Distribution of permeability patterns, Upper San Andres Formation outcrop, Guadalupe Mountains, New Mexico
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Permeability patterns in the subsurface are primary controls on fluid flow. Predictable patterns of permeability can be applied to secondary oil recovery or modeling of solute transport. Conventional field data from well tests are insufficient for accurate modeling of the subsurface environment. The use of analog models from outcrop data with emphasis on analog textures and facies can provide needed insights. Permeability was measured on a 2613 ft. long and 20 - 25 ft. thick outcrop of a single mudstone-bounded carbonate parasequence in a transgressive shelf margin. The four genetic facies were: 1) a mud-dominated deep-water flooded shelf; 2) a coarsening upward, mud-supported ooid and peloid wackestone/packstone shallow shelf; 3) an ooid and peloid grain-supported bar crest; and 4) an ooid and peloid grain-supported, coarsening-upward bar flank. Data were collected in the field with a mini-permeameter and on core plugs from the outcrop. Sampling was performed at 769 locations along 31 vertical transects and in two small-scale grid patterns. Separation distances varied from 325 ft. between the widest spaced vertical transects, down to 1 in. separation within the smallest scale grid. Horizontal variogram analysis for range values indicate a scale dependency based on the sample separation and related variogram step distance, h. Horizontal variogram ranges were 253 ft. (77.8 m) and 748 ft. (230 m) for h of 100 ft. (30.7 m); 22 ft. (6.7 m) for h of 10 ft. (3.1 m); 4 ft. (1.3 m), for h of 1 ft. (0.3 m); and 8 in. (0.2 m) for h of 1 in. (0.03 m). These all exhibited a proportionately high nugget-to-sill ratio of poorly developed spherical variogram models. Development of vertical variograms indicate vertical variability within the parasequence. In populations which represented increasingly less of the whole parasequence thickness, an h of 1 ft. provided vertical ranges of 13 ft., 10.2 ft. and 4 ft. (a nested set), and 2.1 ft. In the 1 in. grid, an h of 1 in. provided a vertical variogram of range 4.2 in. Nugget-to-sill ratios were all moderate with exhibited poorly developed spherical variogram models. A preliminary model based upon the data in a single parasequence can be applied as a grainstone sequence with randomly distributed permeabilities that is bounded above and below by a confining mud dominated layer which may or may not be continuous.