Abstract: Development Strategy in a Fractured Dolomite Reservoir, Permian Basin, Southwestern USA

R. P. Major, Mark H. Holtz

The Permian (Guadalupian) San Andres reservoir at Keystone field, in Winkler County, Texas, is divided into 3 major stratigraphic units and 12 flow units on the basis of multiple upward-shoaling cycles of shallow-water marine to tidal-flat carbonate facies. Minor amounts of siliciclastic material concentrated in tidal-flat rocks impart a gamma-ray signature that allows correlation with well logs. 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, with a 1,500-ft-wide, 100-ft-thick fairway of high saturations having a northwest-southeast trend.

Matrix permeabilities in this reservoir are very low, commonly less than 1 md in rocks having porosities of nearly 10 percent. New wells in areas of highest original oil in place initially produce at rates up to 120 barrels of oil per day. However, these high production rates are commonly short lived and decline an average of 75 percent in the first 6 months. Vertical 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.

Borehole asymmetry and regional stress measurements suggest that the direction of principal compressive stress, which dictates the strike of fractures that effectively transmit fluids, is northeast-southwest. Thus, horizontal boreholes parallel to the strike of maximum oil volume and perpendicular to the strike of effective fractures will maximize primary recovery. Recognition that injected water must load fractures before effectively sweeping the matrix will be critical in designing an effective waterflood.

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas