Abstract The stages of Western Canada foreland basin tectonic evolution are recorded as five stratigraphically distinct depositional assemblages. Each assemblage can be characterized by a series of proximal, medial, and distal facies belts that migrated with and recorded foreland basin regressive or transgressive events. Each facies belt and each assemblage has a unique set of traps and plays controlled by the types of rocks available as reservoirs and seals. Deposits within the facies belts that make up the assemblages are composed typically of stacked progradational sequences that are commonly punctuated by either transgressive or erosional events. The fundamental building blocks of the five assemblages are coarsening-upward progradational sequences that are meters to tens of meters in thickness. The sequences consist of offshore deposits overlain successively by shoreface to shoreline to continental sediments. Most foreland basin oil and gas fields are found in the higher-energy, porous, shoreline-related or fluvial sandstones and conglomerates near the tops of individual sequences. Tectonic subsidence induced by orogenic activity, sea level, climate, and sediment supply controlled the sequence character and sequence distribution and also controlled the geometric relationships among reservoirs, source rocks, and seal rocks. Thus, variations in these controlling factors created differences in trap style within the sequences. For example, distinctive traps originated through a sea level fall that initiated erosion and valley incisement of marine sediments in a progradational sequence, followed by a sea level rise that caused sediment infill of these incised valleys by reservoir sandstones and conglomerates. Because the five assemblages are stratigraphically distinct and represent unique geologic histories, the oil and gas plays of the foreland basin can be grouped within their hosting depositional assemblages as follows. In the Fernie/Nikanassin Assemblage (Upper Jurassic/basal Cretaceous shallow marine to continental deposits), the main plays comprise intra-Jurassic shallow marine, shoreline to fluvial sandstone reservoirs and uppermost Jurassic sandstone erosional remnants isolated by the sub-Cretaceous unconformity. Erosional remnant, channel, and valley-fill traps produce oil and gas in southern Alberta and Saskatchewan (Swift and Success formations), and fluvial channel traps produce gas in the northern Alberta and northeastern British Columbia Deep Basin and Foothills (Nikanassin Formation). In the Mannville Assemblage (Lower Cretaceous fluviodeltaiclittoral deposits), the main plays occur within continental basal sandstones deposited transgressively on the sub-Cretaceous unconformity and within deltaic to estuarine sandy deposits overlying the basal sandstones. Complex geologic relationships created a variety of stratigraphic traps; major petroleum accumulations occur in, for example, Basal Quartz fluvial and estuarine sandstones, Athabasca and Peace River deltaic oil sands, and Dunlevy/Buick Creek fluvioestuarine channel complexes. This assemblage dominates foreland basin gas reserves and unconventional heavy oil reserves. In the Colorado Assemblage (middle Cretaceous marine deposits), the main plays occur in thin, coarsening-upward sandstone sequences commonly overlain unconformably by conglomerates with the sequences pinching out into marine shales. Major oil and gas producers are conglomerates and sandstones of the Viking, Cardium, and Medicine Hat formations: these units dominate foreland basin conventional (light to medium) oil reserves. In the Saunders/Edmonton Assemblage (Upper Cretaceous marine units transitional to lower Tertiary continental deposits), the main plays occur within the basal part of the assemblage in deltaic and fluvial sandstones of the Milk River and Belly River formations, which formations represent the final episode of marine deposition in the foreland basin. The upper part of the assemblage is mainly continental deposits that are essentially barren of petroleum because of a lack of source and seal rocks. The Cypress Hills/Hand Hills Assemblage (upper Tertiary continental deposits) consists of thick, gravelly, continental strata that represent proximal molassic deposits and is currently considered poorly prospective for hydrocarbons. Although channel sandstone and conglomerate reservoir facies are abundant, petroleum potential is limited by the near-surface and surface location of the assemblage, by a lack of petroleum source rock and seal rock facies, and by low thermal maturities.

Abstract The Jurassic of the Canadian Rocky Mountains and foothills comprising the Fernie group and the lower part of the Kootenay and Nikanassin formations is subdivided on a paleontological basis into a number of units which correspond to zones and stages of the Northwest European standard section. The following stages are shown to be present by their index fossils—part of the Sinemurian, the Toarcian, the middle Bajocian, the lower Callovian, at least part of the Oxfordian and Kimmeridgian, and the upper Portlandian. A most significant feature of the Fernie group is its incompleteness, only n or 12 of the 59 Northwest European ammonite zones being proved to be present. Ammonite zones belonging to the Hettangian, parts of the Sinemurian, and the Pliensbachian are definitely absent. Stratal equivalents of the lower and upper Bajocian, Bathonian, upper Callovian and parts of the Oxfordian, Kimmeridgian, and Portlandian are not indicated by index megafossils and may be entirely absent or represented by sediments without megafauna. A variety of different facies is developed which can only be correlated by index fossils common to different facies districts. Some of the most prominent facies units are the Nordegg member, the Paper or Poker Chip shale of the lower Fernie, the middle Bajocian Rock Creek member, the Callovian Corbula munda beds and Gryphaea bed which are equivalent to the Grey beds, the Oxfordian Green beds, and the Passage beds which are mainly of Kimmeridgian age. Correlations of the Rocky Mountains and foothills Jurassic with the Jurassic deposits in British Columbia, Yukon, Prince Patrick Island, and Manitoba are established. Absence of index megafossils in the Jurassic of the Alberta, Saskatchewan, and Manitoba plains renders correlation of these areas with the Fernie group difficult. A tentative attempt has been made to correlate these areas on the basis of existing literature and through the co-operative efforts of the Jurassic symposium committee. Interpretation of the data obtained from field studies has made possible a synthesis of the present Canadian Rocky Mountains and foothills region during Jurassic time, a region which is considered as a non-geosynclinal border zone between the Cordilleran geosyncline in the west and the land Laurentia in the east.

Abstract Enormous volumes of gas (>30 Tcf) are contained within the deepest portions of the Western Canada Foreland Basin, where tight gas-saturated Cretaceous sandstones grade updip into porous water-saturated sandstones. Production has occurred from coarse-grained shoreline sands both near the updip gas–water interface, such as those found in the Elmworth Field, and from low-porosity–permeability reservoirs found deeper in the basin. These basin-centred gas (BCG) reservoirs are characterized by regionally pervasive gas-saturated lithologies, abnormal pressures and no downdip water contact, and occur in low-permeability reservoirs. The keys to Shell's exploration success were an understanding of the stratigraphy, sedimentology and rock properties of the basin, the development of structural, petrophysical and geomechanical models, development of an understanding of the desiccation or dewatering process, the distribution of water within the basin and how the pressure regime evolved, interpretation of 3D seismic, and an aggressive land strategy. The evaluation of structural leads was aided when seismic and geomechanical modelling were combined, thereby aiding in the prediction of zones with a higher probability of encountering favourable reservoir producibility characteristics, that is, areas where a well developed, well connected open fracture network is expected. This multidisciplinary approach has resulted in economic success in regions once thought to be non-productive, and where it was once said, ‘People go broke chasing the Nikanassin’.