Sedimentological Insights into Deepwater Mass Gravity Sands from the North West Shelf, Australia

Kerrie E. Deller, Tobias H. D. Payenberg, and Simon C. Lang
University of Adelaide, South Australia, Australia

Densite deposits, which share characteristics of both turbidites and debrites, represent one of the least known aspects of present day deepwater sedimentology. These deposits are recognised in the northern Dampier and southern Beagle sub-basins located on the North West Shelf of Australia. The region hosts several key hydrocarbon fields which include the Angel gas field and the Exeter/Mutineer oil field complex. The main reservoir interval, the Angel Formation, comprises mass gravity sandstones which are Tithonian to Berriasian in age. These sediments were deposited during a time of active extension associated with continental breakup and seafloor spreading along the western margin of Australia.

Sedimentological interpretation of available cores across the region has revealed that the upper Angel Formation consists primarily of densites. The single event sand units encompass a lower non-cohesive debrite base capped by an upper turbidite-like classic Bouma sequence of sediments. The debritic bases are thick-bedded with sporadic inverse or “stepped” grading. Dish structures formed by escaping pore fluids and floating clasts are also present within these units. Sand injectite features are prevalent throughout the bioturbated silt and shale units, which separate the densite intervals.

The source and transport direction of these sands remains ambiguous. The clean, reworked sands may have originated from the peri-platform regions of the northern Dampier Sub-basin or from shelfal regions located in the southern Beagle Sub-basin. Identification of transport pathways could open up new play opportunities between the source and sink.