Abstract Swan Hills reef complexes are isolated buildups up to 75 m thick that occur on an underlying drowned carbonate platform (approximately 60 m thick) in the subsurface of west-central Alberta and were studied in detail at the Judy Creek and Snipe Lake oil fields. Although these two reef complexes are 85 km apart, 8 to 10 m thick megacycles can be correlated between them. The top of the fourth reefal megacycle is a widespread subaerial unconformity (the intraformational Swan Hills unconformity [ISHU]) that separates an underlying rimmed-reef complex from an overlying ramp-bounded shoal complex. Emergence at the ISHU was a result of a low-magnitude, relative sea level fall. This is substantiated by the following observations: (1) this surface exhibits a lithified nature continuously across both reefs; (2) shallower-water, mainly tidal-flat deposits overlie relatively deeper-water subtidal limestones at the contact; (3) solution vugs filled with marine sediments occur down to 2.3 m below the ISHU; and (4) oxidation of sediments occurs in some cores immediately beneath the unconformity Distinct and unique lithologie changes occur in lagoonal successions in the fourth megacycle below the ISHU. The middle and upper parts of this megacycle consist entirely of shallow lagoonal deposits and totally lack the "deep"-water lagoonal deposits that typify portions of the first three reefal megacycles. These distinct changes record the gradual and progressive loss of accommodation space prior to emergence and suggest that the withdrawal of the sea was not due to a Pleistocene-like, glacial eustatic lowering of sea level. This sea level fall and resulting emergence had little effect on reservoir quality of the limestones underlying the ISHU.

Abstract The Devonian and early Mississippian strata in the Western Canada Sedimentary Basin include a wide diversity of shallow-water carbonate and basin filling carbonate, shale and evaporite facies. Of these, the large Devonian platform-reef complexes are the most spectacular. They occur in magnificent exposures in the Front Ranges of the Canadian Rockies and in the subsurface of Alberta. In the subsurface, these complexes pool many of the largest oil and gas accumulations in Western Canada. This short course is intended to provide a summary of Devonian and early Mississippian deposits in the subsurface of the Western Canada Sedimentary Basin. One of the major goals is to present the evolution of these strata in a sequence-stratigraphic context. The role of sea-level, tectonic and depositional controls on “stacking” and facies patterns are considered. A second major goal is to relate the occurrence of hydrocarbon pools to this sequence-stratigraphic framework.

Abstract Devonian strata of the Western Canada Sedimentary Basin (WCSB) occur in a widespread belt extending from the Arctic Ocean and the Alaska-Yukon border on the north to the southeast through the Northwest Territories, northeast British Columbia, and Alberta into Saskatchewan and southwest Manitoba (Figure 1). This short course manual discusses the temporal and spatial evolution of Devonian carbonates and associated deposits in the WCSB, along with a discussion of early Mississippian strata. The initial 10 chapters of this manual, including this introductory chapter, consider the evolution of Devonian deposits; the 11th chapter deals with the latest Devonian-earliest Mississippian sequence. The 12th and final chapter provides a summary of the entire course. The Devonian of the WCSB is, of course, best known for the occurrence of reefs. These reefs occur in multiple phases of sedimentation and vary according to depositional setting, evolution, related facies associations, areal extent, thickness, fauna and morphology. Many of these reefs are limited in distribution to the subsurface; others are magnificently exposed in the Rocky Mountains. A summary of all of these reefs is provided by Moore (1988a).

Abstract One of the goals of science is to create order out of apparent disorder. Inherent in this pursuit is the need to classify and subdivide naturally occurring phenomena in order to better understand their origin. Stratigraphers since at least the early 20th century have sought to divide lithologic successions into genetic sequences. In regard or application to carbonate strata, stratigraphers have approached this goal from two general perspectives. One group has emphasized the significance of the areal distribution of strata and the nature of their bounding surfaces. Blackwelder in 1909 expounded on a scheme showing the areal distribution and ages of unconformity-bounded, Phanerozoic sequences of a part of the central United States. Sloss (1963) widened the scope of this endeavour and provided a division of Phanerozoic strata over the cratonic interior of North America into six unconformity-bounded sequences. Explicit in both schemes was the recognition that unconformities are of a long duration toward the centre of the craton and disappear into conformable successions along cratonic margins. The utility of Sloss Sequences has gained wide acceptance and has been used as a means for discussing the Phanerozoic evolution of sedimentary basins, for example the Western Canada Sedimentary Basin (Ricketts, 1988).

Abstract Many ancient biohermal carbonate reefs occur on areally extensive, shallow marine carbonate platforms. Because these reefs are imposing structures in outcrop and contain significant hydrocarbon reserves in the subsurface, they have been studied in detail. These studies have concentrated on their facies, paleontology, reservoir attributes, and more recently on reef evolution. Only a few studies have focused on their origin. Furthermore, the internal anatomy of carbonate platforms, from which these reefs evolved, has received only minor attention in the WCSB. As a consequence, models explaining the temporal and spatial distribution of biohermal reefs are very speculative. This paper addresses the question of the origin of large Devonian platform-reef complexes in the WCSB. Why do these reefs form during specific episodes and in certain locations? Large areally extensive biohermal reefs occur in two main stratigraphic intervals in the WCSB: the late Givetian Swan Hills reef complexes of west-central Alberta and the early to middle Frasnian reefs of both east-central and west-central Alberta. These reefs are distinctly different from either earlier Keg River reefs or later Nisku reefs in that they attained much greater areal extent, commonly in excess of 10 km 2 and up to 600 km 2 , and nucleated on shallow-water carbonate platforms. Keg River and Nisku reefs, on the other hand, are normally less than 1 km 2 in size and formed on underlying carbonate ramps.

