Modeling Reservoir Heterogeneity Within Outer Ramp Carbonate Facies Using an Outcrop Analog, San Andres Formation of the Permian Basin

Abstract

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 wells 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 distribution was evaluated using a field permeameter and conventional measurements on small core plugs taken along vertical and horizontal outcrop traverses and from the slim-hole cores. Permeability is related to the variable development of depositional cycles within a small-scale sequence and also to diagenetic overprinting. Geostatistical models of permeability variation, honoring the geologic and petrophysical data, were constructed and input into a waterflood simulator to understand the interactions between heterogeneity and flow. Variograms constructed from conventional plug analyses of permeability for a vertical outcrop transect contain a significant small-scale signal (noise), as seen in the high nugget effect (approximately 50% of the overall sample variance), and have correlation ranges of less than 4.5 m (15 ft). Different 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 short (3-3.5 m; 10-12 ft) when compared to typical interwell distances, supporting a nearly uncorrelated and highly variable permeability model. Using observed short ranges of vertical and horizontal correlation and honoring the vertical transect data, cross sectional, conditionally simulated permeability fields were generated and used in simulated waterfloods to investigate the sensitivities to an oil recovery model and overall fluid injection rate for this style of stratigraphy and cyclicity. Cyclic parts of the section are characterized by a potential for early water breakthrough and relatively high vertical sweep efficiencies. Within the less distinctly cyclic section, waterflood fronts have a fingerlike profile and vertical sweep efficiency is generally poorer.