ABSTRACT
Regional to interwell scales of heterogeneity are exposed in the San Andres outcrops along the Algerita Escarpment in the Guadalupe Mountains of New Mexico. The regional strati-graphic framework is composed of five sequences: a lower to middle San Andres sequence and four upper San Andres sequences. Each sequence is composed of systems tracts that contain a number of parasequences. Stacking patterns of parasequences and of fades and rock-fabric successions within parasequences are systematic and provide the necessary geologic framework for petrophysical quantification of geologic models.
Detailed geologic mapping at the interwell scale in a 2,500-ft-long and 160-ft-high outcrop of the basal upper San Andres sequence in a ramp-crest facies tract shows a geologic model composed of nine shoaling-upward parasequences 10 to 40 ft thick. The parasequences are composed of vertical and lateral successions of dolomitized mud-dominated to gram-dominated rock fabrics. Low permeability mudstones and wackestones, typically found at the base of parasequences, display variable degrees of lateral continuity. High permeability is restricted to grain-dominated packstones and grainstones containing inter-granular porosity that are typically found at the top of parasequences, although mud-dominated packstones and wackestone may also occur at the top of parasequences. Some parasequences are capped by tidal-flat facies which are considered to be tight by analogy with subsurface tidal flat facies. The grainstones of parasequence 7 are unique having high volumes of moldic (separate-vug) porosity and anomalously low permeability values.
Regional to interwell scales of heterogeneity are exposed in the San Andres outcrops along the Algerita Escarpment in the Guadalupe Mountains of New Mexico. The regional strati-graphic framework is composed of five sequences: a lower to middle San Andres sequence and four upper San Andres sequences. Each sequence is composed of systems tracts that contain a number of parasequences. Stacking patterns of parasequences and of fades and rock-fabric successions within parasequences are systematic and provide the necessary geologic framework for petrophysical quantification of geologic models.
Detailed geologic mapping at the interwell scale in a 2,500-ft-long and 160-ft-high outcrop of the basal upper San Andres sequence in a ramp-crest facies tract shows a geologic model composed of nine shoaling-upward parasequences 10 to 40 ft thick. The parasequences are composed of vertical and lateral successions of dolomitized mud-dominated to gram-dominated rock fabrics. Low permeability mudstones and wackestones, typically found at the base of parasequences, display variable degrees of lateral continuity. High permeability is restricted to grain-dominated packstones and grainstones containing inter-granular porosity that are typically found at the top of parasequences, although mud-dominated packstones and wackestone may also occur at the top of parasequences. Some parasequences are capped by tidal-flat facies which are considered to be tight by analogy with subsurface tidal flat facies. The grainstones of parasequence 7 are unique having high volumes of moldic (separate-vug) porosity and anomalously low permeability values.
Over 1,000 permeability measurements were conducted at scales of 1 inch, 1 ft, 5 ft, 25 ft, and 100 ft within the grainstone facies of parasequence 1. Within parasequence 1, distinct variability of petrophysical characteristics occurs at scales well below those of interwell spacing (660 to 1,330 ft). Geostatistical analyses of permeability measurements indicate varying permeability correlation at the different measurement scales. In all cases, however, approximately half of the permeability variability (variance) is due to locally random heterogeneity (nugget effect).
The apparent random permeability distribution in the grain-stone facies suggests that geometric-mean permeability values for specific rock-fabric units can be used as input into reservoir simulators. This idea was tested by comparing numerical simulations of waterflood using conditional permeability realizations with numerical simulations using a geometric-mean permeability distribution. The results were similar, indicating that the short-range permeability correlation does not affect the cumulative production characteristics.
The geologic model was quantified for input into a numerical simulator using rock-fabric specific geometric-mean permeability and arithmetic-mean porosity and saturation values. Statistical analysis of permeability data from all parasequences shows significant differences in the geometric-mean permeability of rock-fabric classes. Average porosity and water saturation values were added using rock-fabric specific porosity-permeability and porosity-water saturation transforms developed from subsurface and outcrop porosity, permeability, and capillary pressure data.
The resulting rock-fabric flow model was used as input into a numerical simulator. The results of flow experiments using this model indicate that the lateral continuity of flow-barrier beds, the distribution of rock-fabrics, and the direction of injection-water flow significantly affects recovery efficiency and location of remaining mobile oil. Laterally continuous permeable grainstone facies are the main conduits for fluid flow and the cause of early water breakthrough. Significant cross flow occurs where permeable grainstones terminate against low-permeability mud-dominated facies trapping unswept oil in an upstream location.
The outcrop results have been applied to the subsurface Seminole San Andres field in West Texas. The upper productive interval in this field has rock-fabrics and parasequences similar to those of the San Andres outcrop. Innovative uses of neutron, density, acoustic, and laterologs were developed to estimate rock-fabrics, separate-vug porosity, total porosity, water saturation, and permeability for wells in a two-section study area. The resulting rock-fabric flow model is similar to the outcrop model.
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