Architecture and Permeability Structure of a Composite Valley Fill in the Cretaceous Fall River Formation of South Dakota

The author of this research mapped strata, facies, and permeability trends through a compasite valley-fill standstone in the Fall River Formation, exposed in Red Canyon, South Dakota. Findings demonstrate the complexity of depositions formed in low-accommodation basin settings. The Fall River Formation is a 45-m-thick layer of fluvial-dominated valley fills and shore-zone strata deposited on the stable cratonic margin of the Cretaceous Western Interior Seaway. Fall River deposits in Red Canyon, located in the southwest corner of South Dakota, expose a cross section of a 3.5-km-wide valley-fill sandstone and laterally adjacent marine deposits. Mapping of changes in stratal architecture, facies, and permeability along this cross section records rock heterogeneities that can influence subsurface fluid flow and reservoir production behavior. The 3.5-km-wide, 30-m-thick compound valley-fill sandstone cuts into a succession of meters-thick, upward-coarsening, marine shore-zone strata and muddy alluvial-plain deposits. Each valley-fill basal erosion surface records an episode of valley incision and relative sea-level fall; the gradual progression from fluvial to more estuarine deposits upward within each fill records relative sea-level rise. All fills, dominantly channel deposits, are either capped by marine-flooding surfaces or truncated by overlying valley fills. The dominance of channel deposits, the gradual change to more estuarine facies in the upper parts of fills, and the location of flooding surfaces at valley-fill tops all suggest that sediment supply initially kept pace with relative sea-level rise and that valleys filled during late marine lowstand and transgression, not during subsequent highstands. Permeability is controlled by facies architecture and stratigraphy. Distinct permeability contrasts occur between (1) the marine deposits and the composite valley-fill sandstone, (2) individual valley fills that compose the composite sandstone, (3) different types of smaller scale sediment bodies within each valley fill, and (4) different lithofacies within individual sediment bodies. These contrasts generally decrease across progressively smaller scales of strata. The largest scale variations might be correlated in subsurface reservoirs, whereas smaller scale variations need to be modeled statistically. Recently proposed facies models have focused on variations in the relative strength of tide, wave, and river currents as controls on valley-fill deposits. Relative rates of sediment supply and basin accommodation change, however, and the shift in this ratio along the depositional profile during multiple-scale cycles of relative sea-level rise and fall are equally important controls on the style of valley-fill deposits. This study demonstrates the importance of defining rock-property trends in a sedimentologic framework when characterizing complex valley-fill reservoir sandstones.
Brian J. Willis

Willis, B. J., 1998, Architecture and Permeability Structure of a Composite Valley Fill in the Cretaceous Fall River Formation of South Dakota: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations No. 251, 61 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation

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