Abstract: Mississippian carbonates of the southern Canadian Foothills host 19 major oil and gas reservoirs and were formed by the juxtaposition of two primary facies assemblages: (I) a shallow, warm-water assemblage characterized by ooid grainstones, calcified algae, peloids and evaporites, and (2) a more widespread cool/deep-water assemblage made up of bioclastic fragments of bryozoans, echinoids, brachiopods and sponge spicules. The carbonates were deposited in a paleo-equatorial setting, on a lemperarure-stratified, high-energy ramp swept by nutrient-rich, cold upwelling waters. Episodic shifts in facies belts driven by changes in relative sea level resulted in long-term cyclic straligraphy. Foothills Mississippian reservoirs are sheet-1 ike al a large scale but are internally complex due to facies changes related to high-frequency alio- and auto-auiocyclic processes. Surface, shallow- and deep-burial diagenesis and tectonic fracturing amplified heterogeneities and resulted in complex reservoir compartmenialization. The degree to which diagenesis affects porosity depends largely on original sediment composition and mineralogy, which in turn is related to position on the Mississippian ramp. Below the thermocline, cool-water sediments were dominated by organisms with calcitic skeletons. Resulting strata lack extensive early vuggy, moldic and framework porosities that typify shallow, warm-water systems and form relatively low porosity (< 12%), low permeability (1-250 md) reservoirs. These include: 1) limestone stratigraphic traps with primary intergranular and shelter porosity in deep-water bryozoan grainstone and roudstone buildups and 2) fractured dolostones and limy dolostones with secondary intercrystalline and moldic porosity in mid- to inner-ramp echinoid/bryozoan tempestites and megaripple shoals. Warm-water sediments above the thermocline were a variable mixture of bioclasts and non-biogenic grains of calcite, dolomite, silica, aragonite and Mg-calcite composition. Associated reservoirs include: 3) stratigraphic traps in micrilizedooid grainstones, 4) fractured silty peritidal dolomites and peloidal grainstones and 5) siliciclastic coastal dune com- Most Foothills Mississippian reservoirs so far discovered occur in mid- to inner-ramp cool-water crinoid/bryozoan facies. Reservoir development is controlled by dolomitization and development of secondary intercrystalline, leached moldic and vuggy porosity. Exploration fairways are defined by trends that intersect optimal depositional and structural sellings. The best producing wells occur at the leading edge of thrust imbricates in areas of enhanced fracture permeability and high matrix porosity. These reservoirs are typical not only of the southern Fooih>Hs belt but also of Mississippian carbonates of the Peace River Embayment, Will iston Basin, Central Montana Trough and Illinois Basin. Distribution of reservoir units is highly patterned and compares favorably to modern analogues described from the south coast of Australia. Understanding details of sediment production, distribution, packaging and diagenesis in analogous Modem and Tertiary cool- and warm-water carbonate systems is invaluable for optimizing exploration and development success. This paper describes Mississippian strata from the Rocky Mountain Foothills in the southwest corner of Alberta, Canada. Samples from outcrops in the Livingstone and High Rock Ranges and subsurface cores from Coleman, Quirk Creek, Turner Valley and Moose Mountain gas fields (Fig. 1) are interpreted in a regional stratigraphic and sedimentologica) framework for the Mississippian established by Bamber et al. (1981), Richards (1989) and Richards et a). (1994). Facies descriptions build upon the excellent and thorough sedimentological analysis of Mundy et al. (1992). In this paper, we document evidence that 1) Mississippian sediments in the southwestern Foothills were deposited under temperature stratified conditions and that 2) the distribution and productivity of hydrocarbon reservoirs, while ultimately determined by tectonic configuration, is equally dependent upon original depositional facies and diagenetic processes.

