Abstract

Several cored sequences through anomalously thick Doig sandstone body reservoirs (ATSB’s) in the Sinclair and Valhalla (East) fields display significant lateral and vertical sedimentological and stratigraphic discontinuities. These ATSB’s are interpreted as being the products of shoreface sourced mass wasting events that deposited relatively coarse grained siliciclastic and carbonate sediments in the outer shelf to shelf margin setting. Middle Triassic (Doig - Halfway) siliciclastic depositional systems prograded from east to west, with the oldest ATSB’s occurring in the vicinity of the Wembley field, and the youngest in the Sinclair and Glacier fields. Isolith maps of the sandstone bodies suggest that the sandstones are confined to elongate (10’s km), narrow (1-2 km) trends, which are deposited subparallel to distant Halfway Formation shorelines. Localisation of the elongate ATSB’s occurs at the palaeo-shelf margin due to the elastic failure (slumping) of fine grained semi-cohesive marine muds deposited in this setting.

The recognition of marine flooding surfaces in the vertical cored sections and correlation of sedimentary facies, internal and external to the anomalously thick sandstone bodies suggests that deposition of the sandstones occurred contemporaneously with the deposition of the enclosing sediments. Mass wasting events (i.e. sediment gravity flows) are interpreted to be the source of sediment for the ATSB’s. In general, the thick sandstones (up to 60 metres thick) consist of a series of aggrading, normally graded sediment gravity flow deposits probably generated from periodic failure of shoreface sediments as a result of storm wave or tectonic loading of the shoreface. Lateral and vertical superposition of facies within the sandstone bodies suggest that the accumulation of sediments within the anomalously thick sandstone bodies was episodic. Internally, the ATSB’s are characterised by a distinctive westward fining from coarse bioclastic sandstone to fine grained sandstone and siltstone. Sedimentary structures present indicate that the deposition of sediments occurred in a unidirectional flow (current ripples, upper flow regime plane beds, and rare planar tabular and trough cross stratification). The tops of the regressive anomalously thick sandstone bodies are reworked by storm and wave processes and are characterised by hummocky cross stratified, wave rippled, and burrowed strata.