Hydrocarbons of experimental and natural gas hydrates, Gulf of Mexico continental slope

Ors. Geochem. Vol. 26. No. 3/4, pp. 289-293, 1997 ~L"1997 ElsevierScience Ltd All rights reserved. Printed in Great Britain PII: S0146-6380(97)00001-6 0146-6380/97 $17.00 + 0.00

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NOTE Hydrocarbons of experimental and natural gas hydrates, Gulf of Mexico continental slope R O G E R S A S S E N and I A N R. M A C D O N A L D Geochemical and Environmental Research Group (GERG), Texas A&M University, College Station, TX 77845, U.S.A. (Received 4 October 1996; returned to author for revision 5 November 1996," accepted 20 December 1996)

Abstract--Gas hydrate was experimentally precipitated for the first time in the deep sea using natural starting materials. The experiments were performed using a research submarine on the Gulf of Mexico continental slope at a water depth of 540 m at 9.0-9.2°C. Starting materials were thermogenic hydrocarbon gases similar in composition to gases in underlying subsurface reservoirs. These gases vented to the water column in association with sea-floor mounds of natural yellow-orange gas hydrate (structure It). The vent gases were captured in transparent Lexan tubes at ambient conditions, and precipitation of white to yellow gas-hydrate dendrites and coatings was seen to occur within minutes. Experimental ~3 hydrates show 6 C of methane and CI-C5 hydrocarbon compositions that are similar to the vent gases from which they precipitated. @ 1997 Elsevier Science Ltd Key words--hydrocarbons of gas hydrate, ~13Cof methane, gas vents, Gulf of Mexico

INTRODUCTION Gas hydrates are ice-like crystalline substances containing hydrocarbon and non-hydrocarbon gases. They are stable at the high pressures and low temperatures c o m m o n l y found in continental slope sediments and certain other natural environments. Gas hydrates are sufficiently abundant worldwide that production of "greenhouse" methane (Ci) from their decomposition during global warming could be a factor in global climate change (Kvenvolden, 1993). Structure I hydrates (body-centered cubic lattice) occur c o m m o n l y in nature (Sloan, 1990), whereas structure I1 and structure H hydrates are uncommon. Structure II gas hydrate (diamond lattice) containing Cl C4 thermogenic hydrocarbon gases was first sampled by piston cores in 530 5 6 0 m water depths on the G u l f of Mexico continental slope offshore Louisiana at 2 7 ° 4 7 ' N and 91°30'W (Brooks et al., 1984). Identification of the hydrate as structure II was based on the relative abundance of the C3 and i-C4 hydrocarbons (Brooks et al., 1984), and corroborated using solid-state nuclear magnetic resonance ( N M R ) (Davidson et al., 1986). Evidence for the natural occurrence of structure H gas hydrate (hexagonal lattice) was first reported at 5 4 0 m water depth at the Bush Hill locality on the G u l f continental slope offshore Louisiana (27°47.5'N and 91°15.0'W) by Sassen and M a c D o n a l d (1994). Identification of structure

H hydrate was based on abundant i-Cs, which represented 41.1% of the total C1-C5 hydrocarbon distribution of the sample. This note documents the hydrocarbon geochemistry of the first gas hydrates experimentally precipitated from natural sea-floor gas vents at the ambient pressures and temperatures of the deep sea. Geologic a n d geochemical setting

The study area is a well-documented natural laboratory for the study of gas hydrates. The faulted seep mound at Bush Hill is about 500 m wide with relief of about 40 m. Oil and gas migrate to Bush Hill near Jolliet Field along faults connected to subsurface reservoirs at 2-3 km sub-bottom (Sassen et al., 1993). Sediments contain abundant biodegraded oil, gas hydrates, H2S, CO2, and authigenic carbonate rock derived from precipitation of CO2. The crest of Bush Hill is colonized by chemosynthetic organisms including bacterial mats, vestimentiferan tube worms, and methanotrophic mussels ( M a c D o n a l d et al., 1989). Persistent natural oil slicks appear on satellite remote sensing images of the sea surface over Bush Hill (MacDonald et al., 1993). Lens-shaped deposits of yellow-orange gas hydrates breach the sea-floor at numerous locations on the crest of Bush Hill (MacDonald et al., 1994). The hydrates form sediment-draped mounds 30 50 cm high and up to several metres in width, with 289

290

Note

Fig. 1. Experimental precipitation of gas hydrate. Image shows gas venting from the crest of a hydrate mound covered with white bacterial mats (site 1). The 7.5 cm diameter Lexan tube is held by the submarine's robot arm in the gas bubble train, and freshly precipitated hydrate is visible just above the water contact within the tube. Object at extreme right is a camera system.

exposed hydrate visible at the edges of mounds. Copious streams of gas vent continuously to the water c o l u m n in association with hydrate mounds. The sea-floor m o u n d s can undergo episodic changes in size a n d m o r p h o l o g y over a period of m o n t h s ( M a c D o n a l d et al., 1994; unpublished data). Samples were collected from three Bush Hill sites within approximately 20 m. The h y d r a t e m o u n d of Site I has been described previously ( M a c D o n a l d et al., 1994). The hydrate m o u n d of Site 2 is approximately 15 m east of Site 1. Site 3 is a gas vent that issues t h r o u g h a bed of m e t h a n o t r o p h i c mussels ( M a c D o n a l d et al., 1994), a n d is approximately 20 m n o r t h of the other two sites. EXPERIMENTAL

