Depositional setting and reservoir-scale architecture of sandstone bodies of the Green River Formation in Evacuation Creek, Dragon quadrangle, eastern Uinta Basin, Utah

Abstract

The Green River Formation is an Eocene lacustrine deposit that is present in several Rocky Mountain basins. In the Uinta basin, the Green River Formation has produced large amounts of oil and gas from many fields, the largest being the Greater Altamont-Bluebell field in the northern margin of the basin, the Monument Butte and Natural Buttes fields in the central region, and the Greater Red Wash field in the northeastern part of the basin. In addition, the Green River Formation contains one of the largest oil shale deposits in the world. The Uinta and Piceance basins are estimated to contain 1.32 trillion barrels and 1.53 trillion barrels respectively of total in-place oil shale resource. This study focuses on littoral to sublittoral sandstone deposition of the Green River Formation in the eastern Uinta basin of Utah, in Evacuation Creek, near the Utah-Colorado border. This area contains extensive and continuous outcrop of the Green River Formation exposed in steep cliffs and gullies, and allows for the study of these units at the reservoir-scale. Litho-stratigraphically, the area contains the transition of the marginal lacustrine Douglas Creek Member to the open lacustrine Parachute Creek Member. The outcrops here contain very fine to medium-grained sandstone, siltstone, organic rich mudstone, carbonate grainstone and microbialite that reflect changes in depositional conditions in Ancestral Lake Uinta. This area was subject to frequent changes in lake level due to changes in climate and tectonics. This study aims to 1) interpret the depositional setting of the sedimentary package over the stratigraphic interval, and 2) describe the depositional architecture of these sand bodies in multiple dimensions at the reservoir-scale. The data collected includes measured sections along the 2 mile long outcrop in the valley cliffs and gullies. Measured sections cover two laterally continuous sand intervals over the outcrop area, which define the study unit. Photo panoramas were taken to outline important contacts and lithobody geometries, and to interpolate facies distribution between measured sections. Samples have been taken from representative facies for thin section analysis. The study interval can be divided into two units bounded by sequence boundaries at the sharp and non-erosive contacts between oil shale and the microbialite facies association and distal delta front facies association. Sequence boundary contacts are described as erosive and incisive in the hinterland in other studies. These boundaries can be traced along the entire outcrop and have been traced into the basin in core and well logs. Five facies associations are interpreted: 1) sharp-based mouth bars, 2) aggradational mouth bars, 3) distributary channel, 4) distal delta front, and 5) oil shale and microbialites. Facies associations 1-4 are comprised of sand to silt-dominated facies, and are the direct result of clastic inputs into the lake from rivers. Facies association show evidence for varying wave influence during sedimentation as seen in ripple forms that occur in the outcrop, but there is little evidence for significant longshore drift processes or developed shoreface successions. The depositional processes of the sandstone intervals in the study area are river dominant with wave influence in the sharp-based mouth bars, aggradational mouth bars, and distal delta front. Distributary channel are proximal to the mouth bar facies associations, and are comprised mainly of sharp-based, non-channelized, trough cross-bedded sandstones, that are locally tar stained. The trough cross bedded sand body is interpreted as proximal, non-channelized portion of the distributary network of channels. Porosity from distributary channelfacies contains the highest porosity values and simplest porosity distribution due to the laterally continuous nature of the trough cross-bedded facies within the unit. Distributary channels are therefore a continuous, less compartmentalized reservoir analogue. Facies distribution in both sharp-based mouth bars and aggradational mouth bars is complex and leads to complex porosity distribution and reservoir compartmentalization. Facies distribution in sharp-based mouth bar facies and resulting porosity distribution contrasts with aggradational mouth bars. Sharp-based mouth bars are in direct lower contact with distal delta front,are laterally extensive in distribution, and contain a generally consistent grain size grain size profile. Sharp-based mouth bar contain a greater vertical net to gross sand reservoir facies compostion. Aggradational mouthbars are comprised of multiple coarsening upward units. As a result, in aggradational mouth bars, lateral facies relationships are important in addition to vertical relationships. Facies containing the coarsest sediment and highest porosity are vertically segregated by lower energy deposition and finer sediment. These bars have a lower net to gross reservoir facies composition. Distal delta front facies associations are comprised of low porosity facies. Sediment in distal delta fronts grade from fine and very fine sand facies to silt and mud facies. The distal delta front contains thin rippled sands, and structureless sands in the proximal setting, and laminated and homogeneous mudstone in the distal setting. The oil shale and microbial carbonate facies associations are comprised of oil shales of varying organic richness, microbial carbonates, and thin sandstone and silt beds. This facies association contrasts with sand dominated facies associations and occurswhen sediment input is reduced or when the shoreline transgresses during higher lake levels.