| Description |
Sedimentary cyclicity is a common, if not ubiquitous, feature of lacustrine deposits and lacustrine hydrocarbon systems worldwide. The Green River Formation is one of the most heavily studied ancient lacustrine successions, with numerous investigations on cyclicity, especially as related to temporal orbital variations and lake level. However, a recently drilled core (486 m thick) through nearly the entire Eoceneaged Green River Formation from the eastern Uinta Basin provides a unique opportunity to expand characterization on the internal facies patterns of fine-grained lacustrine cycles and the external sedimentological variability between cycles by examining a complete stratigraphic record of cyclicity through a particularly mudstone-rich section. In addition, the large-scale stratigraphic architecture of cycles provides insight on depositional processes and sedimentological variability influenced by different paleogeographical lake margins. The range of cycles and cycle-scale mudstone heterogeneities recorded in this study can be used to help understand basin-scale facies distributions and mineralogical fluctuations relevant when predicting meter-scale source rock and reservoir facies variation. Using detailed sedimentologic description integrated with high-resolution geochemical analyses (x-ray fluorescence, x-ray diffraction, and bulk organic geochemical analysis), we identified eight different sedimentary type cycles, including four distal mudstone-dominant cycles. In conjunction with detailed core analysis, several iv measured sections were constructed and correlated to previous study sites to create a regional north-south cross section along the eastern Uinta Basin, just west of the Douglas Creek arch. The sedimentological variations between cycles and large-scale stratigraphic architecture are primarily a result of the paleodepositional characteristics of the eastern margin, specifically shoreline gradient, distance from shore, the dominance of carbonate versus siliciclastic sedimentation and wave versus river depositional processes. Within cycles, the depositional control on shallowing dictates cycle-scale facies transitions. In deltaic and nearshore siliciclastic environments, increased sediment supply, and deltaic autogenic processes drive shallowing and carbonate deposition. Within littoral carbonate- and distal mudstone-dominant cycles, where most organic-rich rock is found, hydroclimatic-driven lake level fluctuations force shallowing, resulting in decreasing total organic carbon (TOC) content and increasing carbonate content. However, increased depth and persistent stratification decreases the observable effects of lake level on mudstone-dominant cycles, creating a unique cycle signature. |