Abstract Reef complexes situated on carbonate platforms contain a disproportionate amount of hydrocarbon reserves. In Western Canada, approximately half of the oil and gas pools with recoverable reserves greater than 16 X 10 6 m 3 (100 million barrels) of oil or its gas equivalent are Devonian reef complexes. Cores and wireline logs from producing and outlying wells from these pools provide perhaps an unparalleled opportunity to examine the evolution of reef complexes in all three dimensions. Most of the hydrocarbon bearing Devonian reef complexes in the WCSB are isolate features situated on areally widespread carbonate platforms. They include the Middle Devonian (late Givetian) Swan Hills reef complexes in west-central Alberta and the Kee Scarp buildup at Norman Wells in the Northwest Territories; and the Late Devonian (Frasnian) Leduc reef complexes in both east-central and west-central Alberta. The previous chapter dealt with the evolution of the underlying carbonate platform and its relationship to reef inception and localization. This chapter focuses primarily on the evolution of isolate platform-reef complexes. The Swan Hills buildup at Judy Creek in west-central Alberta is used as an example to illustrate the evolutionary development of these complexes.

Abstract Thick basinal sequences comprising very fine-grained sediments are common in the geological record. For the most part they present a monotonous succession of poorly defined lithologies. Although not the direct focus of exploration efforts these intervals are important in terms of permeability seals and more importantly the accumulation of hydrocarbon source sections. This article examines the nature and controls on basin filling during the Upper Devonian - Early Mississippian of the WCSB. It focuses primarily on the Ireton and Duvernay intervals of the Woodbend sequence (see Chapter 8). However, conclusions and general inferences are then related to other basin-fill successions, namely the Waterways Formation of the Beaverhill Lake Group (Chapter 7), the Winterburn shales of the Winterburn Group (Chapter 9) and finally the Exshaw - Banff shales of the early Mississippian (Chapter 11). Volumetrically basin-fill successions comprise a significant portion of the Upper Devonian to Lower Mississippian section in the WCSB. They may be grouped into three types based on gross lithology: 1) Evaporitic 2) Carbonate-rich 3) Shale-rich

Abstract The Upper Elk Point Subgroup is comprised of a succession of shallow-water carbonates, (mainly basin-filling) evaporites and some siliciclastics up to over 400 m in thickness. This succession represents the initial open-marine inundation of the Alberta "Basin". Upper Elk Point strata overlie a more restricted assemblage of shallow-water carbonates, evaporites and siliciclastics of the Lower Elk Point Subgroup and are overlain by deposits of the Beaverhill Lake Group (discussed in Chapter 7). It is particularly well known for two reasons. Firstly, isolate reefs of the Keg River and more recently Winnipegosis formations provide significant hydrocarbon reservoirs sealed by evaporitic successions. Keg River reefs of the Rainbow area in northern Alberta are particularly well known. Secondly, deposits of the Muskeg and Prairie formations comprise one of the most widespread and thick evaporite successions in the world. Evaporites vary in composition from calcium carbonate to potash salts. Most of the succession, up to the base of the Watt Mountain Formation, is interpreted to comprise part of a Devonian first-order cycle, as discussed in Chapter 2. The position of the base of this megacycle is unknown, but questionably occurs at the base of the Cold Lake salt. Anhydrites and restricted carbonates of the Chinchaga Formation, overlying the Cold Lake salt, would then be part of the transgressive phase of the Upper Elk Point megacycle. The overlying Watt Mountain Formation, a succession of dominantly siliciclastics, is the uppermost unit of the Upper Elk Point Subgroup, but is interpreted by us to be part of the Beaverhill Lake megasequence. This succession, therefore, is discussed in the ensuing Beaverhill Lake chapter.

Abstract This succession incorporates the entire Beaverhill Lake Group and the underlying Watt Mountain Formation. These units are interpreted to comprise a first-order cycle. Collectively, the progression of strata in this megasequence corresponds to an overall transgressive-regressive succession. Thicknesses of these strata range from nil along the Peace River Arch where these units onlap out, to over 200 m in east-central and northern Alberta. This pattern reflects greater rates of subsidence away from the Peace River and Western Alberta arches. Figure 1 depicts the general paleogeography of the Beaverhill Lake megasequence in the WCSB. This map illustrates the distribution of both shallow-water and basinal facies. In Alberta the dominant element is a deep-water basin that extends as far south as south-central Alberta. Because of the truncation of Beaverhill Lake strata along the pre-Jurassic and pre-Cretaceous unconformities, the configuration of the connection of this basin to the deep-water seaway in the Northwest Territories is unknown. The basin is fringed by shallow-water platform carbonates on both its east and west sides. The platform carbonates along the southeast side of the basin grade into the more restictive carbonate-evaporite successions of the Souris River Formation. Shallow-water carbonates along the west side of the basin and south of the Peace River Arch include backstepping platform and bank successions. These units onlap and ultimately cover the Western Alberta Arch. Of particular note in this area is the occurrence of the Swan Hills platform margin at Caroline. A giant gas-condensate pool was discovered here by Shell in 1985 which ranks as the most significant hydrocarbon discovery in Devonian rocks in the WCSB in the last quarter century. Further north, isolate Swan Hills reef complexes occur on a more areally extensive carbonate platform, basinward of time-synchronous bank development. Collectively, more oil is pooled in these reef complexes than any others in the WCSB.