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Sedimentological and Stratigraphic Controls on Reservoir Architecture and Connectivity in a Variable Fluvial System: Mesaverde Group, Greater Natural Buttes Field, Uinta Basin, Utah

Ellen Wilcox, Edmund R. Gustason III, and Matthew J. Pranter
1110 Monroe Drive, Unit B, Boulder, CO

The Mesaverde Group of the Greater Natural Buttes (GNB) Field in the Uinta Basin, Utah represents a succession of single-story channel, multistory channel, and crevasse splay deposits that form tight-gas sandstone deposits. Reservoir models of these deposits explore how sandstone-body connectivity varies by environment of deposition and stratigraphically. The Mesaverde Group, a 2000 – 2500 ft (610 – 762 m) thick system, is the main producing unit within the GNB field, located along the eastern flank of the Uinta Basin, Utah. Production from the Mesaverde Group is primarily within the lenticular and highly discontinuous stacked fluvial sandstones. Research has subdivided the Mesaverde Group into four intervals of distinct depositional environments, including estuarine-salt marsh, well-developed floodplain, mixed load braided and meandering stream environments. The ~23 mi study area is located along the eastern edge of the GNB field where the changing depositional environment preserves sandstone bodies with unique characteristics in the Mesaverde Group. To assess the stratigraphy and reservoir characteristics, data from 1650 ft (503 m) of core from 6 wells and well logs from 389 wells on a variable 10-ac [4-hectare] to 40-ac [16-hectare] spacing are used in conjunction with outcrop sandstone-body dimensional data from the surrounding area. Core descriptions covering all four intervals of the Mesaverde Group were used to determine key facies (fourteen), facies associations, and architectural elements (ten). A core-to-log comparison was used to develop criteria to calculate lithology logs from conventional well logs. The core interpretations and calculated lithology logs were compared to existing outcrop data and observations to evaluate stratigraphic variability within the area. Three-dimensional models of the Mesaverde Group provide estimates of the spatial variability for sandstone deposits within different depositional environments. The significance of what constitutes a reservoir-quality sandstone and its impact on static connectivity are investigated using various scenarios. One scenario models only single-story and multistory channel deposits as reservoir-quality sandstones while a second scenario models all sandstone deposits (channel deposits, crevasse splays, and sand bars) as reservoir-quality sandstones. The different scenarios explore the effect on static connectivity and potential impact on reservoir performance.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013