Note
Exclusion of 2-methylbutane (isopentane) during
crystallization of structure II gas hydrate in sea-¯oor
sediment, Gulf of Mexico
Roger Sassen *, Stephen T. Sweet, Debra A. DeFreitas, Alexei V. Milkov
Geochemical and Environmental Research Group (GERG), Texas A&M University, College Station, Texas 77845, USAReceived 28 July 2000; accepted 11 September 2000 (returned to author for revision 30 August 2000)
Abstract
Structure II gas hydrate is abundant across the central Gulf of Mexico continental slope. Sediment that directly overlies nodular gas hydrate was collected with a piston core at1920 m water depth in the Atwater Valley (AT) 425 area of the lower continental slope. The gas hydrate has C1±C5 molecular and isotopic properties consistent with
structure II gas hydrate that crystallized from relatively unaltered thermogenic vent gas. The gas hydrate contains mainly methane, ethane, propane and butanes, with 2-methylbutane (isopentane ) as a minor component (< 0.2%). Sediment that closely overlies the gas hydrate (within < 1 m) is characterized by an anomalous abundance of 2-methylbutane (as much as 9.6%). Because the molecular diameter of 2-2-methylbutane is too large for structure II gas hydrate, the 2-methylbutane appears to accumulate preferentially in adjacent sediment as a direct consequence of massive gas hydrate crystallization. The 2-methylbutane is interpreted to be a molecular marker of recent or ongoing net accumulation of structure II gas hydrate. Abundant 2-methylbutane in sediment also could be a precursor to the natural occurrence of structure H gas hydrate, and other new gas hydrate structures not yet discovered in the geologic environment.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Gas hydrate; Structure II gas hydrate; Hydrocarbons; 2-Methylbutane; Isopentane; Seeps; Recent sediment; Gulf of Mexico
1. Introduction
Gas hydrate is an ice-like crystalline mineral in which hydrocarbon and non-hydrocarbon gases are held within rigid cages of water molecules. Structure I gas hydrate has a body-centered cubic lattice, structure II gas hydrate has a diamond lattice, and structure H gas hydrate has a hexagonal lattice (Sloan, 1998). Structure I gas hydrate, which occurs in the Gulf of Mexico and many other basins, is frequently dominated by bacterial methane (Kvenvolden, 1995). Thermogenic gas hydrates of the Gulf slope contain oil-related hydrocarbon gases that migrate to shallow sediments from deeply buried
Mesozoic petroleum source rocks (Wenger et al., 1994). Naturally occurring structure II hydrate mainly includes C1±C4 hydrocarbons (methane through butanes) and
structure H hydrate includes C1±C5 hydrocarbons
(methane through 2-methylbutane) as major compo-nents (Sassen and MacDonald, 1994).
In contrast to simple bacterial methane, thermogenic hydrocarbons preserve complex information on the ori-gin of gas hydrates because multiple hydrocarbon molecules of varying properties are held within the crystal lattice (Sassen et al., 1999a). The Gulf of Mexico is one of the few areas outside of the Caspian Sea (Ginsburg and Soloviev, 1998) where oil-related struc-ture II gas hydrate is abundant at shallow depth in sea ¯oor sediment. In earlier studies, attention focused on the molecular and isotopic properties of thermogenic vent gas and the structure II gas hydrate commonly
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* Corresponding author. Fax +1-979-862-2361.
encountered in the Gulf of Mexico (Brooks et al., 1986; Sassen and MacDonald., 1994, 1997; Sassen et al., 1998; 1999a,b). Less attention has been given to free hydro-carbon gas present in sediment sampled in direct asso-ciation with structure II gas hydrate (Brooks et al., 1986; Sassen et al., 1999b).
Molecular fractionation during crystallization of structure II gas hydrate from vent gas decreases the relative abundance of methane and concomitantly increases the relative abundance of hydrate-forming ethane, propane, and butanes (Sassen et al., 1999b). Other hydrocarbons such as 2-methylbutane are likely to be preferentially excluded during crystallization of structure II gas hydrate because molecular diameters are too large to ®t within the gas hydrate cage. We hypo-thesize on this basis that crystallization of structure II gas hydrate could aect the composition of residual free gas in a closed or semi-closed system. One objective of the present study is to document the molecular and iso-topic properties of hydrocarbon gases from sediment overlying structure II gas hydrate in a piston core from the lower Gulf slope. Another objective is to consider implications of the results with respect to the occurrence of structure H gas hydrate, and new gas hydrate struc-tures not yet discovered in nature.
