High concentration methane hydrate in a silt reservoir from the deep-water Gulf of Mexico

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 (S_h), 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 (V_p) (2515–3012 m s⁻¹) and are interbedded with clayey silt sections that have lower S_h (2%–35%) and lower V_p (1684–2023 m s⁻¹). Clayey silt intervals are composed of thin laminae of silts with high S_h within clay-rich intervals containing little to no hydrate. Degassing of single-lithofacies sections reveals higher-resolution variation in S_h than is possible to observe in well logs; however, the average S_h 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.