common-depth point (CDP) techniques, which were being developed and used in the Western Canada Sedimentary Basin. The well that was crucial to the discovery of these two formations was drilled in 1955, and yielded gas as well as some salt water. Reef buildup of the Strachan and Ricinus gas field reefs are and respectively. Distribution and thickness The Leduc Formation occurs as discrete, discontinuous reef "buildups" in a line following the Woodbend shelf margin from Drumheller in central Alberta to the Peace River Arch area in northern Alberta. The formation is absent in inter-reef areas, and buildups can reach from to in thickness. Relationship to other units In central Alberta the Leduc Formation conformably overlies the platform limestones and dolomites of the Cooking Lake and Beaverhill Lake Formations. In northern Alberta near the Peace River Arch it rests on older red beds or on the Granite Wash. The Leduc reefs are surrounded by shales of the Duvernay and Ireton Formations and the Woodbend Group that were deposited in non-reefal, open marine environments. The Leduc reefs are the same ages as, but not contiguous with, the reefs of the Cairn Formation farther west. Because the Leduc reefs are not exposed at the surface, the extensive outcrops of the Cairn reefs in the Canadian Rockies have been studied to increase the understanding of the Leduc reefs. Near the Leduc Formation there is also the Swan Hills Formation. These two formations hold some similarities, such as the types of rocks and certain diagenetic processes. Rock types in both regions are dominated by limestones and dolomites. Dolomitization has taken place in both formations however it is more dominant in the Leduc Formation. Evaporites such as Anhydrites are also more present in the Leduc Formation, however they are also present in the Swan Hills Formation. The Swan Hills Formation also holds some differences to the Leduc Formation. The porosity types in both formations differ, and the types of fossilized biota also differ. Vuggy, moldic, intercrystalline, and fracture porosities are present in the Leduc Formation whereas the primary porosities in the Swan Hills Formation are interparticle and interfossil. One of the primary fossilized biota in the Leduc Formation are stromatoporoids, whereas the primary fossil type in the Swan Hills Formation are Amphipora. Fossil content The following fossils have been reported from the formation: Bivalves Megalodon sp. Modiomorpha sp. Turbinopsis sp. Brachiopods Conocardium sp. Cranaena sp. Schizophoria