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Carbonates comprising the approximately correlative Upper Devonian Birdbear and Nisku Formations were deposited over an immensely broad, shallow shelf that extended from eastern North Dakota over an immensely broad, shallow shelf that extended from eastern North Dakota to central Alberta. Major oil production is concentrated in Nisku pinnacle reefs bordering the central Alberta basin and Birdbear stromatoporoid banks and biostromes bordering the Williston basin in northeastern Montana. Birdbear fields are located on structures coincident with favorable reservoir facies. Birdbear porosity, formed upon dolomitization, is dominated by interconnected intercrystalline and microvugular voids, skeletal molds, and leaching-enhanced intraskeletal voids. Fractures and vugs are rela ively unimportant. Diagenesis and the distribution of reservoir facies, in turn, were controlled by the sequence and local geography of depositional settings and early burial conditions. Accordingly, depositional and diagenetic models are fundamental to geological exploration for Birdbear reservoirs.
Depositional models over the region are varied, but all have in common early marine inundation of a nearly featureless shelf and coastal plain, followed by universal gradual shoaling by vertical sediment accretion culminating in an extremely restricted depositional setting. Mud-rich depositional textures and contained fossil assemblages demonstrate the predominance of low-energy, shallow subtidal to supratidal waters of more or less elevated salinities. Normal marine shelf conditions were exceptional. Depositional models are constructed as vertical facies sequences, each subject to local and regional facies changes. One such model sequence, from the base upward, progresses from restricted intertidal mud flats (commonly brecciated mudstone), semirestricted Amphipora banks (wackestone a d current-bedded packstone), normal marine lagoons (nodular-bedded mudstone and wackestone), semirestricted stromatoporoid biostromes (in-situ laminar and tabular stromatoporoid boundstone), restricted intertidal mudflats (laminated, stromatolitic mudstone), to supratidal flats and evaporitc ponds (nodular-mosaic anhydrite and mudstone). Amphipora bank and stromatoporoid biostrome facies, which bear the highest porosity values, are especially prone to lateral facies change.
Principal events of a generalized Birdbear diagenetic model include desiccation, brecciation, and caliche development in mudstone; displacive growth of nodular and nodular-mosaic gypsum; and lithification by pervasive dolomitization. These early events were accompanied throughout by physical compaction and loss of nearly all primary porosity. Development of most secondary porosity occurred during dolomitization. Deep-burial diagenesis encompassed fracturing; gypsum-to-anhydrite transformation accompanied by fracture and pore filling and replacement by anhydrite; hydrocarbon generation and migration; and pressure-solution formation of stylolites and microstylolites. These latter events are characterized by chemical compaction, cementation, and reduction of secondary porosity. Any speci ic diagenetic model and resultant reservoir characteristics, however, are valid only for that particular set of rock fabrics and burial settings embodied in any given depositional model.
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