MODELING RESERVOIR HETEROGENEITY WITHIN OUTER RAMP CARBONATE FACIES USING AN OUTCROP ANALOG, SAN-ANDRES FORMATION OF THE PERMIAN BASIN

Variably cyclic, fusulinid-rich, outer ramp facies of the Permian San Andres Formation are exposed along the Algerita escarpment, Guadalupe Mountains, New Mexico. We have used the outcrop exposures and cored we lls drilled adjacent to the outcrop to assess reservoir- and interwell -scale variability of permeability as a potential analog for carbonate reservoirs in the Permian basin and elsewhere. Permeability distribut ion was evaluated using a field permeameter and conventional measureme nts on small core plugs taken along vertical and horizontal outcrop tr averses and from the slim-hole cores. Permeability is related to the v ariable development of depositional cycles within a small-scale sequen ce and also to diagenetic overprinting. Geostatistical models of perme ability variation, honoring the geologic and petrophysical data, were constructed and input into a waterflood simulator to understand the in teractions between heterogeneity and flow. Variograms constructed from conventional plug analyses of permeability fog a vertical outcrop tra nsect contain a significant small-scale signal (''noise''), as seen in the high nugget effect (approximately 50% of the overall sample varia nce), and have correlation ranges of less than 4.5 m (15 ft). Differen t vertical variogram characteristics are displayed by cyclic and less distinctly cyclic parts of the San Andres. Variograms constructed for horizontal transect data from three distinct stratigraphic units have nearly identical properties. Overall, the ranges of correlation are sh ort (3-3.5 m; 10-12 ft) when compared to typical interwell distances, supporting a nearly uncorrelated and highly variable permeability mode l. Using observed short ranges of vertical and horizontal correlation and honoring the vertical transect data, cross sectional, conditionall y simulated permeability fields were generated and used in simulated w aterfloods to investigate the sensitivities to an oil recovery model a nd overall fluid injection rate for this style of stratigraphy and cyc licity. Cyclic parts of the section are characterized by a potential f or early water breakthrough and relatively high vertical sweep efficie ncies. Within the less distinctly cyclic section, waterflood fronts ha ve a fingerlike profile and vertical sweep efficiency is generally poo rer.