HR: 1340h
AN: OS33B-1468    [Abstracts]
TI: Methane Content and Distribution of Natural Gas Hydrate Accumulations in the Deep-Water Basins of the Bering Sea
AU: * Barth, G A
EM: gbarth@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025 United States
AU: Scholl, D W
EM: dscholl@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025 United States
AU: Childs, J R
EM: jchilds@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025 United States
AB: Seismic reflection images from the deep-water (>3500 m) Aleutian and Bowers Basins of the Bering Sea indicate an abundant presence of natural gas and gas hydrate. Distinctive velocity-amplitude anomalies, or VAMPs, stand out as both velocity pseudostructures and gas bright spots within the otherwise horizontal and uniform sedimentary reflection sequences. These are interpreted as methane chimneys overlain by interstitial gas hydrate caps. Hundreds of VAMPs have been imaged throughout the Bering Sea; several thousand are inferred to exist. Ongoing USGS development of an interpretive seismic database presents an opportunity to quantify the hydrate content of individual VAMPs and to explore the distribution of major and minor anomalies relative to basement topography, silica diagenesis features, ancient subduction boundary structures and sediment sources. We present quantitative estimates of the size and methane content of representative large VAMP structures, based on seismic reflection interval-time anomalies. Time-average and frame-component effective medium velocity models are used to relate hydrate concentration to velocity anomaly. For this specific case, differences between the two models are minimal for hydrate concentrations <35% of pore space. To facilitate modeling of sediment dominated by diatomaceous ooze, grain-scale elastic moduli for diatom frustules are back-calculated to be ~5 GPa, assuming shear and bulk modulus are equal. Maximum velocity anomaly observed within the VAMPs is +235 m/s in the hydrate zone, relative to a background P-wave velocity of 1600 m/s. This corresponds to hydrate concentration ~40% of pore space (or ~20% of bulk rock). Hydrate distribution appears to be lithologically controlled within a section of alternating turbidite and diatomaceous sediments. It is preferentially located in a zone ~40 to 90 m above the gas hydrate BSR. Free gas is most concentrated immediately below the hydrate BSR, which lies at ~360 m bsf. Evidence for free gas extends downward in the section to >1 km depth. Individual VAMPs studied range from 4.9 to 8.6 km in diameter. These represent the large end of the variety of VAMP features observed throughout the basins. Equivalent thickness of hydrate in the sediment column is ~25 to 47 m near the anomaly centers. The gas hydrate zone of each individual large VAMP studied is estimated to contain an equivalent free gas volume >1 Tcf at standard conditions.
DE: 3004 Gas and hydrate systems
DE: 3022 Marine sediments: processes and transport
DE: 3025 Marine seismics (0935, 7294)
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005