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.

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