The Permian (Guadalupian) San Andres reservoir at Keystone field, Winkler County, Texas, is divided into three major stratigraphic units and twelve flow units on the basis of an analysis of multiple shoaling-upward cycles of shallow-water marine to tidal-flat carbonate facies. These rocks are now thoroughly dolomitized and cemented with anhydrite and gypsum. The distribution of original oil in place was mapped both laterally and vertically. Most of the resource is in the upper five flow units, and the original-oil-in-place map of these upper flow units indicates that the highest concentration of hydrocarbons is in the center of the study area.

Porosities in this reservoir are nearly 10%, and permeabilities are generally less than 1 md. Despite these low matrix permeabilities, recently drilled wells initially produced at rates as high as 120 bbl of oil per day, although these rates declined an average of 75% in the first 6 months. Primary recovery from this reservoir is only 8% of original oil in place.

Subvertical fractures in this reservoir are visible in cores and on a microimage log. Early floodwater breakthrough occurred without increased oil production in a pilot waterflood. These production characteristics, combined with direct observations 

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

¹Manuscript received March 14, 1996; revised manuscript received October 14, 1996; final acceptance February 18, 1997.

²University of Texas at Austin, Bureau of Economic Geology, Austin, Texas 78713-8924.

Funding for this study was provided by the Office of the Governor of Texas. We are grateful to R. C. Richards for access to data and to R. C. Richards, S. E. Laubach, and C. A. Barton for discussion. John Lorenz reviewed the manuscript for the AAPG Bulletin. Published with permission of the director, Bureau of Economic Geology, University of Texas at Austin.