Why Hydrate-linked Velocity-amplitude Anomaly Structures are Common in the Bering Sea Basin: A Hypothesis
David W. Scholl, Ginger A. Barth, Jonathan R. Childs, 2009. "Why Hydrate-linked Velocity-amplitude Anomaly Structures are Common in the Bering Sea Basin: A Hypothesis", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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The thick sedimentary sequence (2-12 km [6500-39,400 ft]) underlying the abyssal floors (3-4 km [9800-13,100 ft]) of the Bering Sea Basin is shal-lowly (<360 m [<1181 ft]) underlain by large (>2 km [>6500 ft] in diameter, ~200 m [~656 ft] thick) deposits of concentrated methane hydrate. Mound-shaped bodies of hydrate are displayed on seismic reflection records as velocity-amplitude anomaly (VAMP) structures imaged as velocity pull-ups overlying pushdowns. The VAMPs are numerous (hundreds to thousands) and occur across an area of approximately 250,000 km2 (96,525 mi2).
The abundance of VAMP structures is conjectured to be a consequence of high rates of basinwide planktonic productivity; of preservation of organic matter; biosiliceous sedimentation; of silica diagenesis; and of high heat flow; and deposition of a thick (700-1000 m [2296-3281 ft]), upper section of perhaps latest Miocene but mostly glacial-age (early Pliocene and Quaternary) turbidite beds and diatom ooze. Stacking of this upper Cenozoic sequence of water-rich beds heated underlying diatomaceous deposits of Miocene and older age and enhanced the generation of thermogenic methane and the diagenetic conversion of the opal A of porous diatom beds to the denser and contractionally fractured opal-cristobalite tridymite phase of porcellaneous shale. Silica transformation expelled large volumes of interstitial and silica-bound water that, with methane, ascended through the shale via chimneys of fracture pathways to enter the porous (~60%) upper Cenozoic section of diatom ooze and turbidite beds. Ascending methane entered the hydrate stability field at approximately 360 m (1180 ft), above which concentrated deposits of methane hydrate formed as either pore-filling accumulations or more massive lenses.
The deposition of high-velocity methane hydrate above a multitude of chimney structures transporting low-velocity, gas-charged fluids toward the sea floor is posited to account for the widespread recording of VAMP structures in the Bering Sea Basin.
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In September 2004, the American Association of Petroleum Geologists (AAPG) convened a Hedberg Research Conference in Vancouver, British Columbia, Canada titled "Natural Gas Hydrates: Energy Resource Potential and Associated Geologic Hazards." As a continuation of the Hedberg Research Conference in Vancouver, the conveners of the conference and the editors of this Memoir have worked with more than 150 authors and coauthors to prepare this Memoir on gas hydrates. This publication follows the goals of the Hedberg conference; however, the contents of this Memoir were expanded to include all aspects of gas hydrates in nature. This Memoir contains 39 individual contributions, ranging from long topical summaries to shorter focused research papers. This Memoir has been published in two parts, with digital versions of all the complete research papers included on the enclosed CD. The hardcopy portion of the Memoir includes abstracts and several key figures for each of the contributions along with a complete copy of a gas hydrate technical review. The digital portion of this Memoir has been organized into a series of topical sections consisting of review articles, marine gas hydrate papers, and gas hydrate laboratory and modeling studies. Because of the rapidly emerging worldwide interest in gas hydrates, this comprehensive treatise on the geology of gas hydrates will be valuable to both the gas hydrate research community and exploration/development geologists working in arctic and deep marine environments.