2. Samples and methods
The Atwater Valley (AT) 425 study site is an isolated sea-¯oor vent feature on the lower Gulf of Mexico slope (27 34.10 N and 88 29.70 W) in1920±1930 m water
depth. The site is part of a distinct geologic province near the downdip limit of the Gulf of Mexico Salt Basin known as the Mississippi Fan Foldbelt (Weimer and Buer, 1992). The AT 425 site is related to faults over-lying allochthonous salt shallow in the sediment section. Core AT 425-1 was acquired at1920 m water depth using a 6 m (7 cm interior diameter) piston coring device equipped with a 900 kg weight. Deformed, oil-stained, and gas-rich hemipelagic mud occurred overlying a gas hydrate zone at 4.2±4.4 m sediment depth, the core's maximum penetration depth. Four samples of sediment (20 cm core sections) overlying the gas hydrate were collected at depths of 1.4 m (ATS-1), 2.0 m (ATS-2), 2.6 m (ATS-3), and 3.4 m (ATS-4). Intact globular white to amber colored gas hydrate was recovered from 4.2 m depth in the core (ATH-1).
Intact gas hydrate was preserved in liquid nitrogen. Sediment samples were canned and held atÿ20C until
analysis for C1±C5 headspace gases and extractable
hydrocarbons. Detailed analytical procedures for C1±C5
gas chromatography, and measurement of isotopic properties of hydrocarbon gases from sediment are described by Sassen et al. (1999b). Procedures for extractable heavy hydrocarbons are given in Sassen et
al. (1993).d13C values are reported as parts per
thou-sand (%) relative to the PeeDee Belemnite standard (precision=0.2%).
3. Gas hydrate and sediment
The molecular distribution of the gas hydrate sample (ATH-1; Table 1) is diagnostic of structure II gas hydrate in that ethane, propane, and butanes are pre-sent in signi®cant abundance relative to methane (Sloan, 1998). Methane is the main component (88.4%) of the C1±C5distribution. Thed13C (ÿ49.4%) of the methane
is consistent with a thermogenic origin. Relative abun-dance of other hydrocarbon components is: propane, ethane, 2-methylpropane (isobutane), butane. Pentanes are detectable and include 2-methylbutane (0.2%) with minor amounts (< 0.1%) of pentane and 2,2-dimethyl-propane (neopentane). The d13C values of gases,
including ethane (ÿ38.1%), propane (ÿ32.1%), 2-methylpropane (ÿ32.4%), butane (ÿ28.6%), and 2-methylbutane (ÿ29.6%), are consistent with an origin from relatively unaltered thermogenic gas from a sub-surface petroleum system (Sassen et al., 1999b). The gas hydrate also contains visible crude oil as inclusions, and the odor of H2S is obvious during hydrate
decomposi-tion.
The concentrations of C1±C5hydrocarbon gases from
the AT 425-1 core range from 15,232 ppm to 107,139 ppm (Table 1). Methane is the main hydrocarbon com-ponent in the sediment (mean=94.4%). The meand13C
of the sediment methane isÿ51.1%, a value similar to the hydrate-bound methane (ÿ49.4%) (Table 1). The relative abundance of other hydrocarbons in the sedi-ment is: 2-methylbutane, ethane, 2-methylpropane, pro-pane, butane, pentane, 2,2-dimethylpropane. The molecular distribution of sediment hydrocarbons is consistent with the preferential bacterial oxidation of straight-chain hydrocarbons in that branched-chain hydrocarbons appear to be preferentially preserved (Winters and Williams, 1969; James and Burns, 1984). The isotopic properties of the sediment hydrocarbons also indicate bacterial oxidation. The meand13C values
of 2-methylbutane, 2-methylpropane, propane, and butane in the sediment are consistent with bacterial oxidation because they all are enriched in13C compared
to the gas hydrate (Table 1). During bacterial oxidation,
12C is preferentially used from the hydrocarbon reactant
pool, resulting in enrichment of13C in residual
hydro-carbons (Coleman et al., 1981). C15+ chromatography
authigenic carbonate nodules and the odor of H2S is
obvious.