ABSTRACT Scaled physical models demonstrate how a thrust sheet accommodates itself to displacement over ramp systems consisting of two frontal-ramp segments linked by a vertical tear fault or a gently dipping lateral ramp. Four stratigraphic units of alternating competence, composed of thin layers of plasticine and silicone putty, rest on a rigid base. Ramp systems are cut in the basal (competent) stratigraphic unit during assembly. The models are built at a linear scale ratio of ≈10 -6 (1 mm ≈ 1 km) and deformed in a centrifuge at 4000 g . Matched models are serially sectioned transversely or longitudinally, to constrain the structure in 3-D. Structures in the hanging wall and footwall are diagnostic of the geometry of the ramp system. The thrust sheet develops prominent structural culminations associated with the frontal ramps. The culminations terminate abruptly above the buried transverse structure, and associated folds plunge in opposite directions off the ends of the culminations. The leading edge of the competent lowest unit in the thrust sheet tends to become overturned toward the foreland. In plan view, it mimics the geometry of the ramp system. However, the leading-edge segments become curved, reflecting a decrease in thrust displacement on each frontal-ramp segment toward the lateral structure. The transverse structures appear to be rotated in response to 3-D strain comprising longitudinal gravitational spreading and stretching of the ramp-anticline culminations, as well as local longitudinal spreading of the footwall of the trailing frontal ramp adjacent to the transverse structure. Both frontal ramps propagate laterally past the transverse structure. The leading frontal ramp propagates into the footwall beneath the main thrust surface, and the trailing ramp propagates into the thrust sheet itself. Lateral propagation of the ramps generates local fault-propagation fold structures that are potential hydrocarbon traps. The models suggest that a transverse link between two frontal ramps is unstable. Once it has rotated out of the transport direction, the linking fault is no longer favorably oriented to transfer slip between the frontal-ramp segments, so it is abandoned and the two frontal ramps propagate laterally to form a lap-joint displacement-transfer zone. This begs the question: Can a tear fault or lateral ramp develop as a link between two frontal ramps in laterally uniform strata, or must such a structure be localized by a transverse heterogeneity such as a preexisting fault or facies boundary? Some features of the models are also seen in the Limestone Mountain area, southern Alberta Foothills, where the Brazeau Thrust fault and its splays ramp upward laterally to the southeast through lower Paleozoic carbonates. The Limestone Mountain and Marble Mountain culminations terminate abruptly to the southeast, at these blind lateral ramps. In strike section, the lateral structure appears to change its orientation along strike, from southwest to northeast, from a shallow northwest-dipping lateral ramp to a steep and even overturned (southeast-dipping) tear fault. With its variable dip, the Brazeau lateral ramp resembles a combinationof the lateral-ramp and tear-fault physical models, but the variable dip is inferred to be a primary feature rather than a result of out-of-plane rotation. Serial transverse and longitudinal sections of the models provide constraints on 3-D geometry that will aid geoscientists in interpreting wells and seismic sections through these complex structures and other similar situations, in the Canadian Rockies and other fold-thrust belts.

ABSTRACT Seismic structural modeling provides a useful means toward understanding the complex geometry associated with detached-style fold and thrust deformation. Routine seismic methods do not tightly constrain details of structural interpretation, but methods incorporating geometric and seismic modeling allow these details to be inferred. Geometric models measure spatial elements of the folds and faults, and check them for internal consistency, usually against an assumed condition of material balance. Seismic models measure the effects of ray-path bending through a complex, structurally defined velocity field. The seismic model predicts the presence (or absence) of reflections and their pattern on unmigrated sections. These patterns in the modeled data can be checked against the patterns observed in the real data. Iteration between the two models allows the interpretation to converge toward a mutually acceptable solution. Structural analysis of a seismic profile across the Quirk Creek gas field from the Foothills belt of the Canadian Rocky Mountains is used to illustrate this iterative method of seismic interpretation. The internal geometry of thrust sheets, essentially opaque on the basis of routine examination of a migrated seismic profile, is developed with the aid of geometric and seismic modeling techniques. As with many model-based approaches, proposed solutions in a given geometric or seismic model are nonunique. Used in combination, however, they each check solutions independently and thus can more narrowly constrain the range of acceptable interpretations. The incorporation of synthetic seismic modeling greatly improves the accuracy of interpretation for the structurally complicated Quirk Creek gas field.

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.