Abstract

The outcrop distribution, geometry, lithofacies, and vertical gamma-ray signature of Dakota Formation fluvial channel deposits in eastern Grand County, Utah, provide insight into the architecture and lithological variability of Dakota reservoir sandstones in the southern Uinta Basin. In this area, the Dakota Formation consists of two unconformity-bounded depositional intervals. The lower Dakota interval is incised as much as 30 ft into the underlying Cedar Mountain Formation whereas upper Dakota fluvial channels are incised as much as 65 ft into the lower Dakota. In some places, the lower Dakota interval has been completely eroded and upper Dakota channel deposits rest directly on the Cedar Mountain Formation.

Individual channel sandstone bodies consist of single story, upward-fining packages of cross-stratified and horizontally-stratified conglomerate and coarse- to fine-grained sandstone. The orientation of internal bounding surfaces indicates deposition by migrating in-channel dunes and downstream/laterally migrating bar forms. Average paleocurrent orientations indicate north-northeast directed paleoflow. Measured flow-perpendicular widths of observed Dakota channel bodies are between 600 and 3000 ft. Maximum channel form thickness ranges from 25 to 45 ft. Width/thickness ratios of these deposits range from 24 to 67. In eastern parts of the study area, however, vertical and lateral amalgamation of individual channel forms produce sheet-like complexes that are up to 100 ft thick and 5000 ft wide, with width/thickness ratios between 50 and 125.

The observed outcrop geometry of Dakota Formation fluvial complexes is compatible with the observed distribution of reservoir sandstones in the subsurface of the southern Uinta basin. Most gas production from the Dakota in the region is associated with amalgamated channel sandstones. Amalgamated fluvial channel complexes are likely the product of deposition within incised valleys near the base of the Dakota, or within channel belts during deposition of the upper parts of the formation.