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Three-Dimensional Stratigraphic Complexity Within Mixed Eolian-Fluvial Successions: Implications for Reservoir Connectivity


On-going exploration of conventional hydrocarbon plays is increasingly focused towards the development of geologically complex reservoirs for which stratigraphic heterogeneity is difficult to predict. Many such current reservoirs, and an increasing proportion of likely future ones, are characterized by sedimentary bodies that accumulated as mixed eolian-fluvial systems that competed and interacted synchronously. Well-known reservoir examples include the Permian Unayzah Formation of Saudi Arabia, the Permian Rotliegend Group of the North Sea, the Triassic Ormskirk Sandstone of the East Irish Sea, the Jurassic Norphlet Sandstone of the Gulf of Mexico, and the Cretaceous Agrio Formation, Argentina. These mixed depositional systems typically exhibit highly variable lateral and vertical facies configurations that preserve complex juxtapositions of architectural elements composed of stratal units with markedly variable reservoir properties. Such stratigraphic partitioning is intrinsically difficult to predict from limited subsurface data. As such, there exists a requirement for more sophisticated geological models to better account for reservoir architecture and connectivity. This work uses outcropping case-study examples of eolian-fluvial interactions from the Triassic Sherwood Sandstone Group of the UK and the Permo-Pennsylvanian Cutler Group of southeast Utah, USA, to develop a suite of predictive models that depict common styles of stratigraphic complexity within eolian-fluvial systems. Studied successions accumulated in response to a variety of system interactions, deposits of which are preserved at a range of spatial scales from 100–104 m: (i) short-lived and localized fluvial reworking of eolian dune deposits in response to flash flood events; (ii) eolian reworking of fluvial deposits via winnowing; (iii) the fluvial exploitation and possible damming of open interdune corridors; (iv) the flooding of isolated (spatially enclosed) interdune hollows in response to an elevated water table. Identified types of interactions are characterized within a spatial scheme whereby occurrences can be used as a predictor of relative position within the larger-scale zone of transition between coeval eolian dune-field and fluvial systems. Application of this spatial scheme allows for prediction of the type of eolian-fluvial interactions expected for a range of paleogeographic settings, thereby serving as a tool for ranking exploration targets within larger prospect areas.