IDENTIFYING FRACTURE ORIENTATION IN A MATURE CARBONATE PLATFORM RESERVOIR

The Permian (Guadalupian) San Andres reservoir at Keystone field, Wink ler County, Texas, is divided into three major stratigraphic units and twelve flow units on the basis of an analysis of multiple shoaling-up ward cycles of shallow-water marine to tidal-flat carbonate facies. Th ese rocks are now thoroughly dolomitized and cemented with anhydrite a nd gypsum. The distribution of original oil in place was mapped both l aterally and vertically. Most of the resource is in the upper five flo w units, and the original-oil-in-place map of these upper flow units i ndicates that the highest concentration of hydrocarbons is in the cent er 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 aver age of 75% in the first 6 months. Primary recovery from this reservoir is only 8% of original oil in place. Subvertical fractures in this re servoir are visible in cores and on a microimage log. Early floodwater breakthrough occurred without increased oil production in a pilot wat erflood. These production characteristics, combined with direct observ ations of fractures, indicate that productivity is dependent on fractu re permeability. Horizontal boreholes perpendicular to the strike of e ffective fractures and within the part of the reservoir that contains highest remaining oil will maximize primary recovery. Borehole breakou ts and regional stress measurements suggest that the direction of prin cipal horizontal compressive stress is northeast-southwest, and natura l fractures that strike in this direction are most likely to be open a nd capable of transmitting fluids.