Although phylloid algal mounds have been studied for 50 year, much remains to be determined concerning the ecology and sedimentology of these Late Paleozoic carbonate buildups. Herein we perform a digital outcrop study of the well-known Middle Pennsylvanian Lower Ismay mound interval in the Paradox Basin because outcropping mounds along the San Juan River are cited as outcrop analogs of reservoir carbonates in the Paradox Basin oil province of Utah and adjacent states. The principal field area is the Eight Foot algal field located at river mile 19.2 on the San Juan River, approximately 14 miles SSW of Bluff, Utah. The Lower Ismay section is exposed on both sides of the river for 1.4 miles. Mechanisms for mound formation are still a heavily debated topic and even now aren't fully understood. While this study does not seek to solely answer this question, it does shed some light on the argument. A combined total station-LIDAR survey of the exposed Eight Foot mounds indicates that the mound field is comprised of 83 individual and composite mounds that have an average height of 10.9 meters and peak spacing of 48.8 meters. Further, statistical examination of survey data reveals a correlation between mound height and east-west alignment, showing that shelfward mounds were slightly taller than their more basinward counterparts.. However, other shape parameters do not appear to vary systematically across the algal field. Curve-fitting indicates that the overall mound morphology does not differ significantly from a Gaussian surface indicating that mounds are conical in shape. This suggests that mounds did not form under the influence of directional currents such as waves or tides. Yet, Ivanovia-fragment packstone and grainstone facies typical of the mound interval suggest a high-energy depositional setting.

The Aneth Field in the Paradox Basin (SE Utah) has produced nearly 500 MMbbls of oil from phylloid-algal and oolitic carbonate reservoirs of the lower and upper Desert Creek (Paradox Formation, Middle Pennsylvanian) sequences, respectively. The oil resides in a 150 to 200 foot-thick isolated carbonate platform located in a distal ramp setting on the southwest margin of the Paradox Basin. The horseshoe-shaped platform is roughly 12 miles in diameter with an aerial extent of approximately 144 square miles. Evaluation of the platform-to-basin transition on the leeward (southern) margin of the Aneth Platform, the focus of this study, was made possible through Resolute Energy's 2017 donation of well data and core to the Utah Geological Survey Core Research Center. The lower Desert Creek sequence ranges from 50 to 100 feet in thickness and produces from a succession of phylloid-algal, boundstone-capped parasequences in the Aneth Platform. The upper Desert Creek sequence is generally thinner across the platform and is characterized by a succession of oolite-capped parasequences, except on the southern margin of the platform where it ranges from 80 to 115 feet in thickness. The upper Desert Creek thick resulted from southward shedding of platform-derived carbonate sediment and lesser amounts of quartz silt and very fine sand off the low-angle southern platform margin slope. A nine-mile-long, north-south-oriented stratigraphic panel constructed from log and core data permits characterization of thickness and facies trends through the upper Desert Creek from platform (north) to slope to distal basin (south) in the Ratherford unit. In the southern margin, five novel facies for the Aneth Field were analyzed, described, and interpreted using a sequence stratigraphic framework, all of which represent deposition on a gravity-influenced platform-edge slope. It is interpreted that the slope facies association was deposited during transgression and highstand and was generally a result of oversteepened slopes as a function of the carbonate factory on the platform being highly productive. Slope and basin facies range from proximal rudstone and floatstone to thin, graded distal turbidites, the latter of which extend at least five miles into the basin. Compaction of the muddy and fine-grained allochthonous sediment followed by pervasive calcite and anhydrite cementation has destroyed any primary porosity in the platform-derived slope-to-basin sediments.