Abstract: The Carson Creek North Field is an Upper Devonian isolated reef-rimmed buildup in the Swan Hills Formation of the subsurface Western Canada Sedimentary Basin. The Swan Hills Formation belongs to the Beaverhill Lake Group, which contains three regionally defined sequences. The sequence boundaries at the base of the middle sequence (BHL2.1 SB) and the upper sequence (BHL3.1 SB) are present within the Carson Creek North buildup, dividing it into three evolutionary stages (lower atoll stage, upper atoll stage, and shoal stage). The BHL2.1 SB is a locally exposed surface at the top of the lower atoll stage, during which the Carson Creek North buildup evolved from a low-angle ramp to a steep-sided reef margin enclosing a restricted lagoon. The BHL3.1 SB is a regional subaerial exposure surface at the top of the upper atoll stage, also known as the intra-Swan Hills unconformity (ISHU). The upper atoll stage consists of backstepping reef margin cycles, and the shoal stage overlying the ISHU contains backstepping ramps culminating in drowning of the Carson Creek North buildup; therefore, the ISHU occurs within a continuously backstepping margin succession. The ISHU demonstrates ~13 m of paleotopographic relief within the Carson Creek North buildup, yet it is not associated with a regressive margin. The Beaverhill Lake Group also contains shallow-water carbonates in the southeastern part of the Western Canada basin (Eastern Shelf) that are partly equivalent to the Swan Hills Formation. The Eastern Shelf carbonates are separated from Carson Creek North by more than 100 km across the Waterways shale basin. A regressive facies succession is present just below the BHL3.1 SB in the Eastern Shelf area, followed by an intrabasin lowstand corresponding to a hiatus during which paleotopography developed on the ISHU at Carson Creek North. The Eastern Shelf succession thus contains the missing regression across the ISHU at Carson Creek North. The lowstand was followed by gradual regional flooding that included deposition of the Carson Creek North shoal stage ramps. Stratigraphic comparisons among Carson Creek North, the Swan Hills Formation, and the Eastern Shelf areas indicate that regional differential subsidence patterns and initial basin floor topography were the most likely reasons for the divergent stratigraphic architecture. The origin of the ISHU itself remains unresolved, but it could be linked to a global eustatic event near the base of the Frasnian Stage, enhanced by tectonic activity in the Western Canada Basin.

Abstract Four case histories of textural and reservoir analyses of selected Paleozoic carbonate cycles and reef complexes of the Western Canada basin have been utilized in the formulation of a carbonate rock classification chart. This chart is presented to illustrate the relationships of grain, matrix, and cement variants of carbonate rocks, to porosity and permeability determinations, and should satisfy the requirements of an oil geologist or reservoir engineer. Large stratigraphic accumulations of oil have been discovered at or near the Paleozoic subcrop of the Mississippian “Midale” carbonate cycle in southeastern Saskatchewan. Apart from scattered, vuggy, algal encrusted strand-line deposits, most of the carbonates of the “Midale” producing zone consist of sorted, silt-sized material and skeletal or nonskeletal limestones which have a very finely comminuted, commonly dolomitized, limestone matrix with intergranular and chalky porosity. Effective reservoir porosity is controlled by the relative distribution, grain size, and sorting of this matrix. Major hydrocarbon (oil and gas) reserves have been found in the Mississippian “Elkton” carbonate cycle, both in the foothills belt and along the subcrop, in southwestern Alberta. Effective reservoir material of this cycle was found to consist mainly of the dolomitized equivalent of originally coarse, uncemented skeletal limestone, and skeletal limestone with a variable amount of generally porous, finely comminuted (granular) skeletal matrix. Primary porosity was very important in the control of dolomitization, which began with the replacement of this matrix by euhedral rhombohedrons, and finally affected the coarse skeletal material (now generally indicated by leached fossil-cast outlines). These porous dolomites grade laterally in a predictable way into tight, relatively nondolomitized, well sorted, coarse skeletal (locally oolitic) limestones with original high interfragmental porosity now completely infilled with clear crystalline calcite. This lithification by cementation took place early in the history of carbonate sedimentation of this area and before secondary dolomitization processes took effect. The transgressive, reef-fringed, limestone banks or platforms of the Upper Devonian Beaverhill Lake formation in the Swan Hills region of Alberta have been found to contain major reserves of oil and gas. Successive rims of organic lattice, stromatoporoidal and algal, atoll-like “buildups,” with granular matrix, separate generally medium to dark brown pelleted lime muds containing abundant amphiporids, and intercalated lighter colored lagoonal carbonates, from open marine shales and nodular, argillaceous, crinoid- and brachiopod-rich limestones. The most effective reservoir material along the reef fronts or terraces consists of vuggy organic lattice, algal encrusted amphiporids (minor developments), uncemented skeletal or nonskeletal limestone, and reworked stromatoporoidal, algal, and amphiporid material with intra- organic vugs, embedded in a porous, well sorted, micro to finely granular matrix. Matrix grain size and sorting studies are essential to exploration and secondary recovery problems, as the granular material grades laterally into chalky or micrograined limestones, which were laid down under lower energy conditions. Matrix granularity ratio outlines are considered to be superior to ecological maps (percentage of algal and stromatoporoidal material) in the prediction of shoal areas. The highly productive Nisku, regressive, dolomitized biostromal-evaporite complex of the Edmonton and Red Deer areas of Alberta, contains numerous stratigraphic and/or structural traps. Zonation of Nisku dolomites has been accomplished by crystal size and shape studies in combination with identification of vestiges of original organic structures and skeletal or nonskeletal grain outlines. The morphological expression of the underlying Leduc reef platforms and carbonate buildups in the Duvernay formation strongly influences facies variations of the Nisku carbonate-evaporite unit. An algal, stromatoporoidal, coralline, organic lattice chain associated with generally coarse, dolomitized, clastic carbonates with a porous, granular matrix is developed on the Rimbey-Meadowbrook trend, and forms a front to a shale- limestone facies deposited under open marine conditions to the northwest. To the east of this barrier, a complex pattern of fringing organic and clastic carbonate shoals separate locally silled, lagoonal deposits of evaporites and brown carbonate muds containing abundant secondary anhydrite-replaced amphiporids. The shoal and lagoonal carbonates throughout most of this area are overlain by anhydrite or anhydride dolomite sheets, which were precipitated in the wake of an overall regressive Nisku sea.