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Characterization of Complex Facies and Stratal Architecture of Organic Rich Mudstones of the Upper Cretaceous Second White Specks Petroleum System, West-central Alberta, Canada


The Upper Cretaceous Second White Specks petroleum system is actively being explored as an emerging shale oil resource play across western Alberta. Historically, several highly productive vertical oil wells (+1 million barrels) testify the prolific character of the Second White Specks petroleum system, although often dismissed as an unpredictable fracture controlled play based on poor production in offsetting wells. The petroleum system is comprised of a 500-1200m thick succession with several organic rich mudstones, separated by more siliciclastic mudstones. These units are separated into the siliceous, organic rich Fish Scales Formation, the non-calcareous, siliciclastic rich Belle Fourche Formation and the calcareous, organic rich Second White Specks Formation. Characterizing and mapping the reservoir properties of the various facies aid in identifying the various light oil fairways to be exploited by multistage hydrological fractured horizontal wells. Abundant wells and cores in the study area help establish a sequence stratigraphic framework on which regional cross-sections reveal a complex stratal architecture within the mudstone dominated strata of the Second White Specks petroleum system. The resulting lateral and vertical variations in sedimentary facies and depositional environments is illustrated by para-sequence isopach maps, complemented by facies distribution and facies isopach maps. Understanding how facies are spatially distributed proves essential when considering reservoir fairway trends throughout the study area. Sedimentary bedforms (current and wave ripples, graded beds, hummocky cross stratification) documents a depositional setting predominantly just below or above storm wave base, with deposition from a wide range of traction currents. Collectively, evidence from a sequence stratigraphic and sedimentological point of view suggests significant heterogeneity previously unappreciated within the Second White Specks petroleum system. Detailed petrographic analysis reveals unique pore throat systems that characterize each sedimentary facies. Pore throats exist as a result of differing fracture intensity, grain dissolution and interparticle porosity, providing insight into storage and flow characteristics for each facies. The importance of mapping facies distributions that correspond to unique reservoir properties is critical to establishing a working reservoir model for the Second White Specks petroleum system.