Sequence Stratigraphic Reinterpretation of the Virgilian Indian Cave Sandstone as Incised Valley Fills, and Potential for Unexplored Updip Hydrocarbon Resources

Steven A. Fischbein^1,3, Christopher R. Fielding¹,  and R. M. Joeckel². (1) Department of Geosciences, 214 Bessey Hall, University of Nebraska,  Lincoln, NE 68588-0340, (2) Conservation and Survey Division, School of Natural Resources, Hardin Hall, University of Nebraska, Lincoln, NE 68583-0996, (3) Nebraska Department of Environmental Quality, 1200 N St, Suite 400, Lincoln, Nebraska  [email protected]

Keywords:  Incised-Valley-Fill, Fluvial, Estuarine, Sandstone, Glacioeustacy

Thick (<30 m), shoestring sandstone bodies within Upper Pennsylvanian cyclothems of midcontinent USA frequently lack clear stratigraphic context. Previously, these sandstones were interpreted as deltaic deposits, but several have recently been reinterpreted as incised valley fills (IVF). The Virgilian (Upper Pennsylvanian) Indian Cave Sandstone (ICS) of southeast Nebraska and environs is reinterpreted here as multiple, multi-storey IVFs deposited  during  two separate episodes. ICS lithosomes, linear bodies ~ 2,000 m wide and <30 m thick, have relatively steep sides and flat bases. Basal erosion surfaces of ICS bodies correspond to well-developed paleosols on adjacent interfluves and indicate sequence boundaries. The lower parts of ICS bodies comprise fining-upwards successions of trough-crossbedded sandstones interpreted as tidally influenced fluvial deposits. These sandstones are overlain by heterolithic units (with local coals) containing restricted trace fossil assemblages, interpreted as upper estuarine facies. Recognized sequences are correlated with published regional sequence stratigraphic frameworks, suggesting ICS bodies record fifth-order relative sea-level fluctuations of at least 30 m.

Outcrops along the Missouri River bluffs studied here are likely indicative of more regionally continuous ÒshoestringÓ sandstone bodies. Given the ICS is a reservoir target down-depositional-dip in Kansas, the sequence stratigraphic model presented herein is generally applicable to the exploration and exploitation of similar Pennsylvanian shoestring sandstone bodies in the midcontinent. This new ICS play concept in the ultra-low accommodation setting of the high-shelf platform may yield heterogeneous reservoirs up to 30 m thick with high net-gross and few extensive mudstone permeability barriers in the lower sandstone-dominated fluvial-to-estuarine association, and low net-gross, with significant three-dimensional permeability barriers in the upper estuarine association. Reservoir heterogeneity of ICS bodies was strongly influenced by their sea-level history.