MARSHALL, MICHAEL C., and STEPHEN D. BENNETT, Conwest Exploration Company Limited, Calgary, Alberta, Canada
The Devonian Swan Hills Formation near Caroline, Alberta, contains four major shoaling-upward reefal carbonate stages related to relative sea level fluctuations. Each stage was initiated by a relative sea level rise culminating in a stillstand. Distinct facies zones developed toward the end of each shoaling-upward stage.
At least three of the four reef-building stages were terminated by a relative sea level fall and are capped by thin green shale layers that act as excellent chronostratigraphic markers. Subaerial exposure, freshwater leaching, and karstification are particularly evident in the Caroline Beaverhill Lake "A" pool.
The Swan Hills complex at Caroline may be described as a low gradient accretionary rimmed margin reef. A narrow reef margin zone of massive stromatoporoid boundstones acted as a breakwater to produce low energy back-reef environments. Five distinct lithofacies have been identified in this complex: reef margin boundstones, Amphipora floatstones, lagoonal mudstones, lagoon shoal floatstones/grainstones, and green shales.
Porosity in the Caroline Beaverhill Lake "A" pool resides within Swan Hills Stage II dolomitized Amphipora facies. Relative sea level rises throughout Swan Hills time caused backstepping of the reef complex producing bank margins in Stages III and IV that also contain stromatoporoid (Amphipora) facies.
The Caroline Beaverhill Lake "B" pool discovered in December 1989 (Shell et al. Caroline 6-36-34-6 W5M) confirmed the prospectivity of Swan Hills Formation upper reef-building stages. Swan Hills carbonate platform cyclicity may be used in conjunction with existing lower Swan Hills gas fields to search for upper stage Swan Hills exploration prospects.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)