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The Influence of Faults on the Deposition of Large Lacustrine Carbonate Mounds in the Green River Formation of Utah and Wyoming: Implications for Understanding Spatial Distribution and Reservoir Quality


The distribution, petrology, and reservoir potential of large lacustrine carbonate mounds in the Eocene Green River Formation (GRF) provide useful insights into similar deposits in Lower Cretaceous pre-salt reservoirs offshore Brazil and Africa. Large, multi-meter-scale carbonate mounds in the GRF of Wyoming and recently identified mounds in Utah, appear to coincide with fault-derived spring discharge zones, where spring waters enriched with Ca2+ mixed with alkaline lake waters and precipitated CaCO3. Furthermore, fluids that emanated from these faults likely accounted for much of the post-diagenetic alteration of these facies. Here, we examine the distribution of primary GRF carbonate mounds and their diagenetic overprints in relation to regional and local tectonics. The carbonate mounds of the GRF, typically 3-10 m in size, occur in lake margin deposits that often interfinger with siliciclastics of the underlying Wasatch Formation. Large dome-shaped mounds composed of biotic and abiotic textures (digitate stromatolites, travertine) imply subaqueous spring deposits (SSD). Important paleohydrologic indicators sometimes associated with SSD are subaerial spring deposits (ASD) that exhibit gravity-controlled precipitated CaCO3 (flowstones and chemical dendrites) and contain Sr isotopic groundwater fingerprints. Stratigraphic stacking of both SSD and ASD, and associated intra-mound grainstones, reflect marginal lake level oscillations near spring discharge zones. In addition, several lower GRF subsurface giant (5-30 m thick) ‘algal’ mounds on the southeastern corner of the Uinta Basin (including the West Willow Creek oil field) include interbedded microbialite and intra-mound grainstone facies that cover a triangular area of 5 km2. Basin-scale evaluation suggests the distribution of large GRF mounds are controlled mainly by deep-seated faults related to basement structures. Some mounds are aligned parallel to reverse faults; some are clustered and isolated near normal and thrust faults. Commonly, post-depositional hydraulic fracturing produced cross-cutting fractures and veins, filled with travertine and silica cements, and caused brecciation and sometimes enhanced secondary porosity. The giant Uinta Basin algal mounds do not display typical hydraulic brecciation, but log and core interpretations imply penecontemporaneous normal faulting. Nevertheless, we argue that faults influenced the location of spring mounds in the GRF and likely in the pre-salt.