S a m p l e collection D u r i n g dives of the J o h n s o n Sea-Link (JSL) research s u b m a r i n e to Bush Hill in August and September of 1995, samples of naturally occurring vent gas, naturally occurring gas hydrate, a n d experimentally-precipitated gas hydrate were recovered from 540 m water depth. W a t e r temperatures d u r i n g experiments were 9.0 9 . 2 C . Experiments and sampling activities were recorded at the sea floor using conventional 35-ram cameras a n d a video camera. Samples of vent gas were collected by use of 7.5 cm (i.d.) Lexan tubes 3 0 c m in length, which

were clear, closed at the top, and fitted with a Tshaped handle for m a n i p u l a t i o n by the mechanical arm of the s u b m a r i n e (Fig. l). A nylon mesh was placed across the b o t t o m of the tube to exclude floating debris. To collect vent gas, the open bottom of the tube was held in the b u b b l e stream until gas displaced sea water. Hydrate precipitation commenced at the gas/water interface in the tubes and m o v e d upward, leaving a m o r p h o u s coatings a n d dendritic crystals of white to yellow gas hydrates a d h e r i n g to the inner walls (Fig. 2). Residual vent gas was allowed to escape from the Lexan tubes to the water c o l u m n by up-ending the tubes. The Lexan tubes were then reoriented in a quiver on the s u b m a r i n e to retain free gas formed as the hydrate precipitates decomposed during ascent of the submarine to the sea surface. Experiments resulted in a b o u t 25 100 ml of free gas (at surface conditions) from hydrate decomposition. Additionally, samples of naturally occurring h y d r o c a r b o n s venting to the water column from hydrate m o u n d s were simply collected in Lexan tubes, and stored during ascent in a quiver on the submarine. Naturally occurring gas hydrates were collected in a specially designed pressure vessel which contained a m b i e n t sea water. Buoyant hydrate masses of hydrate were transferred to the pressure vessel using an inverted metal cup m a n i p u lated by the mechanical arm of the submarine. The

Note

291

Fig. 2. Close-up of experimentally precipitated gas hydrate (site 2). Note the sharp lower edge of the white-to-yellow hydrate, which corresponds to the gas/water interface in the Lexan tube. Ambient temperature = 9.17°C.

cover of the pressure vessel was sealed, using the mechanical arm, to maintain the hydrate at near seafloor pressure and temperature during ascent to the surface. Hydrate gases were later collected for analysis through an escape valve on the top of the pressure vessel as hydrate decomposed in a cold room. Samples of gases from experimentally precipitated gas hydrates, natural vents, and naturally occurring gas hydrates were transferred under sea water to uncontaminated glass containers, which were sealed and refrigerated until analysis. Intact gas hydrate samples were preserved in liquid nitrogen for other analyses including NMR.

Methods The CI C5 hydrocarbon gases were separated using a Hewlett-Packard 5710 gas chromatograph with a 2-m packed alumina column (Kennicutt et al., 1988). A standard Ct Cs hydrocarbon calibration mixture was used. The composition of CI Cs hydrocarbons is expressed as normalized per cent (Table 1). Following chromatographic separation, individual hydrocarbons were combusted to CO2 in a Craig-type combustion system. Analysis of 613C was performed using a Finnigan MAT 251 Isotope Ratio Mass Spectrometer and appropriate standards (Kennicutt et al., 1988). The 613C values are given in Table 1 as per mil (%0) relative to the Pee Dee Belemnite standard (PDB).

RESULTS AND CONCLUSIONS

The hydrocarbon compositions of five gas samples from three different vents at Bush Hill are similar (Table 1). The C1 C5 hydrocarbons of the vent gases are dominated by methane (Ci = 91.1 94.7%), The 613C values of Cl are within the narrow range o f - 4 2 . 4 to -45.6%0 PDB, values consistent with a thermogenic rather than a bacterial origin (Kvenvolden, 1993). The C2 and C3 hydrocarbons are present, with C2 >C3 in each sample. The C4 and i-C5 hydrocarbons are also present, but n-C5 is absent or below detection limits. Three samples of massive natural hydrate lenses from vent sites 1 and 2 were collected (Table 1). The Cl Cs hydrocarbons of the hydrate gases are dominated by Ci (71.7 80.2%). The 613C values of Cj are in the range o f - 3 6 . 3 to -39.9%0 PDB, indicating a thermogenic origin. The C2 and C3 hydrocarbons are both present in similar but relatively high percentages compared with the vent gas (Table 1). The i-C4 and r/-C 4 hydrocarbons are also present in higher percentages than in vent gas. The i-C5 hydrocarbon is present above detection limits, but n-C5 is absent or below detection limits. Preliminary NMR of an intact hydrate sample preserved in liquid nitrogen is consistent with structure II hydrate (Ripmeester, J., personal communication). Muds associated with the massive structure II hydrate lenses appear to contain structure H

