Permeability structure of a compound valley fill in the Cretaceous Fall River Formation of South Dakota

University of Texas at Austin, Bureau of Economic Geology, Austin, TX, United States

A cross section through a compound valley fill sandstone in the Lower Cretaceous Fall River Formation is exposed in Red Canyon, on the southwest flank of the Black Hills in South Dakota. A mapping of changes in stratal architecture, facies, and permeability along this cross section records rock heterogeneities that can influence subsurface fluid flow 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. The sandstone becomes less permeable, on average, both upward and laterally outward from its axis to its margin. The sandstone contains four discrete valley fills; each fill is floored by a major unconformity surface. The second fill has a permeability mean and variance substantially greater than that of the first fill, and it shows the most pronounced large-scale vertical and lateral permeability trends. The third and fourth fills are progressively narrower, finer grained, more heterolithic, and, on average, less permeable. This retrogradational pattern juxtaposes poor reservoir quality valley fills on older, higher quality, valley fill sandstones. The permeability structure within individual valley fills reflects the geometry and stacking of internal sediment bodies and the distribution of lithofacies within these bodies. Deposits lower in valley fills are more permeable and typically lack lateral permeability trends, whereas deposits higher in fills are less permeable and exhibit more pronounced lateral permeability trends over several tens to hundreds of meters. Permeability variations along internal sediment bodies are less pronounced than those across valley fills. This study demonstrates the importance of defining rock property trends in a sedimentologic framework to characterize complex valley fill reservoir sandstones.

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