ABSTRACT The Uinta Basin of eastern Utah is an intermontane basin that contains an ~2-km-thick succession of mostly carbonate-rich mudrock assigned to the Eocene Green River Formation. In the southwest part of the basin, along Nine Mile Canyon and its tributary canyons, the middle member of the Green River Formation contains numerous interbedded sand bodies. Previous researchers have interpreted these sand bodies variably as lacustrine deltaic mouth bars, terminal fluvial distributary bars, and various types of fluvial (delta plain/floodplain/braid plain) bar. Using some modern western U.S. lakes as partial analogues, and taking into account the overall lacustrine basin context of a widely fluctuating, wave-influenced, alkaline-lake shoreline, we again interpret many of the sand bodies to be fluvial in origin. Several sand bodies both truncate and are capped by brown to red-maroon and variegated weak to noncalcareous mudstone with root and desiccation structures, indicating terrestrial deposition well away from the lake shoreline. Others display steep cutbanks from which noncalcareous, inclined heterolithic stratification laterally accreted as fluvial side bars. Utilizing helicopter-based light detection and ranging (LiDAR) data, we investigated additional sand bodies that may be better examples of deltaic mouth bars. In contrast to the more commonly documented highstand progradational mouth bars of marine and open lake settings, these sand bodies are interpreted to have originated as late-lowstand or transgressive system tract fluvial channels that were then flooded and modified by waves following lake transgression. These examples illustrate that any large-scale sandy bed form present in the general vicinity of a closed basin’s fluctuating lake shore may be expected to have formed under more than one set of environmental conditions. A revised set of guidelines is therefore presented to aid in the interpretation of lacustrine deltaic mouth bars.

ABSTRACT Scanning electron microscopy (SEM) has revolutionized our understanding of shale petroleum systems through microstructural characterization of dispersed organic matter (OM). However, as a result of the low atomic weight of carbon, all OM appears black in SEM (BSE [backscattered electron] image) regardless of differences in thermal maturity or OM type (kerogen types or solid bitumen). Traditional petrographic identification of OM uses optical microscopy, where reflectance (%R o ), form, relief, and fluorescence can be used to discern OM types and thermal maturation stage. Unfortunately, most SEM studies of shale OM do not employ correlative optical techniques, leading to misidentifications or to the conclusion that all OM (i.e., kerogen and solid bitumen) is the same. To improve the accuracy of SEM identifications of dispersed OM in shale, correlative light and electron microscopy (CLEM) was used during this study to create optical and SEM images of OM in the same fields of view (500× magnification) under white light, blue light, secondary electron (SE), and BSE conditions. Samples ( n = 8) of varying thermal maturities and typical of the North American shale petroleum systems were used, including the Green River Mahogany Zone, Bakken Formation, Ohio Shale, Eagle Ford Formation, Barnett Formation, Haynesville Formation, and Woodford Shale. The CLEM image sets demonstrate the importance of correlative microscopy by showing how easily OM can be misidentified when viewed by SEM alone. Without CLEM techniques, petrographic data from SEM such as observations of organic nanoporosity may be misinterpreted, resulting in false or ambiguous results and impairing an improved understanding of organic diagenesis and catagenesis.

Abstract Green River Formation lacustrine deposits in the eastern portion of Lake Uinta formed in two sub-basins (the Piceance basin and the Uinta basin) and represent mixed siliciclastic-carbonate and organic-rich lake deposits deposited during the Eocene climate optimum. The formation is comprised of organic-rich and organic-poor mudstone, siliciclastics, and carbonates, formed in a shallow to deep (tens of meters), stratified lake environment. Integrated sequence stratigraphic analysis using gamma logs, Fisher Assay plots, core, and outcrop has resulted in a predictive framework for organic-rich oil shale distribution, reservoir characterization, and hydrocarbon systems analysis. Lacustrine strata are characterized by three types of (meter to decimeter) depositional cycles: (1) Type 1 cycles formed in a littoral/sublittoral zones and comprise progradational siliciclastic-rich deposits that pass upward into progradational to aggradational carbonate shoal and microbial carbonate and are capped by mud-to silt-sized sublittoral deposits. In the profundal zone, two types of depositional cycles occur: (2) Type 2 cycles start with lean oil shale, pass upwards into siliciclastic turbidites, and are overlain by rich oil shale deposits. (3) Type 3 cycles initiate with evaporites and mixed lean and rich oil shale that is overlain by rich oil shale. Stacked depositional cycles form depositional sequences meters to tens of meters thick. Eleven upward-deepening depositional sequences have been described and are divided into periods of low, rising, and high lake that are separated by sequence boundaries, transgressive surfaces, and main flooding surfaces, respectively. The development of depositional cycles and sequences in these lacustrine basins appear to be strongly affected by climate changes and respective inflow; i.e., during times of low inflow (low lake level) siliciclastic and nutrient input into the lake decreased. In contrast, the highest input of siliciclastics and nutrients occurred during increased and high inflow (rising and high lake level). Low lake level is marked by thin marginal deposits and lean oil shale and at times, evaporite deposition in profundal areas. Increased runoff is marked along basin margins by sharp-based sandstones and carbonates. In the profundal area, rich oil shale overlay lean oil shale.