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 of fractures, indicate that productivity is dependent on fracture permeability.

Horizontal boreholes perpendicular to the strike of effective fractures and within the part of the reservoir that contains highest remaining oil will maximize primary recovery. Borehole breakouts and regional stress measurements suggest that the direction of principal horizontal compressive stress is northeast-southwest, and natural fractures that strike in this direction are most likely to be open and capable of transmitting fluids.

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