Quantifying Reservoir Heterogeneity Through Outcrop Characterization: 1. Architecture, Lithology, and Permeability Distribution of a Seaward-Stepping Fluvial-Deltaic Sequence, Ferron Sandstone (Cretaceous), Central Utah

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The internal architecture of natural gas reservoirs fundamentally controls production efficiency and the volume of gas unrecovered at abandonment. To better understand reservoir complexity, we investigated relations between sandstone architecture and permeability structure of landward-stepping (wave-modified) Ferron deltaic sandstones exposed in central Utah. Deltaic sandstones extend 4 mi along sediment-transport direction in the landward-stepping Ferron genetic sequence GS 5. Most sand was deposited in transgressive, delta-front, and distributary-channel facies. Distributary channels constitute the principal reservoir facies because mean permeability in distributary-channel sandstones is approximately twice that of delta-front and transgressive sandstones, and because distributary-channel sandstones are well developed. Channel architecture, bounding-surface character, and permeability distribution change systematically from landward to seaward position in the system. Near the landward limit, mean permeability is 300 md, mud occurs as clasts along channel-flank bounding surfaces, and permeability systematically decreases upward. Near the seaward extent of the system, mean permeability is 750 md, mud is segregated into discrete strata-bounding sand bodies, and vertical permeability trends are uniformly high. Statistical analysis shows that lithofacies are the fundamental sandstone architectural units. Similar lithofacies have similar permeability character, regardless of position in the facies tract. Variable preservation of lithofacies controls permeability distribution throughout the channel system. Semivariogram analysis shows that vertical and horizontal permeability correlation distances correspond to distances between bounding surfaces and to sand-body dimensions. Diagenetic overprint is minor, owing to low burial temperatures.


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