Abstract

An outcrop-based high resolution sequence stratigraphic study of Pennsylvanian strata on the exposed Paradox shelf has provided insight into the three-dimensional architecture and distribution of both phylloid algal mound reservoirs and their associated facies in the subsurface. In addition, we relate the timing of mound growth and porosity development to high-frequency sea-level changes. The Desert Creek and Ismay producing intervals of the Paradox Formation (Desmoinesian), exposed along the San Juan River in southeastern Utah, are characterized by high-frequency cyclic repetition of carbonate and siliciclastic facies controlled primarily by high-frequency changes in relative sea level. These cycles are generally characterized by a basal sandstone unit, followed by the deposition of black “shale” source rocks recording a subsequent major flooding event. A well-defined shallowing-upward trend among carbonate facies at the top of the cycle is interpreted as deposition during highstand through the early stages of the ensuing fall. Phylloid algal mounds, which form primary reservoir facies in some of the cycles, developed from earliest to latest highstand. Subsurface distribution and geometries of capping evaporites indicate local deposition in intermound depressions on the shelf during latest highstands as well as throughout lowstand and the earliest part of the transgressive phase in the basin.

Lateral variability within the shelfal cycles is observed in both the distribution and thickness of facies. This variability in reservoir and associated facies occurs within tens or a few hundreds of meters, as well as on a kilometer scale, and is a function of antecedent topography, facies types, exposure, unfilled accommodation space, and the amplitude and frequency of relative sea-level change. Landward and seaward shifting of facies belts across the shelf by tens of kilometers, for example, occurs in response to 4th-order (100-400 k.y.) relative sea-level changes. An awareness of facies shifts at this scale can help direct and refine exploration efforts. In addition, rapid lateral variability in the thickness of algal mound reservoirs and associated facies observed in 4th and 5th-order (<100 k.y.) depositional cycles may influence both exploration and production scale efforts in the basin. These results, coupled with facies analysis indicating that many of the cycles did not fill all available accommodation space, suggest that cyclostratigraphic correlations across the basin based on a limited amount of vertical data must be applied with caution.