ABSTRACT: Keg River Sandstones: The Braid-Delta Component of a Mixed Siliciclastic/Carbonate Shelf Sequence, Peace River Arch Area, Alberta

Scott Oldale

The Keg River sandstones were deposited as a series of braid deltas which prograded into an extensive, broad, low-energy carbonate shelf fringing the emergent Peace River Arch in northern Alberta during the Middle Devonian. Three depositional environments--braided alluvium, coastal plain, and subtidal shelf--are recognized based on nine distinct lithofacies identified from a core study of 85 wells incorporating 1350 m of core.

The paleotopography of the braid plain was controlled by pre-Middle Devonian normal block faulting and subsequent erosion of that surface. The conjugate fault sets are both normal and parallel to the northeast-southwest orientation of the Peace River arch, creating a radial drainage pattern during the Middle Devonian. Subsequent tectonic activity associated with uplift of the arch during the Givetian caused a relative sea level fall, allowing coarse-grained arkosic sediment to prograde basinward. Seas quickly transgressed and confined the coarse clastics within the estuarine and fluvial channels. This rise in sea level created a characteristic fining-upward clastic sequence. Two first-order fining-upward sequences, 10-15 m, have been identified and can be correlated to the carbonate s elf stratigraphy. Each of these first-order sequences is composed of numerous second-order fining-upward sequences 1-4 m thick.

Keg River sandstones form very significant reservoirs along the Peace River arch at oil fields such as Utikuma, Red Earth, Otter, and Evi. These pools are formed by stratigraphic/structural traps. Coarse-grained sandstones depositionally pinch out against or drape paleotopographic basement highs. Post-Givetian rejuvenation of preexisting faults produce structural closure. Seismic has been used effectively in the exploration of Keg River sandstone reservoirs, first to map structural closure and second to delineate the Precambrian paleotopography and the presence of clastics by the use of seismic modeling.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990