Permian (Wolfcampian) Admire C": New Exploratory Potential in the Northern Denver Basin"

Abstract

Significant frontier exploratory potential exists in the lower Wolfcampian Admire C unit, a regionally continuous, porous dolomite interval occurring at depths of 7800-8500 ft (2379-2592 m) in western Nebraska, United States. Based on core, mud-log, thin-section, and SEM (scanning electron microscope) image analysis, as well as both regional and detailed local mapping, the Admire C has been found to be the most prospective of several reservoir zones in this region based on (1) consistency and occurrence of oil shows; (2) superior reservoir quality and distribution; and (3) established patterns of production. Continuity of the reservoir directly reflects transgressive-regressive cycles responsible for carbonate deposition in a shallow, intermittently restricted basin (Alliance basin) affected by glacio-eustatic sea level changes. Highest reservoir quality is concentrated in higher energy packstones and rare grainstones that exhibit a combination of intercrystalline, moldic, and vuggy porosity. The Admire C unit has low-to-moderate permeabilities (0.5-30 md) that restrict short-term flow capacity. A number of wells, however, have shown recoveries in the 75,000-130,000 bbl range at low and decreasing rates of annual decline, suggesting that they are in contact with a larger, more continuous reservoir than was previously thought. Source beds include black shales within the Pennsylvanian and Wolfcampian sections. Little free water exists in the Admire C zone, suggesting mobile water may have been displaced by hydrocarbon migration. A number of intriguing similarities between the Admire C and the Ordovician Red River B reservoir in the Williston basin imply that exploration strategies successful in the latter might be applied to the Admire.