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Applied Ichnology in Modeling Mississippian Reservoirs, Upper Midale Beds, Weyburn Oilfield, Saskatchewan: How to Use Paleobiology in Predictions on Dolomitization, Characterization and Compartmentalization?


Traditionally, applications of ichnology utilized the ichnofacies concept as a basic framework for studies on sedimentology and genetic stratigraphy to build paleonvironmental models useful in petroleum geology. At Weyburn Oilfield, Mississippian reservoirs in the Upper Middle Beds consist of extensively burrowed dolomudstones, characterized by a distal-Cruziana ichnofacies, indicating deposition occurred in a deeper-water, outer ramp setting. Yet, widespread bioturbation and dolomitization suggest significant paleobiological controls on diagenesis, including development and characteristics of the reservoir; and, origins of baffles and barriers defined by local textural heterogeneities. An important question arises: how to build on the strengths of ichnofacies modelling to advance economic applications in reservoir geology? To construct biogenic-diagenetic reservoir models, bioturbate texture mapping is proposed using both upscale ichnofacies concept, and its downscale thin-section petrographic data in an integrated approach for understanding paleobiological controls on origins of dolomite fabrics.

This study on reservoir geology clearly shows that building on the advantages of the ichnofacies concept requires consistency in applying its genetic significance to analyses of bioturbate textures at petrographic scale. Such downscaling in approach is facilitated by classifications of bioturbate textures and associated diagenetic signatures on the basis of genetic criteria. This approach yields an independently-derived paleobiological data set that is consistent with biotic processes relevant to the ichnofacies. No one-to-one ichnogeneric links are suggested between genetic classes in thin-section petrography and trace fossils. Genetic relationships in textural characteristics at petrographic scale facilitate interpretations of intrinsic paleobiological controls on diagenesis; and, variations in intercrystalline porosity-permeability linked to dolomite fabrics associated with biogenic structures. Hence, textural classification schemes provide paleoecological tools for understanding origins and variations in reservoir quality within paleobiologically-influenced dolomites; and, facilitate predictions on flow dynamics in modified substrates at ichnofacies scale. Applications of these tools in mapping bioturbate textural heterogeneities in an integrated genetic framework provide insights on local variations in diagenetic fabrics that characterize dolomite reservoirs, and define their internal architecture, where productive zones and barriers highlight differences in fluid-pressure compartments.