Depositional Model for the San Andres Formation, Roberts Unit, Wasson Field, Yoakum County, Texas

Abstract

The Permian San Andres reservoir at Roberts unit produces from approximately 250 ft of anhydritic dolostones. The reservoir interval, which is more than 500 ft below the top of the San Andres Formation, consists of fossiliferous and pelletal/peloidal dolowackestones and dolopackstones. They were deposited in a shallow-marine environment with local shoaling conditions. Toward the top of the reservoir, intertidal and supratidal deposits interfinger with the subtidal units and form the lateral and overlying seals. A sponge-bryozoan bank lithofacies is recognized within the subtidal deposits at Roberts unit. The banks consist of dolomitized mud-rich boundstones dominated by bryozoans, sponges, and crinoids. Interbedded fossiliferous dolowackestones, dolopackstones, and dolograinstones are common. The restricted nature of the San Andres in the western part of Roberts unit (i.e., shoreward of the banks) indicates that the banks baffled wave energy and inhibited current circulation on the platform, resulting in a mud-dominated, restricted lagoonal facies with very low faunal diversity. The sponge-bryozoan banks occur within a narrow belt across the central part of Roberts unit and continue into the adjacent Willard unit. Their distribution has a distinct northeast-soutwest trend that parallels the subjacent Abo shelf margin reef trend, suggesting that the Abo reef trend influenced subsequent bankmore » development.« less