above, the diffusive flux J sand of Equation (1) is a minimum value, and a higher flux reduces the time needed by short migration. 288

7 Moreover, the sediment age at 300 mbsf is computed assuming a constant sedimentation rate between a zero-age seafloor and an age of Ma at ~1000 mbsf. The Pleistocene and Holocene Terrebonne Basin sediments contain less sand than older sediments [Frye et al., 2012], suggesting a decrease in sedimentation rate. If sedimentation rates followed a decreasing trend in the last few 100,000 years, the sediment age at 300 mbsf could be older than ka. [19] We conclude that the time necessary to move enough methane by diffusion into the sand layer does not exceed and is comparable with the sediment age. Short migration is fast enough to result in the observed amount of gas hydrate in the sand layer. 4. Discussion [20] We have shown that short migration is a feasible mechanism to explain the gas hydrate content of the Unit A sand, because there is enough methane in the surrounding hydrate-free zones and enough time for transport by diffusion. In contrast, Boswell et al. [2012] and Frye et al. [2012] argue that fluid flow transported the methane to the Unit A sand. Boswell et al. [2012] and Frye et al. [2012] used seismic data to map the Unit A sand over a significant distance and found a reflection phase reversal at the inferred BGHS, indicating free gas in the sand layer below. Fluid flow from this free gasbearing portion of the sand layer was interpreted to be the migration mechanism for gas hydrate formation in the sand above the BGHS. Besides Unit A, many other thin sands (~3 m or less) occur throughout Hole H and Hole G, most of which are saturated with gas hydrate. Boswell et al. [2012] suggest that many of these sands may have been filled with methane via short migration, because they do not connect with any visible source of free gas on seismic data. We argue that free gas could be present in the Unit A sand below the BGHS in the case of short migration. If microbial methane is generated pervasively within the strata-bound mud interval, short migration can transport it and form gas hydrate in the Unit A sand over the entire basin. Once the dipping sand layer is buried by continuing sedimentation below the BGHS, the gas hydrate will dissociate into free gas, causing a reflection phase reversal. Thus, although thin, isolated gas hydrate-bearing sands are most likely sourced from short migration, a laterally extensive sand layer that exhibits a phase reversal across the BGHS may also be sourced from short migration. [21] There are three ways methane could be provided to the Unit A sand and the strata-bound mud interval: both are fed by fluid flow, fluid flow supplies the sand and microbial methane supplies the fractures, or microbial methane furnishes both hydrate accumulations (with short migration methane filling the sand). It seems highly unlikely that the Unit A sand is supplying methane to the fractured mud interval because the Unit A sand is only found in Hole H, whereas gas hydrate in fractured mud is found in Hole H, Hole G, and the industry hole. Moreover, in Hole H, there are hydrate-free zones separating the gas hydrate accumulations in the sand and in the strata-bound interval (Figure 3). [22] If the methane in the Unit A sand is transported significant distances via fluid flow, whereas the methane in the fractures is generated in situ, the occurrence of the hydrate-free zones surrounding the sand is unexplained. In contrast, short migration explains (in fact, requires) hydrate-free zones to surround the sand [Malinverno, 2010; Rempel, 2011]. The microbial methane that supplies the sand is generated in these zones, but the pore water methane concentration never reaches the level required to form gas hydrate in fine-grained sediment. We also note that the 2.5-m-thick Unit A sand is not uniformly filled with gas hydrate. Instead, the highest hydrate saturations in the sand are found at the top and bottom next to the hydrate-free zones, whereas the middle of the sand layer has lower saturations (Figure 3). In a diffusion-driven short migration scenario, Rempel [2011] predicts that gas hydrate accumulations should form high-saturation spikes at the edges of a coarse-grained layer, adjacent to hydrate-free zones in the surrounding fine-grained sediment. The model by Rempel [2011] closely reflects the observations within the Unit A sand. [23] Although we argue that most of the evidence supports short migration for the formation of gas hydrate in the Unit A sand, we cannot completely exclude the possibility that the sand is supplied with methane via fluid flow. Coring the strata-bound fracture interval and the Unit A sand could provide the information needed to determine methane origin with certainty. For instance, stable carbon isotope analyses would allow to distinguish between a microbial and a deep, thermogenic origin for the methane [e.g., Whiticar, 1999]. 289

8 5. Conclusions [24] In short migration, microbial methane generated within fine-grained sediments, where gas hydrate formation is inhibited, is transported by diffusion into adjacent coarse-grained layers, where gas hydrate deposits accumulate. The 2.5-m-thick gas hydrate-bearing Unit A sand layer in Walker Ridge Block 313, Gulf of Mexico, is surrounded by hydrate-free zones that are expected if methane migrates by diffusion from these fine-grained intervals into the sand. Short migration is a viable mechanism to explain the gas hydrate content of the Unit A sand because there is enough methane available and enough time to diffuse methane from the adjacent fine-grained sediments. Acknowledgments [25] We gratefully acknowledge the hard work of the Gulf of Mexico gas hydrate JIP Leg 2 planners, co-chiefs, science party, and crew of the semisubmersible drilling vessel Helix Q4000. We thank David Goldberg and reviewers for useful comments and Barbara Anderson for resistivity models. 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