ABSTRACT

The Middle Triassic Halfway Formation of the western Alberta subsurface is interpreted as a prograding barrier island shoreline deposit. A detailed sedimentological study based on 130 cored sequences and 300 well logs in the Wembley area (Twp. 72-73, Rge. 7-9, W6) has enabled us to delineate the geometry of reservoir units, interpreted as tidal inlet fill, upper shoreface, and flood-tidal delta sandstones.

Complete shoreface sequences average 15 m in thickness and form mappable trends tens of kilometres along depositional strike, but are only continuous for a few kilometres across dip, with the intervening areas having been reworked by one or more migrating tidal inlets. The strike-elongate inlet-fill sequences cover more than 50 per cent of the field area. They are typically 10 m thick and exhibit the best porosities due to leaching of bioclastic material in the lower part of the fill, but the downcutting of successive inlets makes the reservoir sands laterally discontinuous. Inlet sands extend updip into flood tidal delta sandbodies, which average 4 m in thickness and pinch out in lagoonal muds. Although also averaging 4 m in thickness and showing much greater lateral continuity than the other reservoir units, the upper shoreface sandstones do not exhibit biomouldic porosity and constitute a less productive unit. An understanding of the architecture of the various reservoir components present in a barrier island shoreline system is essential when planning a secondary recovery program.

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