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Characterizing Organic Rich Mudstone Facies of the Upper Cretaceous Second White Specks Petroleum System, Implications for Reservoir Fairway Distribution Across West-Central Alberta, Canada

Abstract

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. This study reveals complex stratal architectures comprised of 14 parasequences, each with mappable facies assemblages that characterize these heterolithic mudstone deposits. Sequence stratigraphic and sedimentological facies relationships highlight a series of stacked and intercalated mudstone units comprised of organic, siliciclastic and calcareous sediments largely from locally sourced organic and pelagic fall-out as well as detrital sediments from western and eastern localities. Lateral distribution of sedimentary facies units therefore defines potential reservoir fairways, the foundation for subsequent play fairway characterization. Regional cross sections using 296 well logs across an area, T35–45, R3W5–9W5, established a sequence stratigraphic framework to evaluate parasequence stacking patterns. 10 cores and 27 petrographic samples help define facies assemblages within the succession. Identifying lateral and vertical variations in sedimentary facies is illustrated by facies isopach maps, complemented by facies distribution maps. These reflect petrophysical facies distributions that correlate to facies described in core and thin section samples. Depositionally, sedimentary bedforms (current ripples, graded beds, bioclastic debris layers) documents a depositional setting immediately below or above storm wave base, with deposition from a wide range of traction currents. This suggests deposition in a relatively high energy, shallow water setting, supporting the interpretation of stacked shelf deposits comprising the Second White Specks petroleum system. In addition, analysis of pore size distribution data across the succession provides insights into pore throat morphologies that can be tied to facies distributions. Pore sizes occur as micro-, meso- and macropores (vol %), typically linked to interparticle and grain dissolution, providing insight into storage and flow characteristics for each facies. Collectively, this helps to identify various light oil fairways to be exploited by multistage hydraulically fractured horizontal wells.