--> Extremely high concentration of methane hydrate in a deepwater silt reservoir from the northern Gulf of Mexico (Green Canyon 955)

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Extremely high concentration of methane hydrate in a deepwater silt reservoir from the northern Gulf of Mexico (Green Canyon 955)

Abstract

We present results from 25 quantitative degassing experiments of pressure core sections collected during Expedition UT-GOM2-1 from Green Canyon 955 in the northern Gulf of Mexico as part of the UT-Austin/DOE Deepwater Methane Hydrate Characterization and Scientific Assessment. We show the hydrate saturation (Sh), the fraction of the pore volume occupied by hydrate, is 79-93% within sandy silt beds between 413 and 442 mbsf in 2032 m water depth. Sandy silt intervals at this site are characterized by consistently high Sh (79-93%) and compressional wave velocity (Vp) (2515-3012 m/s). These intervals are interbedded with sections with lower bulk Sh (2-35%) and Vp (1684-2023 m/s) are composed of thin high Sh beds diluted by clay-rich intervals containing little to no disseminated hydrate, based on high-resolution X-ray computed tomography and Vp scans. The lithofacies-specific degassing sections reveal higher-resolution variation than is possible to observe in well logs; however, the average Sh of 64% is similar to that derived from well logs. The relationship of Sh and Vp suggests hydrate is present as a load-bearing component of the sediment. Gas recovered during these experiments is composed almost entirely of methane, with an average of 94 ppm ethane and <30 ppm propane, butane, and pentane concentrations, suggesting a microbial or highly-altered thermogenic source.