ABSTRACT: Stratigraphic Evolution of a Middle Leduc Reef-Off Reef Transition, Cripple Creek, Front Ranges, Alberta

WONG, P., M. GILHOOLY, M. MCMURRAY, I. MUIR, J. WEISSENBERGER, and L. WEST, Esso Resources Canada Ltd., Calgary, Alberta, Canada

An Upper Devonian, Middle Leduc (Peechee Member) Formation dolomitized carbonate platform to basin transition is well exposed in the Cripple Creek area, in the front ranges of Alberta, 220 km WNW of Calgary. A detailed field study was undertaken to (1) test the utility of upward-shoaling cycles for the internal correlation and subdivision of carbonate reservoirs and (2) to gain an appreciation for the lateral continuity of depositional facies that form reservoirs in the subsurface.

A total of six major upward-shoaling depositional cycles define growth stages of the Middle Leduc at Cripple Creek. Each cycle is between 15 m to 25 m thick and displays the following vertical facies succession: (1) lower to middle foreslope branching and tabular stromatoporoid/branching coral dolorudstones and dolofloatstones, (2) foreslope dolograinstones, (3) margin/upper foreslope stromatoporoid dolorudstones and doloboundstones, (4) reef-flat dolograinstones, and (5) laminoid fenestrae dolopackstones. Cycle boundaries can be traced laterally in outcrop from a platform to a lower foreslope setting.

At Cripple Creek, pronounced progradation within each Middle Leduc cycle has resulted in the development of extensive, tabular reservoir bodies comprised of foreslope to reef flat facies. Reservoir facies associated with successive stages of reef growth are separated by, and can be correlated with, cycle boundaries.

The nature of basinward reservoir facies pinch-out is foreslope angle dependent. For gentle slopes (approximately 10-20 degrees) foreslope reservoir facies pinch-out occurs over horizontal distances of about 100 m. Where foreslope dips are high (approximately 40 degrees), pinch-out is rapid, over distances of 15 m to 20 m.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)