Digital Characterization of Lower Ismay Phylloid Algal Mounds in the San Juan River Gorge, SE Utah

Scott Ritter, Colton Goodrich, and Lincoln Reed

The spectacularly exposed middle Pennsylvanian phylloid algal mounds in the vicinity of Eight-Foot Rapid on the San Juan River have been lauded as outcrop analogs of hydrocarbon-producing carbonate mounds in the Greater Aneth Field and dozens of satellite fields in the Paradox Basin, Utah for many years. Owing to the nature of the exposures, standard outcrop techniques (measured sections and outcrop photography) applied to the Eight-Foot Rapids mound field to-date do not meaningfully characterize, nor quantify critical reservoir parameters. A LIDAR-based survey provides a continuous, high-resolution survey of the outcrops along a 2.7 kilometer-long stretch of the San Juan River. Modeling of selected horizons within the Lower Ismay depositional sequence permit calculation of volumes of key stratigraphic units. Stochastic modeling of the surface defined by the tops of the mounds can be used to determine the shapes of the individual mounds and the spacing of the mounds over a four square mile area. Such models indicate that mounds are relatively equant in footprint, suggesting that current alignment of phylloid algal fragment accumulations (mounds) was not a major process in their genesis. The dominant component of carbonate mounds is non-aligned fragments (blades) of the phylloid genus Eugonophyllum. However, the upper parts of selected mounds are characterized by the occurrence of relatively complete thalli of Eugonophyllum and decimeter-scale in situ Chaetetes heads, indicating a change in depositional conditions/ecology during the latter stages of mound development. Intermound depressions ranging in size from 30 to 40 meters in diameter and up to 6 meters deep were filled with grain-rich carbonate sediment that displays an shallowing-upward change in marine biota within individual intermound pools.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013