292

Note

Table 1. Ci Cs hydrocarbon compositions (% by volume) and c5>C (%o) of C+ from vent gas, structure lI hydrate, and experimentally precipitated hydrate (Exp.) Sample

Ci

c51~C

C2

C3

i-C4

n-Ca

i-C5

Vent Gas (1) Vent Gas (1) Vent Gas ( 1) Vent Gas (2) Vent Gas (3)

93.2 93.5 94.7 94.6 91.1

-43.3 -42.5 -45.6 -43.8 -42.4

4.3 4.3 3.9 3.8 4.8

1.5 1.4 0.7 0.7 1.8

0.3 0.2 0.1 0.1 0.4

0.6 0.4 0.5 0.5 1.2

0.3 0.2 0.2 0.3 0.8

Hydrate (1) Hydrate (2) Hydrate (2)

71.7 80,2 72.1

-36.3 -38.5 -39.9

10.6 9.4 12.4

12.6 7.3 11.4

2.6 1.6 2.3

1.7 1.2 1.6

0.8 0.3 0.3

Exp. (l) Exp. (1) Exp. (2)

92. l 93.9 87.7

-44.3 -45.3 -40.5

4.0 3.5 5.8

2.2 1.2 2.1

0,5 0.3 0.5

0.7 0.6 2.3

0.4 0.4 1.6

The n-Cs hydrocarbon is absent or below detection limits. Numbers in parentheses indicate study sites 1, 2, and 3.

hydrate as indicated by high percentages of i-C5 (unpublished data). The h y d r o c a r b o n c o m p o s i t i o n s of three samples of experimentally precipitated gas hydrate from three separate JSL dives are s h o w n in Table 1, and are similar to vent gas compositions. The Cl C5 h y d r o c a r b o n s of the experimentally precipitated gas hydrates are d o m i n a t e d by m e t h a n e (C1 = 87.7 93.9%). The 513C values of C1 are within the - 4 0 . 5 to - 4 5 . 3 % o P D B range, indicating a thermogenic origin. The C2 and C3 h y d r o c a r b o n s are also present in meaningful percentages, with C ~ > C 3 in each sample. T h e C4 and C5 h y d r o c a r b o n s are generally present at low concentrations, but n-C5 is absent or below detection limits. The Ct C5 compositions of the vent gas samples from all three sites are similar to each other, and to earlier m e a s u r e m e n t s of Bush Hill vent gases in June 1993 ( M a c D o n a l d et al., 1994). in addition, the gi~-;C of Ct from the vent gas samples are isotopically similar to each other (Table 1), and to that of oil-related Ct from underlying subsurface reservoirs of Jolliet Field ( - 4 5 , 8 to - 4 6 . 4 % o P D B : K e n n i c u t t el al., 1988). These data are consistent with rapid leakage to the sea-floor along deeplyrooted faults that tap subsurface reservoirs of Jottiet Field. Phase equilibria models a n d carefully constrained l a b o r a t o r y experiments emphasize that f o r m a t i o n of gas hydrates involves preferential inclusion of specific h y d r o c a r b o n molecules (Sloan, 1990). The structure I1 hydrate sampled at Bush Hilt appears consistent with preferential inclusion of C> C~, iC4, and n-C4 from vent gases (Table 1). The ~513C of C[ from structure I1 hydrate samples is several per rail heavier t h a n the vent gases (Table 1). lsotopically heavy Ct is also noted in structure H hydrate from Bush Hill (-29.3%0 PDB), possibly reflecting bacterial oxidation effects (Sassen and M a c D o n a l d , 1994) on hydrates from chemosynthetic communities. The Ct C5 h y d r o c a r b o n compositions and c513C of C~ from samples of b o t h the vent gas and the ex-

perimentally precipitated gas hydrate are similar (Table 1). This observation is unexpected since b o t h structure I! (this paper) a n d structure H hydrates (Sassen a n d M a c D o n a l d , 1994; Sassen et al., 1994) coexist naturally at Bush Hill. Perhaps rapid experimental precipitation resulted in non-equilibrium hydrate compositions. M o r e sophisticated experiments from research s u b m a r i n e platforms could address the questions these data raise, and significantly enhance our u n d e r s t a n d i n g of hydrate formarion in the deep sea. A s s o c i a t e Editor

J. Curiale

Acknon'lee~4ements Support for research was provided by the NOAA National Undersea Research Center, University of North Carolina at Wilmington. NMR analysis of hydrate by J. Ripmeester, National Research Council of Canada. is appreciated. Discussions with E. Dendy Sloan of the Colorado School of Mines, and helpful guidance in our experimental design from Barun Sen Gupta and Harry H. Roberts of Louisiana State University are also appreciated. Laboratory assistance was provided by Stephen T. Sweet and Javier Alcala-Herrera. The assistance of our reviewers, Keith A. Kvenvolden and Joseph A. Curiale. is appreciated.

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