--> Application of core to refine thick fluvial successions: a case study of the Late Cretaceous Mesaverde Group, Uinta Basin

AAPG Rocky Mountain Section Meeting

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Application of core to refine thick fluvial successions: a case study of the Late Cretaceous Mesaverde Group, Uinta Basin

Abstract

On the east side of the Uinta Basin, gas production comes from the Late Cretaceous Mesaverde Group; which consists of the Neslen, Farrer, and Tuscher Formations. Historically, these sandstone reservoirs have been interpreted as fluvial channel systems. Within the Greater Natural Buttes Field, the Mesaverde Group is regularly undifferentiated due to a lack of significant stratigraphic markers, which has led to varied interpretations. Recently, other authors have proposed that subdivision based on stacking patterns and net-togross trends are continental expressions of sequence stratigraphic tracts identified in outcrop. This research investigates the hypothesis that architectural elements and associated well log response can provide a foundation for accurate subsurface correlations and can be tied to sequence stratigraphic interpretations. The first phase was to conduct a detailed sedimentological and stratigraphic assessment on available core to better define the Mesaverde Group in the subsurface. These results revealed varying channel types and multiple interpreted depositional environments. The second phase was a core-to-log analysis. Based on core, the well log response of architectural elements and the frequency of architectural elements within each formation were modeled for later interpretation in non-cored wells. The final phase was to merge interpretations from core and associated well log response into a sequence stratigraphic model. The architectural elements and depositional environments interpreted from core record a transition from coastal plain to alluvial plain environments and can be tied to sequence stratigraphic tracts modeled by other authors. Based on this study, the Mesaverde Group represents a 4th order sequence stratigraphic evolution from high-stand to low-stand and back to high-stand during a transition from coastal plain to alluvial plain setting. This interpretation is reflected in core, well log, net-to-gross sand ratios, and sand body stacking patterns. These results allow for a refined subdivision of the Mesaverde Group within the Greater Natural Buttes field and points to the importance of core analysis for interpreting depositional environments in the subsurface.