The interplay between cm- and m-scale geological and geomechanical heterogeneity in organic-rich mudstones: Implications for reservoir characterization of unconventional shale plays

In this study, the vertical and lateral geological and geomechanical heterogeneity of the Duvernay shale play of Western-Central Alberta (Canada) is investigated at two different scales (meter and centimeter) using an outcrop-based approach. Twenty-five hand samples from the Duvernay outcrop-equivalent Perdrix Formation were collected along one vertical stratigraphic section and analyzed for elemental composition and mechanical hardness. Data points were acquired at every node of a 1cm x 1cm grid built on the cut face of each sample. Results show that significant compositional and mechanical heterogeneity is present both when comparing different rock samples and when considering all the measurements taken within the same rock sample. Our study shows that intra-sample variation in chemical composition may exceed 20% on a relative basis, and intra-sample variation in mechanical hardness may exceed 50% on a relative basis. Multi-variate statistical models show that a positive relationship is observed between mechanical hardness and XRF Calcium content (carbonate proxy), while a negative relationship is observed between mechanical hardness and XRF Aluminum content (clay proxy). Our analysis shows that heterogeneity in the Perdrix Formation is facies-dependent: mudstones that are relatively lean in Calcium but rich in Silicon and Aluminum have up to twice the compositional and mechanical heterogeneity of facies that are relatively rich in Calcium and poor in in Silicon and Aluminum. Based on the analyzed data and published literature, we support the hypothesis that by decreasing the scale of investigation we increase the magnitude of heterogeneity that can be resolved when measuring organic-rich mudstone composition and mechanical hardness. This understanding can be applied to improve 3-D subsurface models, where upscaling often oversimplifies the reservoir and is unable to adequately resolve reservoir heterogeneity and its influence on reservoir potential and behaviour when undergoing hydraulic fracturing treatment.