Reservoir Sedimentology of the M. Triassic Halfway Fm., Wembley Field, Alberta

WILLIS, ANDREW JOHN, University of Alberta, Edmonton, Alberta, Canada

The Middle Triassic Halfway Formation of west-central Alberta 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 (Townships 72-73, Ranges 7-9, West of Sixth Meridian) 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 kilometers along depositional strike, but are only continuous for a few kilometers 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% 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 down-cutting of successive inlets makes the reservoir sands laterally discontinuous. Inlet sands extend up-dip into flood-tidal delta sandbodies that average 4 m in thickness and pinch out in lagoonal muds. Although showing much greater lateral continuity than the other reservoir units, the upper shorefa e sandstones do not exhibit biomoldic porosity and are a less productive unit. Such 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.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)