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Abstract

Exposures of the Ferron Sandstone Member of the Cretaceous Mancos Shale formation, east-central Utah provide large-scale cross-sectional views of valley-fill sandstone bodies within a well-constrained sequence stratigraphic framework. Quantitative data collected from these outcrops help constrain the modeling of interwell volumes of analogous valley-fill reservoirs and permit a better evaluation of reservoir potential. The study has implications for how heterogeneities should be modeled in analogous reservoirs. Differences in net-to-gross, connectivity, and petrophysical property structure of valley-fill deposits can be related to stratigraphic stacking pattern (progradational versus aggradational) and position along depositional profile (proximal versus distal).

Valley-fill deposits display a general decrease in net-to-gross and sand body connectivity when stacking patterns of delta-front sandstone bodies change from strongly progradational to aggradational or from proximal to distal positions within a valley fill. Valley-fill deposits associated with strongly progradational delta-front sandstone bodies are preserved as relatively homogenous, sand-rich bodies that have a narrow shoestring-like geometry. Sandstone beds are highly amalgamated and mudstone interbeds are generally rare. The principal heterogeneity within these deposits are caused by relatively low-permeability sandstones containing abundant clay clasts that occur along the base and margins of individual channel-form bodies. In contrast, valley fills associated with aggradational delta-front units are preserved as relatively heterogeneous, mud-rich bodies that have a broad ribbon like geometry. Internal heterogeneities result from mudstone drapes that line the base and margin of individual channel-form bodies and from the interbedding of sandstone dominated channel-form bodies with mud-stone-dominated channel fills. In going from a proximal to distal position within a valley-fill deposit the frequency and continuity of mudstone drapes and frequency of mudstone-dominated channel fills increases. Vertical and horizontal variograms indicate permeability is correlated over a distance of about 4-8 m (13-26 ft) vertically and approximately 40-60 m (130-200 ft) laterally. The vertical and lateral correlation ranges are similar to the average channel-form width and thickness.

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