--> Numerical Model for Prediction of Internal Stratigraphic Architecture and Heterogeneity in Tidally Influenced Fluvial Point-Bar Deposits

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Numerical Model for Prediction of Internal Stratigraphic Architecture and Heterogeneity in Tidally Influenced Fluvial Point-Bar Deposits

Abstract

Stratigraphic successions of point-bar elements that accumulate in tidally influenced fluvial settings are typically characterized by vertical and lateral heterogeneity whereby sand-prone packages are draped and partitioned by mud-prone deposits of variable thickness and continuity in response to fluvial and tidal processes that vary spatio-temporally. Although the extent to which mud-prone deposits act to partition reservoirs is difficult to determine from subsurface data, quantification is important to predict reservoir behavior. This study has developed a numerical stratigraphic model that uses a mixed process- and geometric-based approach to predict the 3D distribution of sand- and mud-prone deposits. The model is able to reproduce changes in morphology and architecture of channels and associated point-bar elements (e.g., due to bar expansion, translation and rotation) based on real-world data. Episodes of sand movement occur during times of elevated current; episodes of mud deposition occur as currents wane to zero, thereby enabling suspension-settling and mud-draping of inclined bar fronts. The model accounts for changes in current magnitude and direction arising from both short-term tidal effects (e.g., diurnal flood and ebb currents, spring-neap and annual cycles) and longer-term changes in fluvial discharge (e.g., seasonal and longer-term flood events). The distribution of facies around meander bends varies according to bend tightness and distance from bend apex; this allows for the effective modeling of features such as mud-prone counter point bars. The model also accounts for temporal changes in facies distribution in response to neck and chute cut-off, oxbow lake development, and nodal avulsion that induces abandonment of entire reaches. The model uses a series of look-up tables that reference real-world modern and ancient examples contained within an architectural database to determine the trajectories of different types of meanders and the distribution of different lithofacies. Additionally, the model uses stochastic approaches to depict inherent natural variability in architectural-element size, shape, orientation, distribution and migration trajectory. The model has been employed to demonstrate facies distributions and heterogeneity in both modern tidally influenced fluvial systems (e.g., Gironde) and reservoir successions, including the McMurray Formation, Alberta, Canada and the Mungaroo Formation, NW Shelf, Australia.