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Management of Depositional Environment Uncertainty Based on Facies Observations in Marginal Marine Systems: Allowing for Multiple Plausible Interpretation Scenarios


The typical workflow for depositional environment interpretation in the subsurface using core involves: (1) making observations of lithofacies, ichnology and other paleoenvironmental indicators, and (2) using facies models, in addition to user experience, to make interpretations. Depositional interpretation is important as it is often used to determine geobody dimensions, assess reservoir quality and predict the distributions of barriers and baffles in the subsurface. While this approach has stood the test of time, it can have important limitations: (1) the quality of interpretation can be affected by user experience; (2) a single interpretation, rather than a range of valid alternative interpretations, is the typical result; (3) the interpretation can be consciously or subconsciously biased by the background of the researcher. The full range of depositional uncertainty of a system is, therefore, often not considered, even when data availability is limited. Such gravitation towards a single solution can lead to a misinterpretation of a depositional environment, when an alternative scenario that has not been considered ends up being the correct solution. A modification to the workflow provides an effective methodology for managing the depositional uncertainty in marginal marine systems. The approach uses the element complex categories of the Wave Process and Architectural Classification. Observed sedimentary structures are linked to the range of all possible depositional processes that may have generated them (wave, tide, and fluvial). Some sedimentary structures provide a unique process relationship; wave ripples must be wave generated. However, most sedimentary structures can be related to more than one process; trough cross stratification can be fluvial, wave and tidal in origin. The relative proportions of individual sedimentary structures, and their possible process associations, can be used to establish a limited range of process combinations that characterize an interval. The range of process combinations can in turn be related to a finite number of element complexes (facies associations), which represent possible alternative depositional scenarios. Use of ichnology and facies models can then be used to further decrease the number of possible scenarios. The new workflow can be easily standardized, partially automated and integrated with software systems. We illustrate the uncertainty management aspects of the approach with examples.