EISENBERG, RICHARD A., P. M. HARRIS, and F. J. CONNER, Chevron Oil Field Research Company, La Habra, CA

ABSTRACT: Permeability and Fluid-Flow Patterns in Variably Cyclic Outer-Ramp Carbonates: San Andres Formation, Guadalupe Mountains, New Mexico, Part 1: Geologic Framework and Permeability Controls

Superb outcrop exposures of variably cyclic, fusulinid-rich, outer-ramp facies of the middle San Andres Formation in the Guadalupe Mountains, New Mexico, contain stratigraphic intervals with variable bedding/cyclicity patterns that are analogous to those commonly encountered in San Andres/Grayburg reservoirs of the Permian Basin. Detailed geologic investigations and permeability measurements on outcrop sections and cored wells drilled adjacent to the outcrop area provide a unique opportunity to assess reservoir and interwell-scale variability of permeability and its relationship to depositional and diagenetic fabric. Observed stratigraphically controlled permeability patterns and small-scale variations are honored in conditional fluid-flow simulations (see Part 2 of this paper).

The lowermost 70 feet (21 meters) of the fusulinid-rich interval studied consists of thick-bedded, subtly cyclic, faintly crossbedded crinoid-peloid-fusulinid dolopackstones and dolograinstones. The overlying deposits are 100 to 110 feet (30 to 34 meters) of thin-bedded, noncyclic, crinoid-peloid-fusulinid dolowackestones and dolopackstones. Fifty to sixty feet (15 to 18 meters) of thicker-bedded, peloid-fusulinid dolowackestones and dolopackstones overlie the thin-bedded facies. Four cycles, containing dolomudstone bases and fusulinid-peloid dolopackstone tops, form the uppermost portion of the fusulinid-rich facies in the middle San Andres.

Permeability, measured on core plugs taken along vertical and horizontal outcrop transects and on the cored wells, is related to stratigraphic variation, depositional texture, and diagenetic variation. Mean permeabilities of the various mud-based cycles and the lower fusulinid zones are similar. Within the upper dolomudstone-based cycles, upward-increasing trends of permeability coincide with each cycle. An irregular pattern of increasing permeability with depth is observed within the dolowackestones and dolopackstones of the massive fusulinid zone, whereas a distinct 30 foot-thick (9 meters) zone of highly variable but higher mean permeability occurs within the thin-bedded facies.

Permeability variation superimposed on these gross permeability patterns is related to small-scale depositional and diagenetic heterogeneity. Statistically different mean permeabilities are related to variation of grain, matrix, fusulinid mold, and vug content. Permeability is also related to the abundance of white sucrosic dolomite, irregularly distributed throughout the middle San Andres. This dolomite inhibits the development of fusulinid moldic porosity.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.