Detailed Core Facies Analysis and Well Log Response in the Mesaverde Group, Eastern Greater Natural Buttes Field, Uinta Basin, Utah

This study analyzes and presents a definitive definition of lithofacies, facies associations and their well-log responses for the Neslen, Farrer, and Tuscher Formations (Mesaverde Group) in the southeastern portion of Greater Natural Buttes Field, Utah. Stratigraphic and sedimentological analyses were conducted on 1645 ft of core from 6 wells. Cored intervals include 352 ft from the lower Neslen, 293 ft from the upper Neslen, 560 ft from the Farrer, and 439 ft from the Tuscher. The depositional environment and sequence stratigraphic evolution define the subsurface stratigraphy in the study area, and may be used to further subdivide formations and aid in future reservoir modeling. Eight mudstone-dominated and twelve sandstone-dominated lithofacies were identified and grouped into six facies associations consisting of three channel, two floodplain and one estuarine association. Channel associations were subdivided based on lithofacies assemblage and interpretation of relative stream power. They consist of high-energy, low-energy, and heterolithic (mixed energy) channel deposits. High-energy channel deposits are characterized by cross-bedded sandstone, have an average core thickness of 15.5 ft, and have the greatest reservoir potential. Low-energy channel deposits are characterized by ripple cross-laminated sandstone and have an average core thickness of 6.1 ft. Heterolithic channel deposits are characterized by interbedded sandstone and mudstone facies and have an average core thickness of 16.0 ft. Hypothesis testing of average thickness and gamma-ray response for channel associations indicates that they may be distinguished from each other in the absence of core. Floodplain associations were subdivided based on interpreted water-table conditions and include well- and poorly-drained floodplain deposits. Well-drained floodplain deposits are characterized by fissile and crumbly mudstones, as well as abundant rooting. Poorly-drained floodplain deposits are characterized by darker color, carbonaceous mudstone, and abundance of carbonaceous debris. Estuarine deposits are characterized by lenticular mudstone, bioturbated muddy sandstone and wavy bedded sandstone as well as Planolites, Thalassinoides, and Ophiomorpha trace fossils. Thin fluvial channels, floodplain, and tidal-flat deposits in the lower Neslen interval represent an estuarine complex that grades into a lower coastal plain environment. The upper Neslen interval represents of a coal-bearing upper coastal plain environment with meandering channels crossing poorlydrained floodplains. Thicker channel deposits in the Farrer represent a shift from coastal plain to alluvial plain. The thick channel deposits in the Tuscher Formation are also interpreted to represent deposition on an alluvial plain, but with greater accommodation space than the underlying Farrer as evidenced by a greater preservation of floodplain deposits. The Mesaverde Group in the Greater Natural Buttes area records transition of coastal plain to alluvial plain environments during the 3rd order high-stand systems tract of the late Campanian sequence. The lower and upper Neslen represent the high-stand systems tract of a 4th order sequence. The overlying amalgamated alluvial plain channel deposits of the Farrer represent the late high-stand to low-stand systems tract. Floodplain deposits with brackish-water influences separating the Farrer from the overlying Tuscher and signifies a new 4th order cycle. The Tuscher is a new transgressive to highstand systems tract formed on an alluvial plain