DOLOMITIZATION AND DEDOLOMITIZATION OF A PERITIDAL CARBONATE RESERVOIR; LATE DEVONIAN UPPER LEDUC FORMATION, EAST-CENTRAL ALBERTA, CANADA
Nicholas P. Ettinger, Walter Loogman, Noel P. James, 2017. "DOLOMITIZATION AND DEDOLOMITIZATION OF A PERITIDAL CARBONATE RESERVOIR; LATE DEVONIAN UPPER LEDUC FORMATION, EAST-CENTRAL ALBERTA, CANADA", Characterization and Modeling of Carbonates–Mountjoy Symposium 1, Alex J. Macneil, Jeff Lonnee, Rachel Wood
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Following the early successes of subsurface platform and pinnacle reef exploration in central Alberta, the Upper Devonian Leduc Formation of eastern Alberta has over the last two decades seen sporadic exploration and production, as well as limited research. In the heavy oil belt south of Lloydminster, the uppermost several meters of the Leduc are oil bearing within erosional karst remnants formed by sub-Cretaceous erosion. The highs have been rendered producible by horizontal drilling and the presence of reservoir quality dolomite. The Leduc Formation in east-central Alberta is composed of an impressive 200 m platformal accumulation that typifies the rapid carbonate growth during the Frasnian Stage. From bottom to top, the Leduc becomes increasingly more restricted, as indicated partly by the decrease in skeletal carbonate and increase in restricted peritidal facies. The lower Leduc contains a series of prograding stromatoporoid reefal and interreefal facies, which grade upward into back-reef facies, then finally into restricted lagoonal and peritidal facies of the upper Leduc. Leduc members have been thoroughly correlated in the area, and the youngest two from which oil is produced were sampled across the region for petrographic and geochemical analyses. Contrary to most other models for dolomitization of the inner Leduc platform of the Western Canada Sedimentary Basin, we interpret pervasive dolomitization to have replaced the original mineralogy of the upper Leduc early, during shallow burial, as a result of intraformational brine generation and reflux within peritidal facies. The upper Leduc dolomites recrystallized with burial to produce nonferroan dolomites, with an average δ18O value of −4.1‰ Vienna Pee Dee Belemnite (VPDB), and an average δ13C value of +1.0‰ VPDB. The dolomites are nearly stoichiometric, with cloudy cores and in some samples, clear rims. After initial burial, laterally extensive pre-Cretaceous erosion created an approximate 260 million year gap between Upper Devonian carbonates and upper Lower Cretaceous siliciclastics. The proximity of the sub-Cretaceous Unconformity to the upper Leduc dolomites is interpreted to have led to local dedolomitization in some lithologies, yielding low-magnesium calcites with very negative, meteoric δ18O compositions. The erosion of the overlying seal exposed areas of the upper Leduc to replacement calcite associated with dedolomitization, and an influx of Cretaceous clays, both of which are a detriment to reservoir quality. Proximity of the sub-Cretaceous Unconformity to the Leduc reservoir could increase the likelihood that these processes took place. Resolving the position of the unconformity relative to the upper Leduc reservoir is thus a critical tool in predicting reservoir quality.