GIANNINY, GARY L., and J.A. (TONI) SIMO, Univ. of Wisconsin - Madison, Department of Geology and Geophysics, Madison, WI

ABSTRACT: Kilometer-scale Facies Variability on a Low Angle Carbonate/Siliciclastic Ramp, Lower Desmoinesian of the Paradox Basin, SE Utah

Syn-depositional topography and relative position on the shelf were dominant influences on facies distributions of the high frequency lower Desmoinesian mixed carbonate/siliciclastic cycles on the southwestern shelf of the Paradox basin. Spectacular surface exposures of the Akah oil and gas production interval, in the entrenched meanders of the San Juan River in SE Utah, provide nearly continuous dip and oblique strike sections in a 3 x 10 km area, a scale similar to most Pennsylvanian reservoirs. The Akah consists of 5-15 m thick unconformity-bounded cycles that could easily be misinterpreted as laterally uniform in facies composition and thickness. However, detailed field work reveals that within a cycle, facies composition, thickness, and depositional geometries can change markedly within less than 1 kilometer. For example, one unconformity-bounded cycle shows a lateral change from 8 m of carbonate grain/packstones to 25 m of progradational siliciclastics (60%) and carbonates (40%) over a distance of 800 m in a down-dip direction. In addition, intra-cycle meter-scale coarsening-upwards and shallowing-upwards depositional pulses are highly variable and many are too localized to correlate for more than hundreds of meters.

Detailed three dimensional facies analysis and the recognition of significant facies shifts remain an integral part of architectural reconstruction and of permeability and porosity prediction and are not superseded by cycle thickness variations (i.e., stacking patterns) alone. In constructing sequence stratigraphic and cyclostratigraphic models to understand and explore reservoir scale variation on similar low angle carbonate/siliciclastic ramps. it is critical to utilize three dimensional facies data sets that can incorporate highly variable facies compositions and geometries.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.