Bounding-Surface Hierarchies and Related Sources of Heterogeneity in Seemingly Uniform Fluvial Sandstone Sheets
Holbrook John, Francisca E. Oboh-Ikuenobe, 2002. "Bounding-Surface Hierarchies and Related Sources of Heterogeneity in Seemingly Uniform Fluvial Sandstone Sheets", Sequence Stratigraphic Models for Exploration and Production: Evolving Methodology, Emerging Models and Application Histories, John M. Armentrout, Norman C. Rosen
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Fluvial sandstone sheets may appear uniform initially, but detailed examination tends to reveal sharp lithologic contrasts across bounding surfaces. The mid-Cretaceous Muddy Sandstone of southeast Colorado, USA contains two sandstone sheets and provides examples of such potential permeability barriers. Sediment bodies bound and respective bounding surfaces are, in order of ascending rank: bar and dune deposits (1st- to 3rd-order), nested channel fills (4th-order), channel fills (5th-order), channel belts (6th-order), nested valley-fills (7th-order), valley-fills (8th-order), and sequences (9th-order). Bar and dune through channel-belt deposits commonly have permeability barriers because the episodes of waning flow common to their deposition tend to cause draping of surfaces and filling of scours locally with finer grained deposits. Channel-fills through sequences tend to have permeability barriers related to juxtaposition of permeable and non-permeable lithofacies during successive incision-and-fill events. Fluvial sandstone sheets can have reduced permeability in paleodip orientation that operates on the scale of individual wells because of draping of down-dip-oriented bar- and dune-accretion surfaces. A dip-oriented grain, reducing permeability in the strike direction, may arise on the scale of fields, because of permeability barriers formed at nested channel through higher-order valley and sequence boundaries.