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GAS HYDRATES IN THREE INDIAN OCEAN REGIONS, A COMPARATIVE STUDY OF OCCURRENCE AND SUBSURFACE HYDROLOGY

University of British Columbia. Department of Chemical and Biological Engineering; International Conference on Gas Hydrates (6th : 2008 : Vancouver, B.C.)

To establish the structural and lithological controls on gas hydrate distribution and to assess the potential energy resource and environmental hazards in the Indian Ocean, non-pressurized and pressurized cores were recovered from the Krishna-Godavari (K-G) and Mahanadi Basins offshore east India, and from an Andaman Sea site. The pore fluids were analyzed for: salinity, Cl-, sulfate, sulfide, carbonate alkalinity, Ca2+, Mg2+, Sr2+, K+, Na+, Ba2+, and Li+ concentrations, δ13C-DIC, δ18O, D/H, and 87Sr/86Sr ratios; together with infra-red imaging they provided important constraints on the presence and distribution of gas hydrates, thus on the subsurface hydrology. Evidence for methane hydrate was obtained at each of the sites. Only in the K-G Basin, between the sulfate-methane transition zone (SMT) depth and ~80 mbsf, higher than seawater chloride concentrations are observed; below this zone to the depth of the base of the gas hydrate zone (BGHSZ), chloride concentrations and salinity are lower than seawater value. In the Andaman Sea and Mahanadi Basin, only lower than seawater chloride concentrations are observed, and the shallowest gas hydrates occur at 100-200 m below the sulfate-methane transition zone (SMT) and extend to the depth of the BGHSZ. In the K-G Basin, the highest methane hydrate concentrations are associated with fracture zones in clay-rich sediments and/or in some coarser grained horizons. In the Andaman Sea, however, they are primarily associated with volcanic ash horizons. Assuming dilution by water released from dissociated methane hydrate, chloride and salinity anomalies suggest pore volume occupancies on the order of <1% to a maximum of ~61% at two sites (10, 21) in the K-G Basin and <1% to a maximum of ~76% at the Andaman Sea site. Overall, the percent pore volume occupancies based on pressure core methane concentrations and the chloride concentrations in conventional cores are similar. Variations in sulfate gradients were observed with the steepest gradient having the SMT at 8 mbsf in the K-G Basin and the deepest SMT at ~25 mbsf at the Andaman Sea site. The extreme negative δ13C values of the dissolved inorganic carbon (DIC), ranging from -38‰ to -47‰ at the SMT at some of the sites, indicate that anaerobic oxidation of methane (AOM) is an important reaction responsible for sulfate reduction at these sites. At several sites in the K-G Basin, however, the δ13C-DIC values indicate that organic matter oxidation is the dominant reaction.

Gas hydrates; Pore fluids; Cl- concentration; Sulfate-methane transition zone; Dissolved inorganic carbon

Text

eng

Vancouver : University of British Columbia Library

10.14288/1.0041146

Non UBC

Unreviewed

http://creativecommons.org/licenses/by-nc-nd/4.0/