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Abstract

Oil- and gas-bearing sands occur at multiple stratigraphic levels within the fill of an unidentified mid-slope submarine canyon. Sand-to-shale ratio of the canyon fill is approximately 15% (calibrated to an average gross thickness of ~ 200 m), based on seismic and well control. Individual sand-prone intervals, which vary in thickness from 1 to 25 m, can have excellent reservoir properties (average net-to-gross from base sand to top sand of ~ 70%; average porosity ~ 30%; average permeability ~ 1500 mD). On seismic displays, sands have pod-or tongue-shaped geometries in plan view, with linear to curvilinear internal discontinuities. Sands display a backstepping architecture, although there is a degree of vertical overlap and compensational stacking. Sinuous channel-form geometries, which characterize reservoirs in many slope canyons, are not evident. Seismic cross sections show that sands, visualized as single seismic loops, have flat bases and rugose tops, and occur above a characteristically chaotic, low-amplitude seismic facies. Well logs and whole-rock cores over each of these three reservoir-prone intervals indicate that there is a preferred facies association. This association is a muddy debrite (corresponding to the chaotic seismic facies) overlain by massive sands and composite sandy and/or mixed-lithology breccias, in turn overlain by thin-bedded turbidites, culminating in thin-bedded hemipelagic sediments. Conglomerates punctuate the stratigraphic column but are most prevalent in the lowermost part of the succession.

Based on their seismic character, as well as log, core, and supplementary data, these reservoir intervals are interpreted as sand-prone mass-transport deposits, in which sands and underlying muds were remobilized together. Sandy units are thought to have deformed as a passive “blanket” over more ductile muddy stratigraphy. Remobilization probably occurred during or shortly after sand deposition, possibly in response to rapid loading of undercompacted muds by these sands. Thin-bedded turbidites potentially represent a waning phase of deposition from a turbid plume of suspended sediment, or may be the less-deformed part of the parent deposit. Conglomerates are interpreted to represent local canyon-margin failure. Although mass-transport deposits are seldom considered to be reservoir-prone, the example presented in this paper indicates that they can contain high-quality sands, even in low net-to-gross settings. Furthermore, this example illustrates that slope canyons need not always be filled by sinuous channel systems.

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