The presence of a wedge of offshore permafrost on the shelf of the Canadian Beaufort Sea has been previously recognized and the consequence of a prolonged occurrence of such permafrost is the possibility of an underlying gas hydrate regime. We present the first evidence for widespread occurrences of gas hydrates across the shelf in water depths of 60-100 m using 3D and 2D multichannel seismic (MCS) data. A reflection with a polarity opposite to the seafloor was identified ~1000 m below the seafloor that mimics some of the bottom-simulating reflections (BSRs) in marine gas hydrate regimes. However, the reflection is not truly bottom-simulating, as its depth is controlled by offshore permafrost. The depth of the reflection decreases with increasing water depth, as predicted from thermal modeling of the late Wisconsin transgression. The reflection crosscuts strata and defines a zone of enhanced reflectivity beneath it, which originates from free gas accumulated at the phase boundary over time as permafrost and associated gas hydrate stability zones thin in response to the transgression. The widespread gas hydrate occurrence beneath permafrost has implications on the region including drilling hazards associated with the presence of free gas, possible overpressure, lateral migration of fluids and expulsion at the seafloor. In contrast to the permafrost-associated gas hydrates, a deep-water marine BSR was also identified on MCS profiles. The MCS data show a polarity-reversed seismic reflection associated with a low-velocity zone beneath it. The seismic data coverage in the southern Beaufort Sea shows that the deep-water marine BSR is not uniformly present across the entire region. The regional discrepancy of the BSR occurrence between the US Alaska portion and the Mackenzie Delta region may be a result of high sedimentation rates expected for the central Mackenzie delta and high abundance of mass-transport deposits that prohibit gas to accumulate within and beneath the gas hydrate stability zone.
The accurate placement of a P-S mode‐converted reflection point on a horizontal reflector is important in stacking surface seismic data for enhancement (Dohr and Janle, 1980; Garotta et al., 1985) and in interpreting results from Zoeppritz’s or Knott’s equations (Tooley et al., 1965; Young and Braile, 1976). This note describes an adaptation of Snell’s law to determine exactly the reflection point of a P-S mode‐converted reflection.
Geophysical mapping is presented that suggests a genetic relationship may exist between a band of Glauconitic sandstones and the edge of a large Mississippian structure. A seismic line of late 1970s vintage, covering a thick Glauconitic sandstone reservoir, was reshot with a broader band vibrator sweep, reduced geophone offsets, and shorter arrays. These changes significantly improved the seismic visibility of the sandstone reservoir. However, they also significantly enhanced the vulnerability of the signal to high‐noise environments. Such an environment is identified beneath a 163 000 V transmission line with wooden poles, where ground roll from resonating poles and not induced 60 Hz current is the dominant noise system. Examples are shown of the seismic signatures for both structural and stratigraphically controlled reservoirs in the area.
Abstract:The presence of offshore permafrost in the Canadian Beaufort Sea region has previously been identified from seismic and borehole data. The consequence of such permafrost is the possibility of an underlying gas-hydrate stability zone. In this study the authors present the first evidence for the widespread occurrence of gas hydrate in the offshore portion of the Beaufort Shelf using 3-D seismic data. A reflector of opposite polarity relative to the seafloor was identified at a depth of about 1000 m below seafloor that mimics some of the behaviour of the traditionally seen bottom-simulating reflectors in marine gas-hydrate regimes; however, the reflection identified is not truly bottom simulating, as its depth is rather controlled by the rapidly thinning wedge of submerged permafrost. The depth of the reflector decreases with increasing water depth, as predicted from thermal modelling. The reflection crosscuts strata and marks a zone of enhanced reflectivity underneath, possibly originating from free gas that accumulated at this phase boundary over time as the permafrost and associated gas-hydrate stability zones were thinning in response to the transgression. The presence of a clear and widespread gas-hydrate stability field beneath the permafrost has widespread implications on the region, including deep-drilling hazards associated with the presence of free gas, possible overpressure, and lateral migration of fluids and associated expulsion at the seafloor.Résumé : La présence de pergélisol en milieu extracôtier dans la région canadienne de la mer de Beaufort a été relevée dans le passé grâce aux données de levés sismiques et de sondages. L'existence de ce pergé-lisol soulève la possibilité qu'une zone de stabilité des hydrates de gaz soit présente sous celui-ci. Dans la présente étude, nous offrons les premières preuves de la présence largement répandue d'hydrates de gaz dans la section extracôtière de la plate-forme continentale de Beaufort grâce à des données sismiques 3D. Un réflecteur de polarité inverse par rapport à celui du fond marin a été identifié à une profondeur d'environ 1 000 m sous le fond marin. Ce réflecteur imite certaines des caractéristiques des réflecteurs épousant la forme du fond marin traditionnellement observés dans les contextes d'accumulations d'hydrates de gaz en milieu marin. Toutefois, le réflecteur identifié n'est pas un réel réflecteur de ce type, puisque sa profondeur est régie par la présence du prisme de pergélisol submergé qui s'amincit rapidement. La profondeur du réflecteur diminue avec l'augmentation de l'épaisseur de la colonne d'eau, comme le prévoit la modé-lisation thermique. La réflexion est discordante par rapport aux contacts des strates et marque la limite supérieure d'une zone de réflectivité accrue, résultant probablement de l'accumulation au fil du temps de gaz libre le long de cette limite de phase, pendant que le pergélisol et les zones de stabilité des hydrates de gaz qui y sont associées s'amincissaient en réponse à la transgression marine. La présenc...
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