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Characterization of Duvernay Formation Mudrocks Using Detailed Sedimentological Analysis


The Upper Devonian Duvernay Formation of the Western Canadian Sedimentary Basin is well known as a prolific source rock, but has recently seen growing interest in its potential as an unconventional reservoir. Although knowledge of the character and distribution of quality reservoir facies is in its infancy, early production has exploited thick packages of organic-rich, calcareous, biosiliceous mudstones in reef-distal areas of the basin. Internal sedimentological heterogeneities are tied to reservoir quality and reflect changes in the nature of basinal sedimentation. Identification and characterization of internal heterogeneities allow for the creation of depositional and sequence stratigraphic models, upon which exploration and production strategies may be based. Detailed analysis of four cores, combined with petrographic evaluation using extra-thin thin sections, reveals heterogeneities within homogenous-looking dark grey-brown mudstones. Facies were characterized based on mineralogy, sedimentary structures and fabric on a macro- and micro-scale, microfossil content, cement type and abundance, as well the abundance and character of natural fractures. Correlation of identified facies to wireline logs allows for the extrapolation of interpretation beyond core control. A basin-scale wireline correlation describes the distribution of reservoir and non-reservoir strata and allows for upscaling of core analyses to form a depositional and sequence stratigraphic model. The most basinal deposition is represented by two facies: pyrite-rich, streaky argillaceous mudstone; and faintly laminated to massive argillaceous calcareous mudstone. Increasingly shallower water deposition is represented by parallel-laminated to banded silty calcareous mudstones and parallel- to wavy-laminated silty calcareous mudstone. The shallowest and most oxygenated facies are fully bioturbated calcareous argillaceous mudstones (in the East Shale Basin), reef-derived limestone breccias (in reef-proximal locations), and bioturbated nodular calcareous argillaceous mudstones. Basinal-restricted deposits become more prevalent during transgression. Correlation of facies between cores shows thinning of basinal facies towards basin margins, and a growing predominance of shallower water carbonate-rich facies. Lowstand limestone breccia packages are confined to reef-proximal locations but are equivalent to nodular calcareous mudstones away from buildups.