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ABSTRACT: Hydrate Stability Zone Permanence Along Dynamic Louisiana Offshore

LOWRIE, ALLEN, MICHAEL D. MAX, RHETT HAMITER, and IAN LERCHE

The Louisiana offshore, and northern Gulf of Mexico continental margin in general, is an area of multiple dynamic occurrences at different periodicities given the interrelationship between sedimentation/sea level oscillation/climate and the tectonics of semiplastic salt and shale. Throughout the Neogene, and probably much of the Paleogene, documented glacial--interglacial climate-driven sea level oscillations at 3rd (approx. 106 yrs), 4th (approx. 105 -2(102) yrs), 5th (approx. 103 yrs), and 6th (approx. 102 yrs) order intervals are recorded in offshore deposits, each depositional periodicity as producers of salt/shale, growth- and thrust-fault tectonics, and gravity-driven (large and small) slumping, as in Mississippi Canyon creation.

Hydrates develop as petroleum-derived gases become crystalline, based on appropriate combinations of temperature and pressure. Changes in the temperature/pressure regime increase/decrease the hydrate stability zone. Global climates determine the Gulf physical oceanography (glacial ages increase vertical extent and shallowness of Antarctic Intermediate Water) and sea level (glacial advances lower sea level 125 m during the Quaternary), thus, changing the inputs to the stability zone during at least 4th order frequencies. Salt/shale tectonics would disrupt the sedimentary layers and release the hydrate to become gas and escape to the overlying oceans and atmosphere.

Maximum impact of temperature/pressure changes on a hydrate stability zone would occur during the relatively rapid deglaciation phase and the tectonic dynamics appear most strongly during lowstands, surely at 4th order oscillations. Rapid, apparently warm climate-driven Heinrich events of some 103 year durations during the cold/lowstand phase may contain appreciable sea level lowerings impacting sedimentation patterns. Such sedimentation changes may drive tectonics, in turn rupturing hydrate zones.

Dynamic question concerns rate of hydrate regrowth after disruption. Inputs include migration routes and rates of methane migration from source area, methane reservoirs regionally, recreation of hydrate stability zones, and tectonic fractures/fault "healing" after disruption.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana