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Subseismic Scale Reservoir Complexity in a Multilayered Tight Reservoir: Combining New and Established Well Log Characterization Techniques With Seismic Attribute Analysis Sheds New Light on Reservoir Heterogeneity


In SE Saskatchewan Canada (our study area), the Middle Bakken dolomitic siltstone interval is a prolific tight oil reservoir with ~20 million barrels of oil produced in 2016. Five vertically stacked and laterally continuous facies (2a, 2b, 4, 6, and 7) occur in the interval. Most multi-fractured horizontal wells (MFHws) are landed in facies 2b which has relatively higher porosity and permeability. In our study area, the cumulative production from MFHws with similar completions and hydraulic fracturing characteristics vary significantly, even when laterals are completed in one facies. The objective of this research was to improve the geological characterization of the Middle Bakken in the study area by leveraging well data and post-stack seismic attributes. We completed petrophysical log interpretation in 4 vertical wells (3 cored). Using the bulk density, compressional and shear sonic logs from the 4 wells, we computed dynamic elastic properties: Young’s modulus, Poisson's ratio, and uniaxial compressive stress. With a new well log characterization tool, we performed detailed well log trend analysis and clustering of 20 well logs in the 4 wells. Porosities and permeabilities obtained from log interpretation were cross-plotted against modified Winland correlation lines to highlight the dominant pore throat sizes per facies. Post-stack seismic volume attributes were generated, cross-plotted, and evaluated. Seismic to well comparison was performed using well templates of equivalent attributes that were generated using density and sonic velocity logs. From dynamic elastic properties, we observed that Facies 2b is comprised of 2 mechanically and petrophysically distinct sublayers. Both sublayers have different ranges of dominant pore throat sizes, corroborated by Middle Bakken rate of adsorption results. Both Facies 2b sublayers were also identified using acoustic impedance, RMS amplitude, and curvature crossplots. A time slice of the seismic ant tracking volume at the top of both Facies 2b sublayers shows a variation in the direction and size of lineament features, with longer and larger lineaments observed in the upper Facies 2b sublayer which contains larger dominant throat sizes. Well logs clustering provided new indications that Middle Bakken facies show vertical as well as lateral variations in reservoir properties that are corroborated by variations in seismic attribute crossplots. This study demonstrates a new method for combining well log interpretation and seismic attributes to highlight reservoir heterogeneity in thin multilayered tight reservoirs. Our findings extend the understanding of Middle Bakken geomechanical heterogeneity by demonstrating intra-facies geomechanical and reservoir variations. Indeed, the MFHws production variations could be influenced by the changes in the intra-facies reservoir and geomechanical characteristics observed in this study.