Lorenson, Thomas D. and Collett, Timothy S. (2000): Gas content and composition of gas hydrate from sediments of the southeastern North American continental margin

ODP 164
ODP 164 994
ODP 164 995
ODP 164 996
ODP 164 997


Lorenson, Thomas D.
U. S. Geological Survey, Menlo Park, CA, United States

Collett, Timothy S.
University of Tokyo, Japan

Gas content and composition of gas hydrate from sediments of the southeastern North American continental margin
In: Paull, Charles K., Matsumoto, Ryo, Wallace, Paul J., Black, Nancy R., Borowski, Walter S., Collett, Timothy S., Damuth, John E., Dickens, Gerald R., Egeberg, Per Kristian, Goodman, Kim, Hesse, Reinhard F., Hiroki, Yoshihisa, Holbrook, W. Steven, Hoskins, Hartley, Ladd, John, Lodolo, Emanuele, Lorenson, Thomas D., Musgrave, Robert J., Naehr, Thomas H., Okada, Hisatake, Pierre, Catherine, Ruppel, Carolyn D., Satoh, Mikio, Thiery, Regis, Watanabe, Yoshio, Wehner, Hermann, Winters, William J., Wood, Warren T., Miller, Christine M. (editor), Reigel, Ruth (editor), Proceedings of the Ocean Drilling Program; volume 164; scientific results; gas hydrate sampling on the Blake Ridge and Carolina Rise; covering Leg 164 of the cruises of the drilling vessel JOIDES Resolution, Halifax, Nova Scotia, to Miami, Florida, sites 991-997, 31 October-19 December 1995
Texas A & M University, Ocean Drilling Program, College Station, TX, United States
Gas hydrate samples were recovered from four sites (Sites 994, 995, 996, and 997) along the crest of the Blake Ridge during Ocean Drilling Program (ODP) Leg 164. At Site 996, an area of active gas venting, pockmarks, and chemosynthetic communities, vein-like gas hydrate was recovered from less than 1 meter below seafloor (mbsf) and intermittently through the maximum cored depth of 63 mbsf. In contrast, massive gas hydrate, probably fault filling and/or stratigraphically controlled, was recovered from depths of 260 mbsf at Site 994, and from 331 mbsf at Site 997. Downhole-logging data, along with geochemical and core temperature profiles, indicate that gas hydrate at Sites 994, 995, and 997 occurs from about 180 to 450 mbsf and is dispersed in sediment as 5- to 30-m-thick zones of up to about 15% bulk volume gas hydrate. Selected gas hydrate samples were placed in a sealed chamber and allowed to dissociate. Evolved gas to water volumetric ratios measured on seven samples from Site 996 ranged from 20 to 143 mL gas/mL water to 154 mL gas/mL water in one sample from Site 994, and to 139 mL gas/mL water in one sample from Site 997, which can be compared to the theoretical maximum gas to water ratio of 216. These ratios are minimum gas/water ratios for gas hydrate because of partial dissociation during core recovery and potential contamination with pore waters. Nonetheless, the maximum measured volumetric ratio indicates that at least 71% of the cages in this gas hydrate were filled with gas molecules. When corrections for pore-water contamination are made, these volumetric ratios range from 29 to 204, suggesting that cages in some natural gas hydrate are nearly filled. Methane comprises the bulk of the evolved gas from all sites (98.4%-99.9% methane and 0%-1.5% CO (sub 2) ). Site 996 hydrate contained little CO (sub 2) (0%-0.56%). Ethane concentrations differed significantly from Site 996, where they ranged from 720 to 1010 parts per million by volume (ppmv), to Sites 994 and 997, which contained much less ethane (up to 86 ppmv). Up to 19 ppmv propane and other higher homologues were noted; however, these gases are likely contaminants derived from sediment in some hydrate samples. CO (sub 2) concentrations are less in gas hydrate than in the surrounding sediment, likely an artifact of core depressurization, which released CO (sub 2) derived from dissolved organic carbon (DIC) into sediment. The isotopic composition of methane from gas hydrate ranges from delta (super 13) C of -62.5 per mil to -70.7 per mil and delta D of -175 per mil to -200 per mil and is identical to the isotopic composition of methane from surrounding sediment. Methane of this isotopic composition is mainly microbial in origin and likely produced by bacterial reduction of bicarbonate. The hydrocarbon gases here are likely the products of early microbial diagenesis. The isotopic composition of CO (sub 2) from gas hydrate ranges from delta (super 13) C of -5.7 to -6.9, about 15 per mil lighter than CO (sub 2) derived from nearby sediment.
Coverage:Geographic coordinates:
West:-76.1127East: -75.2807

Economic geology, geology of energy sources; General geochemistry; Atlantic Ocean; Blake-Bahama Outer Ridge; C-13/C-12; carbon; concentration; continental margin sedimentation; D/H; gas hydrates; gas sampling system; hydrogen; isotope ratios; isotopes; Leg 164; marine sediments; natural gas; North Atlantic; Ocean Drilling Program; ODP Site 994; ODP Site 995; ODP Site 996; ODP Site 997; petroleum; sedimentation; sediments; southeastern North America; stable isotopes;