| ABSTRACT
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. | 
