Natural gas hydrates occur on most continental margins in organic-rich sediments at water depths
>450 m (in polar regions >150 m). Gas hydrate distribution and abundance, however, varies
significantly from margin to margin and with tectonic environment. The National Gas Hydrate
Program (NGHP) Expedition 01 cored 10 sites in the Krishna-Godawari (K-G) basin, located on
the southeastern passive margin of India. The drilling at the K-G basin was comprehensive,
providing an ideal location to address questions regarding processes that lead to variations in gas
hydrate concentration and distribution in marine sediments. Pore fluids recovered from both
pressurized and non-pressurized cores were analyzed for salinity, Cl-, SO4
2-, alkalinity, Ca2+,
Mg2+, Sr2+, Ba2+, Na+, and Li+ concentrations, as well as 13C-DIC, 18O, and 87/86Sr isotope ratios.
This comprehensive suite of pore fluid concentration and isotopic profiles places important
constraints on the fluid/gas sources, transport pathways, and CH4 fluxes, and their impact on gas
hydrate concentration and distribution. Based on the Cl- and 18 depth profiles, catwalk infrared
images, pressure core CH4 concentrations, and direct gas hydrate sampling, we show that the
occurrence and concentration of gas hydrate varies considerably between sites. Gas hydrate was
detected at all 10 sites, and occurs between 50 mbsf and the base of the gas hydrate stability zone
(BGHSZ). In all but three sites cored, gas hydrate is mainly disseminated within the pore space
with typical pore space occupancies being 2%. Massive occurrences of gas hydrate are
controlled by high-angle fractures in clay/silt sediments at three sites, and locally by lithology
(sand/silt) at the more “diffuse” sites with a maximum pore space occupancy of ~67%. Though a
majority of the sites cored contained sand/silt horizons, little gas hydrate was observed in most of
these intervals. At two sites in the K-G basin, we observe higher than seawater Cl- concentrations
between the sulfate-methane transition (SMT) and ~80 mbsf, suggesting active gas hydrate
formation at rates faster than Cl- diffusion and pore fluid advection. The fluids sampled within
this depth range are chemically distinct from the fluids sampled below, and likely have been
advected from a different source depth. These geochemical results provide the framework for a
regional gas hydrate reservoir model that links the geology, geochemistry, and subsurface
hydrology of the basin, with implications for the lateral heterogeneity of gas hydrate occurrence
in continental margins.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/2713 |
| Date | 07 1900 |
| Creators | Solomon, Evan A., Spivack, Arthur J., Kastner, Miriam, Torres, Marta, Borole, D.V., Robertson, Gretchen, Das, Hamendra C. |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | text |
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