Vertical and lateral facies variability in organic-rich mudstones at the reservoir scale: A case study from the Devonian Duvernay formation of Alberta, Canada

Abstract

Organic-rich mudstone successions are known to have significant vertical and lateral heterogeneity. This has been well documented in literature at the core scale and the basin scale in many unconventional reservoirs around the world. Investigating geological heterogeneity at the km-to sub-km scale is not always possible because of the dense well coverage necessary for such assessment. Therefore, facies heterogeneity assessment of organic-rich mudstones at the km-scale of investigation is poorly accounted for in literature. Using a case study from the Devonian Duvernay Formation of Western Canada, this study takes advantage of the large amount of well data available in the public domain to perform a heterogeneity assessment of organic-rich mudstone successions at the km-resolution. The five key petrofacies identified through core-to-well log tie – a) organic rich black siliceous mudstones, b) carbonate-rich mudstones, wackestones, and packstones, c) organic-poor clay-rich mudstones, d) organic-rich clay-rich mudstones, and e) interbedded silica-rich and carbonate-rich – were displayed in cross-sections and mapped across the study area for assessment of vertical and lateral heterogeneity. Analysis reveals that the Duvernay Formation has a significant degree of vertical and lateral heterogeneity, which varies depending on the location with respect to the main sediment sources active at the time of Duvernay deposition. These include debris flow channels originating from carbonate reefs, carbonate platforms in the NE that prograded westward towards the study area, and ocean currents that transported detrital clays into the area. In the upper Duvernay Member, parasequences of shallowing upward petrofacies can be correlated across the study area. However, these are locally interrupted by carbonate- or clay-rich petrofacies that comprise a significant part of the succession decreasing the thickness of the organic-rich petrofacies. Our analysis confirms that the Duvernay Formation shows significant lateral changes in occurrence and thickness of petrofacies at the sub-km scale, making it an extremely heterogeneous reservoir. This understanding may be applied to other shale plays worldwide which lack the well control necessary to perform a similar assessment.