Abstract: Log-Based Petrophysics of the Permian Ervay Member of the Park City Formation, Cottonwood Creek Area, Bighorn Basin, Wyoming

Edward B. Coalson, Richard F. Inden

The Ervay Member of the Park City Formation in the Bighorn Basin of Wyoming contains open marine, restricted subtidal, peritidal, and lagoon/salina lithofacies. Most of these lithofacies have a distinctive log "signature", or uniquely identifying collection of log characteristics. The gamma ray log is of particular value in recognizing uranium-rich subtidal rocks. Yet several types of logs are needed to differentiate Ervay lithofacies.

Lithofacies identifications from logs allow reconstruction of the depositional history of the Ervay in the area of Cottonwood Creek Field. The Ervay is of anomalous thickness due to late Permian rejuvenation of a post-Tensleep (PermoPennsylvanian) horst valley. The resulting embayment of the Ervay shoreline filled with subtidal and peritidal sediments during four sea level rises. The microstratigraphy of these four cycles suggests that overall bathymetric relief was very low, but that several modern anticlines (e.g., Worland Dome) were slightly structurally positive during Ervay time. One cycle displays a barrier development not associated with local paleostructure.

Deposition strongly affected reservoir development in the Cottonwood Creek area. Lower intertidal dolomites are the best reservoir rocks, with permeability up to 500 md. Upper intertidal and subtidal dolomites are less permeable (0.1-1 md), but still reservoir rocks. Exposure created diagenetic terrains and cemented sheet cracks that are barriers to fluid flow. The resultant high contrasts in permeability strongly affect primary performance and response to water flooding and acid stimulation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994