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Abstract

The Upper Ferron Sandstone in southern Castle Valley, Utah, is a river-dominated deltaic complex made up of seaward-stepping, vertically stacked, and landward-stepping cycles. These cycles, which consist of delta plain, delta front, and prodelta/offshore facies associations, are partitioned by flooding surfaces that detailed biostratigraphic analysis indicates lack an open marine signature. They are thought to be abandonment flooding surfaces associated with delta lobe switching. The abundance of distributary channel belts associated with all cycles suggests that riverine processes controlled the evolution of their associated delta fronts and that the entire Upper Ferron is a river-dominated system in which marine processes (predominantly waves and storms) played a subordinate role. Autocyclic processes, channel avulsion and lobe switching, controlled the internal architecture and partitioning of the cycles. The stacking pattern was controlled by allocyclic processes, primarily decreasing sediment supply combined with increasing accommodation. No compelling evidence was found to confirm the presence of incised valleys in the Upper Ferron.

Distributary channel-belt sandstones and delta-front sandstones are the principal reservoir facies in the Upper Ferron deltaic complex. Channel-belt sandstones have the best reservoir quality, with an average porosity of 15.6% and average permeability of 116 md. By comparison, delta-front sandstones have an average porosity of 11.9% and an average permeability of 47 md. Rock physics and seismic modeling results indicate that the seismic response of the Upper Ferron is dominated by large acoustic impedance contrasts associated with coals and carbonaceous shales even though they are generally thin and well below typical tuning thicknesses. Distributary channel belts show up as amplitude dimouts of the strong response generated by associated coals and carbonaceous shales. They appear to be the only reservoir facies that can be imaged directly.

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