4. Exclusion of 2-methylbutane
The C1±C5 hydrocarbon distributions of sediment
samples from the upper part of the core (1.4±3.4 m) and
gas hydrate (4.2 m) from the base of the core dier sharply, and the transition is abrupt over a < 1 m depth interval (Table 1). The molecular distribution of the structure II gas hydrate is characterized by compatible hydrocarbons including methane, ethane, propane, and butanes (Fig. 2). In contrast, sediment hydrocarbons are markedly de®cient in structure II hydrate-forming hydrocarbons other than methane (Fig. 2). Excluding
Table 1
Molecular and isotopic properties of hydrocarbons from core AT 425-1. Normalized percentages of C1±C5hydrocarbons are shown,
and isotopic properties are given in parentheses beneath the percentage value
Sample location Depth (m) C1±C5(ppm) % C1 % C2 % C3 %i-C4 %n-C4 %i-C5 %n-C5 % neo-C5 UCM (ppm) Sediment
ATS-1 1.4 107,139 99.3 0.6 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 (ÿ52.8) (ÿ32.8) (ÿ17.2)
ATS-2 2.0 81,822 98.4 1.1 0.1 0.1 <0.1 0.2 <0.1 <0.1 15,433 (ÿ53.7) (ÿ33.5) (ÿ22.4) (ÿ29.5) (ÿ22.4)
ATS-3 2.6 15,232 92.0 1.2 0.2 0.6 <0.1 5.9 <0.1 0.1 4,457 (ÿ46.8) (ÿ27.9) (ÿ27.7) (ÿ25.5)
ATS-4 3.4 72,390 87.8 0.9 0.3 1.2 0.1 9.6 <0.1 <0.1 10,735 (ÿ50.9) (ÿ34.9) (ÿ23.0) (ÿ31.3) (ÿ23.4) (ÿ25.5)
Mean of sediment
69,146 94.4 0.9 0.2 0.5 <0.1 3.9 <0.1 <0.1 10,208 (ÿ51.1) (ÿ32.3) (ÿ20.9) (ÿ29.5) (ÿ22.9) (ÿ25.5)
Gas hydrate
ATH-1 4.2 88.4 3.4 5.4 1.3 1.3 0.2 <0.1 <0.1 (ÿ49.4) (ÿ38.1) (ÿ32.1) (ÿ32.4) (ÿ28.6) (ÿ29.6) (ÿ30.5)
methane, 2-methylbutane is the most quantitatively sig-ni®cant molecule in the sediment overlying the gas hydrate (Table 1). Furthermore, 2-methylbutane is not a signi®cant component of the structure II gas hydrate (Table 1), and the sediment hydrocarbons do not show the characteristics expected from gas hydrate decom-position.
Gas hydrate crystallization eects on vent gas best explain the observed sediment hydrocarbon distribu-tions. Gas hydrate crystallizes rapidly from the high ¯ux of C1±C5 hydrocarbon vent gas in the deep sea,
pre-ferentially removing hydrate-forming hydrocarbons from the residual vent gas (Sassen and MacDonald, 1997). 2-Methylbutane is concentrated by dierence
because it is incompatible with structure II gas hydrate. Findings of the present study suggest that rapid, recent or ongoing crystallization of structure II gas hydrate increases the relative abundance of 2-methylbutane in residual vent gas in the sediment, serving as a transient marker of the crystallization process. Bacterial oxida-tion of residual vent gas may slightly enhance the abundance of 2-methylbutane because branched-chain hydrocarbons are relatively resistant to bacterial oxida-tion (Winters and Williams, 1969; James and Burns, 1984). However, bacterial oxidation is not likely to be an important factor since anomalous abundance of 2-methylbutane is not observed except in close proximity to gas hydrate (Table 1).
5. Implications of 2-methylbutane in sediment
The crystal structure of gas hydrate is controlled by the molecular distribution of the vent gas, and the crys-tallization process either captures or excludes hydro-carbons from the vent gas. Altering the molecular distribution of vent gas by increasing the relative abun-dance of 2-methylbutane opens new possibilities with respect to unusual gas hydrate structures in Gulf sedi-ment. Structure H gas hydrate synthesized in the laboratory encloses larger molecules than structure I or II gas hydrate, including petroleum molecules such as 2-methylbutane (Ripmeester et al., 1987). The phase equilibria results of Mehta and Sloan (1993) suggest that structure II and structure H gas hydrate could co-exist in nature at petroleum seep sites. Although gas hydrate was not directly observed in the upper part of the AT 425-1 core, the possible occurrence of structure H hydrate in the core cannot be excluded. Evidence of structure H hydrate on the ¯ank of a structure II gas hydrate mound in Green Canyon (GC) 185 on the Gulf slope at540 m water depth is presented by Sassen and MacDonald (1994). The gas hydrate from GC 185 is novel in that 2-methylbutane comprises 41.1% of the C1±C5 hydrocarbon distribution. This study provides
the ®rst explanation as to why molecular distributions rich in 2-methylbutane, favoring the formation of structure H hydrate, should occur in proximity to structure II gas hydrate. Structure H hydrate at the buried margins of structure II gas hydrate could be more common than previously thought.
The recent laboratory synthesis by Udachin and Rip-meester (1999) of a new gas hydrate structure encom-passing some characteristics of structure H gas hydrate is pivotal, given the results of the present study. The Gulf of Mexico oers a complex natural laboratory. It is possible that the new gas hydrate structure of Uda-chin and Ripmeester (1999), or as yet unknown gas hydrate structures, will be discovered within the Gulf of Mexico continental slope.
Acknowledgements
This research was supported by the Applied Gas Hydrate Research Program at Texas A&M University. We acknowledge the support of scientists, graduate stu-dents, and crew of the Research Vessel Powell for core sample acquisition. The helpful suggestions of our reviewers, Andrew N. Bishop and Joseph A. Curiale, are greatly appreciated.
Associate EditorÐJ. Curiale
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