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Controls on the Three-Dimensional Depositional Architecture of Tidally Influenced Fluvial Successions


Tidally influenced fluvial deposits are increasingly recognized as important hydrocarbon reservoirs, yet their sedimentology, stratigraphic heterogeneity and architectural complexity make reservoir performance and prediction problematic. To address this issue, a series of detailed quantitative depositional models have been developed that account for sediment dispersal from channelized depositional elements to neighboring floodplain and interdistributary bay fill sub-environments. Morphological data from modern and exhumed systems have been used to build a database from which styles of behavior can be related to a range of controls, including rate of accommodation generation, degree of tidal influence, relative rate and style of sediment fluvial sediment input and nature of backwater effect. Satellite imagery of tidally influenced systems have been used to document the dimensions of lacustrine, channel and splay sub-environments under different climatic conditions and varying tidal influence. Data collected from these studies have been used to document the plan-form architectures present and to constrain likely dimensions of equivalent elements preserved in outcropping successions. Outcrop work has included detailed study of the Upper Cretaceous Neslen Formation, between Floy and Crescent canyons, Utah, which provides excellent exposures. This enables detailed three-dimensional sand-body geometries and relationships to be established together with spatial and temporal changes in sand-body architecture. Architectural-element analysis at various scales has enabled the development of a time-series of palaeogeographical maps and reconstruction of channel-belt orientations, from which changes in process regime have been reconstructed and depositional sub-environments mapped. Traditionally, the observed style of stacking for these deposits has been attributed to changes in the generation of accommodation space, however, the role of autogenic progradation of distributive fluvial systems have become increasingly recognized and could produce similar stacking styles. Tidally influenced systems are complicated and depositional models developed from this study can be used to quantify sedimentary structures and architectures produced at various positions in the fluvial-marine transition zone. Quantitative approaches to depositional modelling are an important step in the development of more reliable models for prediction of reservoir performance.