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High concentration methane hydrate in a silt reservoir from the deep-water Gulf of Mexico
Stephen C. Phillips,1 Peter B. Flemings,2 Melanie E. Holland,3 Peter J. Schultheiss,4 William F. Waite,5 Junbong Jang,6 Ethan G. Petrou,7 and Helen Hammon8
1Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas; [email protected]
2Institute for Geophysics and Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas; [email protected]
3Geotek, Ltd., Daventry, United Kingdom; [email protected]
4Geotek, Ltd., Daventry, United Kingdom; [email protected]
5US Geological Survey, Woods Hole, Massachusetts; [email protected]
6Integrated Statistics, Inc., Woods Hole, Massachusetts; [email protected]
7Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas; [email protected]
8Institute for Geophysics and Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas; [email protected]
We present results from 30 quantitative degassing experiments of pressure core sections collected during The University of Texas-Gulf of Mexico 2-1 (UT-GOM2-1) Hydrate Pressure Coring Expedition at Green Canyon Block 955 in the deep-water Gulf of Mexico as part of The University of Texas at Austin–US Department of Energy Deepwater Methane Hydrate Characterization and Scientific Assessment. The hydrate saturation (Sh), the volume fraction of the pore space occupied by hydrate, is 79% to 93% within sandy silt beds (centimeters to meters in thickness) between 413 and 442 m below seafloor in 2032 m water depth. Sandy silt intervals are characterized by high compressional wave velocity (Vp) (2515–3012 m s−1) and are interbedded with clayey silt sections that have lower Sh (2%–35%) and lower Vp (1684–2023 m s−1). Clayey silt intervals are composed of thin laminae of silts with high Sh within clay-rich intervals containing little to no hydrate. Degassing of single-lithofacies sections reveals higher-resolution variation in Sh than is possible to observe in well logs; however, the average Sh of 64% through the reservoir is similar to well log estimates. Gas recovered from the hydrates during these experiments is composed almost entirely of methane (99.99% CH4, <100 ppm C2H6 on average), with an isotopic composition (δ13C: −60.4‰ and −63.6‰ Vienna Peedee belemnite and δ2H: −178.2‰ and −179.0‰ Vienna standard mean ocean water) that suggests the methane is primarily from a microbial source. A subset of six degassing experiments performed using very small pressure decrements indicates that the salinity within these samples is close to the average seawater concentration, suggesting that hydrate either formed slowly or formed during a rapid event at least tens of thousands of years before present.
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