Abstract Dolomudstones of the Pekisko Formation in western Canada form small but important oil and gas reservoirs. The reservoirs are irregularly shaped bodies 1 km or so wide and commonly 5–8 m thick. Porosity development within the dolomudstones is a complex function of sedimentation, early facies-selective dolomitization and later telogenetic leaching of calcareous components. The carbonate sediment precursor of the dolomudstone, interpreted from relict textures preserved in chert nodules, was a microwackestone with abundant silt-sized skeletal fragments. Dolomudstone reservoirs are comprised of dolomudstones, calcareous dolomudstones, and subordinate interbedded dolowackestones and dolograinstones. Some dolomudstone reservoirs are contained entirely within grainstones. Others are capped by tight fenestral lime mudstone that has been dolomitized locally. Dolomitization has been most intense within the centres of these reservoirs, and dolomudstones grade laterally into calcareous dolomudstones. The association of facies indicates that microwackestones were deposited in subtidal intershoal and lagoonal environments on an inner ramp. Grainstone shoals provided a broad barrier that absorbed wave energy seaward of the lagoon. Fenestral lime mudstones accumulated in peritidal environments in restricted areas of the inner ramp, landward of the lagoon. Dolomitization is interpreted to have been early and selective to the microwackestone facies because it retained permeability or was reactive during early burial. Dolomitizing fluids were most probably derived from overlying formations and made their way downwards through spatially separated conduits. The Pekisko Formation was exposed and sculptured at several Jurassic-Early Cretaceous unconformities. During these times, sandstones and shales were deposited in solution cavities developed within the dolomudstones. Concomitant leaching of calcite increased porosity of the dolomudstone reservoirs.

Abstract Upper Mannville G, U and W pools in the Little Bow field are hosted by separate parallel elongate estuarine sandstone bodies within an incised valley fill Each sandstone body is 3-4 km long, 300-500 m wide, up to 22 m thick, with an average porosity of 22%. Values of horizontal and (vertical) permeability vary widely and average 1324 (125) md in G pool, 2005 (472) md in U pool, and 258 (73) md in W pool. G pool was discovered in 1972 and placed on primary production. Oil production declined gradually and was accompanied by modestly increasing GOR and WOR. U and W pools were discovered in 1982 and 1983 respectively, and produced by primary methods until initiation of waterflooding in 1985. Response to waterflooding these two pools has been a rise, then decline, in the GOR, followed by rapidly rising WOR, to values much greater than those predicted from reservoir modelling, currently up to 10:1 in wells adjacent to water injectors. Despite the wide variation in permeability values and the different production histories, similar proportions of oil have been produced: 9.2% OOIP in G Pool; 10.5% in U Pool; and 9.3% in W Pool. Production response indicates controls by mesoscale and microscale reservoir heterogeneities. Mesoscale heterogeneities include permeable sandstone beds several meters thick that are continuous between adjacent wells, and stochastic shale beds up to 80 cm thick. Rapid breakth of water has occurred in producing wells adjacent to injectors due to channeling in thick permeable sandstone beds between

Abstract Breccia deposited seaward of the margins of Miette and Ancient Wall buildups (Upper Devonian of Alberta) records synsedimentary submarine cementation of carbonate sands. The size, shape and composition of breccia clasts indicates that their source rocks were differentially cemented (nodular), thinly bedded carbonate sands deposited high on the foreslopes, close to the margins of the buildups. Downslope movement of differentially cemented carbonate sand sequences mixed nodules with uncemented carbonate sands and resulted in deposition of breccia beds. The breccia was transported downslope by hybrid sediment gravity flows in which upward movement of fluids, grain-grain interactions, and possibly fluid turbulence supported the clasts. Increased pore pressures necessary for initiating such flows on low slopes were the result of metastable packing of sand grains in thinly bedded sequences and/or overriding currents carrying material from the buildups. Rapid dissipation of excess pore pressures, coupled with low slopes, did not allow sustained flow and as a consequence deposition of the breccias occurred within a few kilometers of the buildups. Carbonate breccia formed by differential submarine cementation and downslope displacement of carbonate sands has not previously been described. One important facet of its origin is that it does not record periods of abrasional erosion of the buildups, but formed at times when rate of sediment supply to the foreslope was relatively high-