Reservoir Characterization of Sand-Prone Mass-Transport Deposits Within Slope Canyons
Published:January 01, 2011
Lawrence D. Meckel, III, Matthew Angelatos, Jos Bonnie, Roderick McGarva, Tony Almond, Neil Marshall, Laurent Bourdon, Karen Aurisch, 2011. "Reservoir Characterization of Sand-Prone Mass-Transport Deposits Within Slope Canyons", Mass-Transport Deposits in Deepwater Settings, R. Craig Shipp, Paul Weimer, Henry W. Posamentier
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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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Mass-Transport Deposits in Deepwater Settings
Historically, submarine-mass failures or mass-transport deposits have been a focus of increasingly intense investigation by academic institutions particularly during the last decade, though they received much less attention by geoscientists in the energy industry. With recent interest in expanding petroleum exploration and production into deeper water depths globally and more widespread availability of high-quality data sets, mass-transport deposits are now recognized as a major component of most deep-water settings. This recognition has lead to the realization that many aspects of these deposits are still unknown or poorly understood. This volume contains twenty-three papers that address a number of topics critical to further understanding mass-transport deposits. These topics include general overviews of these deposits, depositional settings on the seafloor and in the near-subsurface interval, geohazard concerns, descriptive outcrops, integrated outcrop and seismic data/seismic forward modeling, petroleum reservoirs, and case studies on several associated topics. This volume will appeal to a broad cross section of geoscientists and geotechnical engineers, who are interested in this rapidly expanding field. The selection of papers in this volume reflects a growing trend towards a more diverse blend of disciplines and topics, covered in the study of mass-transport deposits.