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Jeffrey W. Nealon1, William P. Dillon1, D.C. Twichell1
(1) U.S. Geological Survey, Woods Hole, MA

Abstract: Identification of the gas hydrate stability zone in seismic-reflection profiles in the Gulf of Mexico

Gas hydrate seems to exist to the base of the gas hydrate stability zone (GHSZ) in the Bryant Canyon area south of Louisiana on the basis of seismic-reflection profile interpretation. Worldwide, the presence of gas hydrate in ocean sediments has most commonly been recognized in profiles by the presence of a bottom simulating reflection (BSR). Classic BSR’s are rare in the Golf of Mexico, but temperature, pressure, and the presence of gas dictate that gas hydrate must form within the sediments. A BSR is commonly thought of as (1.) a single reflection event that (2.) parallels the sea floor. However, as the representation of the base of the GHSZ, neither criterion is necessary. The BSR is cause by the contact between gassy sediments below the base of the GHSZ and non-gassy, gas hydrate-bearing sediments above. This contact commonly appears: (1.) as the top of a discontinuous set of strong reflections from individual gas-charged strata, and (2.) at a variable depth below the sea floor controlled by variations in water depth, thermal gradient, chemistry of interstitial waters and gas chemistry (thus not actually bottom “simulating”). The common low amplitudes of reflections above the base of GHSZ could indicate the normal reflection strengths of gas-free strata or could represent an actual decrease of reflection strength caused by gas hydrate accumulation in the more porous strata; this has become known as “blanking”.

Awareness of the presence of gas hydrate and the depth to the base of the GHSZ is important, because warming or depressurization of hydrates at this level, where gas hydrate is at its phase boundary, can cause release of free gas and water, increase in pressure, and possible drilling problems.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana