Sandstone Deposition in the Eocene Green River Formation of Eastern Uinta Basin, Evacuation Creek: Depositional Environments and Reservoir-Scale Architecture
T. Ryan O'Hara, Kati Tänavsuu-Milkeviciene, and J. Frederick Sarg
Colorado School of Mines
Lacustrine basins across the globe contain large resources due to the occurrence of rich source rock and adjacent clastic reservoir facies. In the Uinta basin the Eocene Green River Formation is productive from several fields in the basin such as: Altamont-Bluebell, Red-Wash, and Monument Buttes. The reservoir characteristics of these fields are dissimilar and can be explained by the contrasting depositional settings and subsequent diagenetic histories. Outcrop studies in Evacuation Creek in eastern Uinta basin offer insight into a relatively understudied portion of the Uinta basin. Evacuation Creek is located near the Colorado-Utah border on the western flank of the north-south trending Douglas Creek Arch. This study is based on detailed description of two sandy siliciclastic units of the marginal-lacustrine Green River Formation, profiled along a 2.5 mile outcrop in the R6 zone of the Rising Lake Stage of Tänavsuu-Milkeviciene and Sarg (2012). Facies analysis is used to interpret the overall depositional setting. Deposition within the study interval ranges from alternating oil shale-microbialites during low sediment input phases to sandy clastic input during rising lake levels when sediment supply was greater and deltaic style deposition dominated. The sandstone intervals in this study area are complex. Described facies associations include mouth bars, delta front, distributary channels, and oil shale-microbialites. These units are mapped across the study interval with high resolution panoramic photography. Depositional environments contain evidence for high sedimentation rates, which include slump and slide features like plastic transportation of sandstone blocks out into the distal delta front resulting in 'Cinnamon Roll' features. Mouth bars are sharp-based and vary from a single phase of development that prograded rapidly to multiphase mouth bar deposition that contain wave and wave-modified current ripple lamination, and coarsening upward sequences. Single phase mouth bars prograde directly onto distal delta front and contain local slides of mouth bar blocks within the laterally extensive bar form. Delta front facies contain ripple facies in proximal settings, and planar to laminated facies from sediment plume settling in distal settings. Distributary channels contain predominantly trough cross-bedded facies in broad, sharp-based packages.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013