ABSTRACT: Depositional Cyclicity and Scaling Petrophysical Parameters for Characterization of Fluid Flow in Carbonate Platform Reservoirs: San Andres Outcrop, Algerita Escarpment, New Mexico
LUCIA, F. J., C. KERANS, and R. K. SENGER, University of Texas at Austin, Austin, TX
San Andres outcrops along the Algerita escarpment in the Guadalupe Mountains of New Mexico are composed of five sequences: a lower to middle San Andres sequence and four upper San Andres sequences. Within a sequence the predictable stacking patterns of cycles in different systems tracts, and of facies and rock-fabric successions within cycles, provide the necessary geologic framework for petrophysical quantification of geologic models.
Rock fabric is a fundamental scale controlling the petrophysical properties of porosity, permeability, and capillarity. Four basic rock fabrics are present in the upper San Andres at Lawyer Canyon: dolograinstone, dolograin-dominated packstone, finely crystalline mud-dominated dolostone, and separate-vug dolograinstone. These four rock fabrics have unique average permeability values and porosity-permeability transforms. Rock fabrics are stacked systematically within the cycles. Detailed geologic mapping in a 2500-ft-long and 160-ft-high outcrop at Lawyer Canyon reveals nine cycles that change vertically and laterally from mud-dominated fabrics to grain-dominated packstone and grainstone. Only cycle 7 is characterized by significant separate-vug porosity (grain molds). Low-permeability dolomudstones and dolowackestone beds, typically found at the base of cycles, display variable degrees of lateral continuity and are important as flow barriers.
Permeability distribution was determined in the grainstone facies of cycle 1 at scales ranging from 1 in. to 100 ft. Variogram analysis of spatial permeability distribution indicates short-range correlation with a relatively high nugget, suggesting that permeability heterogeneity is largely random within a rock-fabric facies. A detailed cross section showing the nine cycles and the vertical and lateral distribution of rock fabrics within the cycles was converted to a permeability flow model using geometric mean permeabilities. The resulting rock-fabric flow model is suitable for input into reservoir simulators for performance prediction studies.
AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)