--> Abstract: Geophysical Evidence for Dense Masses of Methane Hydrate in the Bering Sea Basin; Thoughts about their Origin, Occurrence, and Resource Potential, by D. W. Scholl and P. E. Hart; #90935 (1998).

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Abstract: Geophysical Evidence for Dense Masses of Methane Hydrate in the Bering Sea Basin; Thoughts about their Origin, Occurrence, and Resource Potential

SCHOLL, DAVID W., Department of Geophysics, Stanford University, Stanford, CA; andPATRICK E. HART, U.S. Geological Survey, Menlo Park, CA

Seismic reflection and velocity anomalies indicated that the Bering Sea Basin (3,500-4,000 m) is shallowly (0-450 m) underlain by deposits of methane hydrate. Hydrate occurs as disseminated deposits of low concentration (< 10-15 % of pore space), and as localized more massive accumulations (> approx. 30 %). Dense concentrations are detected in recordings of prominent velocity and reflection-amplitude anomalies known as VAMP structures. These structures, which are recorded within a flat-lying sequence of diatom-rich turbidite beds, are displayed as vertical columns (1-3 km wide) of upward-arched horizons stacked directly above a vertical series of downward-arched horizons (1.0-1.5 km wide). At a subbottom depth of about 400 m, a high-amplitude, negative polarity reflector separates the up- and down-arched horizons, the vertical amplitude of which, respectively, decreases upward and increases downward.

Geophysically, the VAMP structure is a lateral velocity distortion in horizontal beds caused by the superposition of a velocity pull-up directly above a velocity push-down. Geologically, the pull-up is interpreted to be a massive deposit of methane hydrate that has accumulated in porous beds above a column of thermogenic gas ascending toward the sea floor through diagenetically contracted siliceous deposits of Miocene age.

If they are discreet psuedostructures, at least 12,000 VAMPs occur in the Bering Sea. If they are dimensionally simple, and a low concentration of gas (2 %) is called upon to explain the push-down, the minimum basin-wide volume of methane at VAMPs is about 1100 TCF, 80 percent of which is methane hydrate. At a single large VAMP (pull-up = 30 ms, push-down = 80 ms), the minimum volume of methane involved is about 0.3 TCF, which is equivalent to a large gas field.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California