Sequence Stratigraphy Interpretation of the Ervay Member (Permian, Park City Formation), West-Central Wyoming Paleoshelf

CLARK, W. J., Marathon Oil Co., Littleton, CO, P. W. CHOQUETTE, E. E. HIATT, T. R. GARFIELD, and D. A. BUDD, University of Colorado, Boulder, CO, and N. F. HURLEY, Marathon Oil Co., Littleton, CO

The Park City Formation is an important unconformity-bounded, Upper Permian stratigraphic sequence. Established lithostratigraphic correlations within the unit can be reinterpreted within a sequence stratigraphic framework. Three major carbonate-rich parasequences occur. From bottom to top, these are the Grandeur, Franson, and Ervay.

The Ervay is the main hydrocarbon-producing unit within the Park City. In the study area it generally consists of low-energy, restricted- to open-marine wackestones and packstones deposited on a westward-dipping carbonate ramp. Interbedded sandstones are also present. The lower boundary of the Ervay is a disconformity that ranges from a submarine omission surface to an erosional exposure surface. The upper boundary is the Permo-Triassic regional unconformity. The Ervay exhibits several transgressive surfaces and possible thin condensed zones in cores and outcrops. These features, which commonly appear as high gamma-ray markers on logs, are characterized by phosphatic and skeletal lags, marine hardgrounds, and glauconite. Log correlations of these features allow stratal geometries to b determined. In the lower Ervay, transgressive units appear to onlap eastward. In the upper Ervay, geometries are primarily aggradational.

Most Ervay hydrocarbon production is from structural traps. In at least one field, however, updip stratigraphic pinch-outs form traps. At a regional scale, or in analogous systems, the sequence stratigraphic approach may help define other pinch-outs, dolomitized trends, fairways of grainier facies, and areas where source and reservoir rocks are juxtaposed. The sequence stratigraphy of the Ervay parasequence also helps explain the complex reservoir zonations found throughout the study area.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)