HR: 13:55h
AN: OS42B-02 INVITED     [PDF]
TI: Laboratory and Seafloor Experiments with Hydrocarbon Clathrate Hydrates: A Review with Potential Applications to Hydrates in Nature
AU: * Kirby, S H
EM: skirby@usgs.gov
AF: U.S. Goelogical Survey, 345 Middlefield Rd., Menlo Park, CA 94061 United States
AU: Stern, L A
EM: lstern@usgs.gov
AF: U.S. Goelogical Survey, 345 Middlefield Rd., Menlo Park, CA 94061 United States
AU: Brewer, P
EM: Brpe@mbari.org
AF: MBARI, 7700 Sandholdt Rd., Moss Landing, CA 95035 United States
AU: Circone, S
EM: scircone@usgs.gov
AF: U.S. Goelogical Survey, 345 Middlefield Rd., Menlo Park, CA 94061 United States
AU: Durham, W B
EM: durham1@llnl.gov
AF: LLNL, Box 808, Livermore, CA 94550 United States
AB: The in-situ chemical composition, structure, and textural positions of clathrate hydrates within sediments, and their effects on the properties of their host sediments are important features that are rarely, if ever, determined directly. All hydrocarbon clathrate hydrates retrieved from their natural environments for study in the lab at atmospheric pressure have been to some degree altered and/or partially decomposed during transit and handling. Thus interpretation of their structures, textures and properties is not always straightforward. Laboratory and seafloor experiments on synthetic hydrocarbon hydrates and aggregates with sediment are useful for aiding the interpretation of retrieved materials. Here we describe such exploratory efforts at the USGS, MBARI and LLNL: (1) Rock-physics theory and measurements of the physical properties of multi-phase granular materials in general indicate that the textural position (conformation) of hydrates in the pore space of sediment dictate the quantitative effects of hydrates on sediment properties. Our ice-to-hydrate synthesis method permits us to make synthetic sediment/hydrate aggregates with known conformation as well as measure accurately their bulk properties and test the rock-physics theories. Such tests currently seem infeasible using natural samples. (2) Such synthetic samples with known hydrate characteristics and properties can be subjected to known P-T-time pathways that simulate the retrieval pathways of natural samples. The effects of the retrieval process can be evaluated by comparing the starting hydrate characteristics and properties with those of the simulated "retrieved" material. Optical-cell experiments can be potentially useful in evaluating the textural effects of retrieval by direct observation. (3) The governing physical chemistry of our ice-to-hydrate synthesis method is probably different from that in natural environments. We have found, however, that exposure of lab-made samples to the seafloor and their subsequent retrieval within the P-T stability of methane hydrate alters their grain textures as determined by cryogenic SEM. Such samples resemble natural samples taken from the shallow seafloor of the Gulf of Mexico, suggesting that similar processes that lead to rapid textural equilibration are occurring in both settings. (4) Samples of hydrates at vents sampled by ROV's may be quickly decomposed to gas near the seafloor and retrieved for later gas analysis. Such analyses are not as prone to gas partitioning during transit and hence may be more representative of the in-situ hydrate gas compositions.
DE: 4200 OCEANOGRAPHY: GENERAL
DE: 5754 Physical properties of materials
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting