Contrasting Styles of Fluvial-Eolian Interaction: Implications for Reservoir Characterization
Ephemeral fluvial systems that are characterized by a systematic downstream decrease in discharge, channel size, and lateral and vertical connectivity of channel-fill elements, are widespread in semi-arid climatic settings where they commonly interact in their distal parts with eolian sand sheets and dune fields. In low-relief sand sheet environments, unconfined, non-channelized flows are free to spread across broad alluvial plains, but are typically later at least partly reworked by eolian processes. In cases where the water table lies in the shallow subsurface, nodular and laminar paleosols readily develop and the resultant sheet-like architectures are typically heterogeneous, with subtle yet significant changes in sediment porosity and permeability character reflecting: i) the provenance, caliber and style of transport of the fluvial sediments, ii) the degree of subsequent eolian reworking and ‘cleaning’ via winnowing and iii) the extent and style of paleosol development. By contrast, where distal flood waters penetrate eolian dune fields with significant topographic relief, a variety of complex styles of interaction occur, including: i) flow diversion along linear interdune corridors, which encourages the refocusing of flood waters and their penetration deep into dune fields, ii) flow ponding within closed interdunes or along the leading edges of dune fields, iii) the fluvial cannibalization and reworking of eolian dune sands, and iv) the post-flood burial and reworking of former fluvial deposits via renewed eolian processes.
Understanding the likely range of preserved geometries that result from these styles of interaction is important for predicting net reservoir sandbody dimensions and for estimating degree of lateral and vertical connectivity in continental reservoir successions. Specifically, such studies highlight the potential danger of assuming layer-cake correlations between neighboring wells within stratigraphically complex reservoirs. Examples of interaction from the Pennsylvanian-Permian Cutler Group of the Paradox foreland basin of southern Utah and from the Permo-Triassic Sherwood Sandstone Group of the UK demonstrate how long-lived, large-scale ephemeral fluvial systems can episodically partition eolian dune field successions in a geometrically complex manner. Forward stratigraphic numerical modeling results demonstrate how such styles of interaction act to determine internal reservoir character.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009