STOAKES, FRANK, and ROBIN DIXON, Stoakes Campbell Geoconsulting Ltd., Calgary, Alberta, Canada
The Caroline field, discovered in 1986, produces from the updip northeast stratigraphic closure of dolomitized platform margin carbonates of the Devonian Swan Hills Formation in southwest Alberta. It lies at a present burial depth of around 3600 m (11,800 ft) and current estimates place reserves at almost 56,640 x 106 cubic meters (2 tcf) of gas and 4 million cubic meters (250 million barrels) of condensate.
The Swan Hills platform is up to 100 m thick (328 ft) and is comprised of a total of seven major depositional cycles. These may be characterized in ascending order as: Initial ramp (Cycle 1), Platform stage (Cycles 2-5), and Final grain ramp (Cycles 6 and 7).
The first cycle develops as a low angle mud-dominated ramp over the relatively flat Elk Point surface and its break-in-slope lies east of the present field. Cycles 2 through 5 constitute the platform stage where a distinct reef margin separates interior lagoon sediments to the west from foreslope and basin sediments to the east. Cycles 2, 3, and 4 show progressive backstepping to the west with the fifth and final platform cycle appearing to forestep and offlap towards the east. Cycles 6 and 7 mark a return to ramp sedimentation, this time of a more grain-dominated nature, and these backstep again to the west. The platform margin was subsequently buried by basinal shales and limestones of the Beaverhill Lake Group that prograded into the area from the east, downlapping against the drow ed shallow-water carbonates.
The key to porosity preservation at Caroline is dolomitization. Observations suggest that this resulted from burial and subsequent dewatering of the adjacent Beaverhill Lake shales. The resulting pattern of dolomitization is controlled by the original depositional facies within each cycle, making an understanding of the evolution of the platform complex critical in extending and predicting the vertical and lateral extent of individual reservoir zones. Only three depositional cycles (3, 4, and 5) retain reservoir-quality porosity in a 1 to 2 km (0.6-1.2 mi) belt that corresponds in large part to the reef margin and adjacent environments.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)