Sand-Prone Mass-Transport Deposits: Unconventional Reservoirs on the Continental Slope

Meckel, Trey¹ (1) Woodside Energy Ltd, Perth, Australia

Sand-prone mass-transport deposits have seismic, biostratigraphic, and grain size signatures that distinguish them from the more commonly observed channel or lobe turbidite systems that typify slope canyon infill elsewhere. Log and core characteristics of these features are less diagnostic, and are in fact quite similar to those that characterize channel systems; however, when considered in aggregate with the other characteristic elements of mass-transport deposits, they are less ambiguous, and contribute to a fully consistent interpretation. Although there are few known producing analogues for this unconventional reservoir type, sand quality can be excellent, making them potentially important and attractive targets for future exploration, especially in low net-to-gross settings.

Individual sands are visualized as discontinuous seismic reflectors with flat bases, rugose tops. In map view, bodies have pod- or tongue-shaped geometries with internal discontinuities. Numerous supplementary datasets indicate that the sands are associated with localized remobilization and dewatering that occurred during or shortly after deposition, possibly in response to rapid loading of under-compacted shales and sands within the canyon. Cores indicate that there is a predictable vertical succession in these mass-transport deposits: master failure surface, muddy debrite, composite sandy and/or mixed-lithology debrite(s), ‘waning phase' thin-bedded turbidites, and undisturbed ‘background' hemipelagic sediments. Conglomerates, interpreted to represent local canyon-margin failure, punctuate the stratigraphic column.

Outcrop and shallow subsurface examples suggest that such infill is not uncommon in nature; thus, it seems likely that other, similar systems in the subsurface may have simply been (mis)interpreted according to more conventional models of canyon fill.